How To Quickly Determine The (Dell'Orto) Carburetor Main Jet Size
Update: Hey guys, sorry for some confusion on this article. This formula here mostly applies to small engine Dell'Orto PHBG carbs (12-21mm). It might work for others (tho definitely not for Kehin PWK's). Keep this in mind! Generally speaking tho, the default main jet +15% is often the limit for most carburetors!
Determining the correct main jet size for you engine can often be a tedious task. Especially if you don't know where to start. Sure, the carburetor ships with a main jet, but does this match your engine at all? I found that the following formula gives you a near perfect result for tuned engines and is a perfect starting point to determine the correct main jet size!
Let's jump right into the formula as it really is a piece of cake:
Carburetor Diameter in MM x 5
The idea behind this formula is to give you the ideal main jet size for a theoretical perfect engine. A perfect engine is determined by perfectly matching components and ideal condition (sea level, perfect weather, etc.) When using a racing exhaust and an open air filter (along with a good reed valve, etc.) you will notice that this formula will give you pretty much perfect results for any carburetor based engine (both 2-stroke and 4-stroke).
The Stock Engine Example
Let's apply this formula to an example: On a stock scooter you often have a 12mm carburetor installed, along with a restricted stock exhaust and a bulky stock airbox. In most cases the preinstalled main jet is a 55. This makes sense, as the exhaust and stock airbox restrict the engine. If you now go ahead and install a sports pipe and an unrestricted filter mat (not a pure racing filter as a stock engine can't handle this). You will notice that the formula gives you a pretty much perfect setup:
Carburetor Diameter in MM (12mm) x 5 = 60
The reason why the manufacturer installs a 55 main jet is due to the restriction caused by the stock exhaust and air box. It restricts the engine to produce its full potential and therefore requires a lower air to fuel ratio.
The 70cc Sports Engine Example
Another example on a more highly tuned engine. Let's imagine an engine with a Stage6 Sport Pro MK2 70cc kit, a 17.5mm Dellorto carburetor and a Stage6 Sport Pro exhaust, along with a sports air filter and a Stage6 Reed Valve:
Carburetor Diameter in MM (17.5mm) x 5 = 87.5 -> 88
The resulting 88 main jet will be near perfect. Probably a bit too rich but an ideal starting point!
Going a Step Further
If you want to go a bit further with the formula, you can apply a variable to it, which is fluent between 0.9 and 1. The variable can be set according to your setup, for example on a complete stock engine it would be 0.9:
Carburetor Diameter in MM (12mm) x 5 x 0.9 = 54 (remember the 55 from above?)
For a racing air filter and a racing exhaust I always add 0.05 each to the formula. This way I end up with a theoretical value that matches my components. Given our example above for the 70cc engine, let's imagine we would ride the setup with the original air box (maybe slightly tuned). I would then add 0.05 to the 0.9 due to the Stage6 exhaust:
Carburetor Diameter in MM (17.5mm) x 5 x 0.95 = 83.125 -> 84
Again, the size 84 main jet will probably give me very good results for this setup and allows me to quickly determine the perfect main jet!
The Last 5% on Main Jet Determination
Surely there are other factors that need to be kept in mind when choosing the main jet. The elevation above sea level and the outside temperature are certainly affecting the optimal fuel to air ratio. Also, every engine behaves slightly different. Nonetheless, above formula should give a good indication on where to start! Hopefully I'll get around to write a full carburetor setup tutorial soon! Until then, have a look at my How To on Spark Plug Reading and drive safely.
Let me know in the comments which main jet you ride and if the formula matches! Also, if you need help, drop me a comment!
99 Comments
Sam
Jan 30, 2015
I have a polaris outlaw 90 and want to upgrade the carburater from stock to KEIHIN PE20
What main jet and what pilot jet should I use?
What main jet and what pilot jet should I use?
Daniel
Mar 26, 2015
Hi,
I don't know this carb unfortunately thus it is a bit difficult to tell. My formula works best for Delorto carbs. keihin are a bit different unfortunately. Stock settings should be ok to start with tho...
I don't know this carb unfortunately thus it is a bit difficult to tell. My formula works best for Delorto carbs. keihin are a bit different unfortunately. Stock settings should be ok to start with tho...
Marvin
Dec 2, 2016
Hi... I'm from malaysia... I use SOLEX 34 34 Z1 at my peugeot405. Now I am use 122 main jets on 1st n 2nd choke... Is it posibble to change it to larger size like 150 or as calculation it can reach 170... Tq regards...
Daniel
Dec 2, 2016
Hey Marvin,
Please keep in mind the calculation here really is meant for Dell'orto carburetors. Other carbs use way different numbering schemes and this might not work at all.
Please keep in mind the calculation here really is meant for Dell'orto carburetors. Other carbs use way different numbering schemes and this might not work at all.
Simonas
Feb 9, 2015
Hi Daniel, I have 2012 Aprilia sr50r carb version, just have upgraded to stage 6 sport pro, so to keep stock crank I'm using Giannelli Next and stock 17.5 dellorto carb. Fitted this cylinder kit, starts easy and idles as normal , but bogs when trying to take off, seems like it doesn't want to move at all, tried different jet size 65 to 85, it didn't do any changes. I know it came with few base gaskets for adjusting the squish, seems like it's around 0.7mm as required, but doesn't make any changes. Any suggestions? What might be wrong?
alex
Apr 2, 2015
Does the width of carb x 5 work for larger engined bikes too ? Im riding a 125 piaggio and the carbs been Infinately tampered with by the last owner !
Boris
Apr 20, 2015
Hi Daniel, great guide you have here, it gave me something to go on.
What would you use on a 63cc gy6 4-stroke with a Tuning cam, 18.5mm carb, upgraded head and a full flow leovince exhaust?
I tried with a 82 but it was a tad lean, 86 seems too rich, but might have been oil burning of after the build of the engine.
What would you choose? Thanks mate, you save us a lot of pain and engine wear!
Boris
What would you use on a 63cc gy6 4-stroke with a Tuning cam, 18.5mm carb, upgraded head and a full flow leovince exhaust?
I tried with a 82 but it was a tad lean, 86 seems too rich, but might have been oil burning of after the build of the engine.
What would you choose? Thanks mate, you save us a lot of pain and engine wear!
Boris
Johnny
Apr 24, 2015
Hey Daniel, good article.
I bought and fitted a stage 6 70cc mk2 kit & stage 6 sport replica exhaust and a better air filter in the standard air box to my 2008 adly silver fox. Stock jet was #75 and i was told by my suppliers this set up should use around #90 jet. That size causes a rich a/f ratio. I drilled some 6mm holes in the standard airbox which improved things but not much. But if I apply your formula I should use a #85 jet instead which should lean out the rich a/f ratio. What you reckon? Regards John.
I bought and fitted a stage 6 70cc mk2 kit & stage 6 sport replica exhaust and a better air filter in the standard air box to my 2008 adly silver fox. Stock jet was #75 and i was told by my suppliers this set up should use around #90 jet. That size causes a rich a/f ratio. I drilled some 6mm holes in the standard airbox which improved things but not much. But if I apply your formula I should use a #85 jet instead which should lean out the rich a/f ratio. What you reckon? Regards John.
Daniel
Sep 1, 2015
Hi,
Yes, 85 sounds about right to me. Whats the carb stock size tho?
Yes, 85 sounds about right to me. Whats the carb stock size tho?
Jamie
Aug 9, 2015
Hi, sorry for the late post.
I'm planning to use bike carbs on a 2.4 liter straight six engine (Webers are expensive!). What kind of jets should I run? Thanks! and sorry again for the late post.
I'm planning to use bike carbs on a 2.4 liter straight six engine (Webers are expensive!). What kind of jets should I run? Thanks! and sorry again for the late post.
Daniel
Sep 1, 2015
Hey Jamie,
Sorry for my even later reply! I looked into it and indeed quite a few people do that mod. That's pretty cool i have to say. Regarding jetsize you are on your own there tho. I have honestly no idea how jetting on a car works. I would think it's the same, but... :) Good luck with it tho!
Sorry for my even later reply! I looked into it and indeed quite a few people do that mod. That's pretty cool i have to say. Regarding jetsize you are on your own there tho. I have honestly no idea how jetting on a car works. I would think it's the same, but... :) Good luck with it tho!
Kenny
Aug 27, 2015
I have an xr250r with a 285cc piston kit, full Xr's only competition muffler and header air box is not restricted and the carb is a 41mm
Daniel
Sep 1, 2015
Nice!
Pedheadfred
Sep 5, 2015
Hello iam wondering what size jet should be used on a 72cc running a 24mm flat slide Mikiuni carb ?
CE
Sep 11, 2015
Hi;
Have a bws 100 and already purchased a naraku 113 cc kit. Read the jennings book and ported according to explanations. Targeting 180 degree exhaust duration. I'll be using 19 mm dellorto which came with 90 main. Stock exhaust with a downpipe 28 mm id ro fit my exhaust port and holes on the standard airbox. Machined the head to 55 and will adjust the squish to 1mm. First time in tuning. What do you think about the jet size?
Thanks
Have a bws 100 and already purchased a naraku 113 cc kit. Read the jennings book and ported according to explanations. Targeting 180 degree exhaust duration. I'll be using 19 mm dellorto which came with 90 main. Stock exhaust with a downpipe 28 mm id ro fit my exhaust port and holes on the standard airbox. Machined the head to 55 and will adjust the squish to 1mm. First time in tuning. What do you think about the jet size?
Thanks
Daniel
Sep 24, 2015
I think you can even go bigger with the carb given the displacement. But at 19mm something like 90-100 sounds good.
CE
Aug 27, 2016
Hi daniel!
I still couldn't find time to install the cylinder kit. I know the carburetor seems to be small but the original is 16mm. Don't know why but we have tons of bws 100 here they all have 16mm carbs. The new cylinder kit has larger flange 28mm but the original headers is about. 20, think its a kind of restricted, doesn't pass 8k. Do you think its the reason?
I still couldn't find time to install the cylinder kit. I know the carburetor seems to be small but the original is 16mm. Don't know why but we have tons of bws 100 here they all have 16mm carbs. The new cylinder kit has larger flange 28mm but the original headers is about. 20, think its a kind of restricted, doesn't pass 8k. Do you think its the reason?
CE
Sep 11, 2015
Hi again ;
Checked my notes, 183 ex 124 intake timing. Chamfered ports and 1.5 mm base shave to have a chance to play with timings and squish.
Thanks
Checked my notes, 183 ex 124 intake timing. Chamfered ports and 1.5 mm base shave to have a chance to play with timings and squish.
Thanks
Nathan
Sep 15, 2015
I have a 140cc Konker motor with a gio 125 carb. Was wonder what maximum size jet I could put in it.
Thanks,
Nathan
Thanks,
Nathan
renato
Oct 4, 2015
What is the perfec jetting for 28mm carburator.my mc is yamaha 135.
Daniel
Oct 28, 2015
Depends on the carb. Dellorto as explained in the article. Everything else +/- 10% of the original jet.
Justin
Nov 30, 2015
HI I got a quick question I'm running a Yamaha jog with a stage 6 sport pro 70cc ,stage 6 sport pro exhaust,polini cp 23 carb I'm running a 95 main and a 32 pilot, is this sufficient for my setup? It runs fine but slighly bogs in the begining but I jus Want to know if the Jets I'm running is OK for my setup Thanks alot
Daniel
Dec 4, 2015
Hi Justin,
I think the carb is too big for the setup and you waste a lot of petrol. Also I have to admit I don't have any experience with the Polini carbs. Never tried them. I saw they ship with a 110 mainjet tho. I would expect something like 100 would match your setup better, but can't tell for sure. If it bogs in the beginning then it's rather normal when the engine is cold. Not sure if the choke is adjustable?
I think the carb is too big for the setup and you waste a lot of petrol. Also I have to admit I don't have any experience with the Polini carbs. Never tried them. I saw they ship with a 110 mainjet tho. I would expect something like 100 would match your setup better, but can't tell for sure. If it bogs in the beginning then it's rather normal when the engine is cold. Not sure if the choke is adjustable?
shush
Aug 4, 2016
@ Justin and Daniel: I have this same setup in my 04 Zuma. The 110 the CP23 came with is not enough to sufficiently cool the engine! It gets very HOT very fast (climbs over 380 CHT in 30 seconds to a minute), I have to jet even larger (waiting on jets) to keep the thing from getting too hot. The only thing that wasn't mentioned was the air filter, I am running a UNI 2 stage foam filter on mine.
Side note Daniel, the CP carbs are supposed to be super efficient and use less fuel than normal carbs, so your formula may be completely off on these carbs. And my Zuma will rev to 13k fairly easy with the setup at just under 70mph on GPS :-)
Side note Daniel, the CP carbs are supposed to be super efficient and use less fuel than normal carbs, so your formula may be completely off on these carbs. And my Zuma will rev to 13k fairly easy with the setup at just under 70mph on GPS :-)
Husqvarna
Oct 9, 2015
Hi!
I have Husqvarna sm 125 with dep pipes i want upgrade to mikuni or some 38 carburetor from dellorto 28 with stock airfilter (yet). What jet should i need to use?
Thanks!
I have Husqvarna sm 125 with dep pipes i want upgrade to mikuni or some 38 carburetor from dellorto 28 with stock airfilter (yet). What jet should i need to use?
Thanks!
Daniel
Oct 28, 2015
The Mikuni use a different Airjet numbering system. Should ship with a good setup. Go from there +/- 10%
jay jay
Oct 28, 2015
Hey daniel, would you know how to determine the size of a four barrel carb on a 350 truck engine
Daniel
Dec 4, 2015
Hey JayJay,
No sorry. This calculation rule really only applies to Dellorto carbs up to 21mm :-/
No sorry. This calculation rule really only applies to Dellorto carbs up to 21mm :-/
Justin
Nov 25, 2015
Hey Daniel,
Thanks for putting up this site. After tons of searching on this subject this was the only site that cut to the chase and made it user friends. Thanks man! I wanted to throw this by you and get your thoughts. I have a 1980 Honda Express NC50. Everything is stock except I added a new Mikuni VM20 carburetor with a 43mm K&N RC-1070 air filter. I got rid of the oil pump and now using just pre-mixed gas. The main jet is 180 and the idle jet is 22.5. I can get the bike to idle but as soon as I pull the throttle it bogs down and dies. I'm thinking the main get is to large. So, per your calculations above I was thinking of changing the main jet to a 90. I have also read that the idle jet in this carb is to big for a 49cc bike so I might also change that out to a 17.5. Any thoughts or recommendations for this would be great. Thanks man!
Thanks for putting up this site. After tons of searching on this subject this was the only site that cut to the chase and made it user friends. Thanks man! I wanted to throw this by you and get your thoughts. I have a 1980 Honda Express NC50. Everything is stock except I added a new Mikuni VM20 carburetor with a 43mm K&N RC-1070 air filter. I got rid of the oil pump and now using just pre-mixed gas. The main jet is 180 and the idle jet is 22.5. I can get the bike to idle but as soon as I pull the throttle it bogs down and dies. I'm thinking the main get is to large. So, per your calculations above I was thinking of changing the main jet to a 90. I have also read that the idle jet in this carb is to big for a 49cc bike so I might also change that out to a 17.5. Any thoughts or recommendations for this would be great. Thanks man!
Daniel
Dec 4, 2015
Nice bike. It might also be pulling too much air. If its a stock cylinder you get better performance with a stock airbox, or by taping the racing filter off a bit so it pulls more gas in. Try that!
Garth Pecor
Dec 13, 2015
How about snowmobile performance engines ( I have a strictly race 1404 cc ) triple.
make close to 300hp... does the rules still apply? is it per cylinder so 1404 divide be 3??? THX!!!
make close to 300hp... does the rules still apply? is it per cylinder so 1404 divide be 3??? THX!!!
carlo
Dec 26, 2015
hi daniel,
i have a kawasaki leostar six speed, stage 2 tuned pipe installed on it and a full volume crank shaft (originally 120cc) but now its 130cc w/ a mikuni 22mm carb. should i upgrade my carb or get a bigger jettings on it? my jets is 20/100
i have a kawasaki leostar six speed, stage 2 tuned pipe installed on it and a full volume crank shaft (originally 120cc) but now its 130cc w/ a mikuni 22mm carb. should i upgrade my carb or get a bigger jettings on it? my jets is 20/100
Dave
Dec 27, 2015
I have a 26mm mikuni on a 125cc dax, what size main jet would you suggest???
Thanx in advance,
Thanx in advance,
Jayson
Jan 22, 2016
Hi Daniel
I have a Yamaha BWS 100 which I've big bored to 117cc. I'm running a stock airbox but a race pipe. I ran with stock jetting and bike seized at 3/4 throttle. Offhand I'm unsure of the carb size, but using your formula including the "x 0.95" how would I compensate for the extra 17cc with regards to the jetting?
I have a Yamaha BWS 100 which I've big bored to 117cc. I'm running a stock airbox but a race pipe. I ran with stock jetting and bike seized at 3/4 throttle. Offhand I'm unsure of the carb size, but using your formula including the "x 0.95" how would I compensate for the extra 17cc with regards to the jetting?
Jayson
Jan 22, 2016
Keep in mind that I'm in a coastal town right at sea level (Cape Town, South Africa).
kim
Feb 25, 2016
hi i have a 139qmb 49cc with 18mm carb. i change the stock manifold to a performance manifold and gona put a prformance air filter. whats the best main jet and pilot jet?
Daniel
May 6, 2016
I dont think you will notice much of a difference in performance, tho much more gas consumption. Still tho, about 10-15% larger main jet will be needed. Pilot can remain the same.
lito
Mar 2, 2016
What is the best jetting 28mm carb 57.4mm bore 150cc for gy6 planning to bore up to 60mm
steve
Mar 12, 2016
hi I have a gilera stalker got for prodject its got 17.5 carb race pipe standed air box with malosi fome and 76 main jet bike runs and gos off line well but anything after quter gas bogs like it needs more air or fuel bit stuck can anyone help thanks steve
Daniel
May 6, 2016
Probably the main jet is too big.
Asher Hedger
Apr 16, 2016
Hi I was wondering, I have a sym jet euro x 50cc with an air filter, that air filter is 38mm free flow. What size Jets would I need? I have 4 main jets to one side so I was thinking of drilling them, but what would be perfect setup as I constantly am full throttle (who isn't, it's a moped)
Daniel
May 6, 2016
10-15% bigger normally. Tho, you won't notice any performance improvement. Just louder :)
BengtM
Apr 16, 2016
Hi Daniel!
I have got Sym Jet 50 MY2000 w. the "new" engine BL05W-6.
Just purchased a Malossi Sport 70cc cyl.kit and a Malossi( Dellorto ) PHGB 19mm AS.
Using a standard but modified airbox and a unsrestricted exhaust silencer (not the worst kind, since I want to use the bike daily).
The carb. has a 80 mainjet installed.
What are your suggestion for a mainjet?
Acc. to your calc. I have a guess for 90?
I have got Sym Jet 50 MY2000 w. the "new" engine BL05W-6.
Just purchased a Malossi Sport 70cc cyl.kit and a Malossi( Dellorto ) PHGB 19mm AS.
Using a standard but modified airbox and a unsrestricted exhaust silencer (not the worst kind, since I want to use the bike daily).
The carb. has a 80 mainjet installed.
What are your suggestion for a mainjet?
Acc. to your calc. I have a guess for 90?
Daniel
May 6, 2016
Yes, somewhere between 80 and 90 should be fine. I would just get 85 and 90 and then try 90 first, and if its too big go with 85.
Bobby Miller
Apr 24, 2016
Hi Daniel
I'm currently building a piaggio zip 125 witch had an air cooled version of the piaggio leader engine but I've put a 125 4 valve liquid cooled cylinder and head and I'm getting a 30mm flat slide carb but not sure what size jets I'm going to need
I'm currently building a piaggio zip 125 witch had an air cooled version of the piaggio leader engine but I've put a 125 4 valve liquid cooled cylinder and head and I'm getting a 30mm flat slide carb but not sure what size jets I'm going to need
Daniel
May 6, 2016
Hi Bobby,
Always difficult to tell, especially since my rule thing only applies to Dellorto carbs up to 21mm. Tho, the stock configuration of a Keihin carb is always a good starting point!
Always difficult to tell, especially since my rule thing only applies to Dellorto carbs up to 21mm. Tho, the stock configuration of a Keihin carb is always a good starting point!
fuzzly
May 16, 2016
Hi, is there any formula to determine the slowjet/pilot jet
size? Thanks for the article. Very helpful.
Thanks.
size? Thanks for the article. Very helpful.
Thanks.
Jennifer Amo
May 18, 2016
Hi Daniel.
I'M having the hardest time trying to find new Jets for my Deni 125cc Carb.
I have pics of the Old ones. but don't Know how to Send them . please Help:)
main jet has 82C on it
pilot jet has 35C on it
I'M having the hardest time trying to find new Jets for my Deni 125cc Carb.
I have pics of the Old ones. but don't Know how to Send them . please Help:)
main jet has 82C on it
pilot jet has 35C on it
vader
May 23, 2016
I habe a question, is the area or the diameter of
Main and Pilot Jet to add?
Exampel: CVK 24, Main 102, Pilot 38.
For the diameter of 24 the Jet sie should bei 24x5x0.9 = 1,08
Thus is 0,930mm2
1,02 > 0,817mm2
0,38 > 0,113mm2
Then it World bei perfect sized?
Main and Pilot Jet to add?
Exampel: CVK 24, Main 102, Pilot 38.
For the diameter of 24 the Jet sie should bei 24x5x0.9 = 1,08
Thus is 0,930mm2
1,02 > 0,817mm2
0,38 > 0,113mm2
Then it World bei perfect sized?
terence buckless
Jun 3, 2016
Hi I have a ajs j's 125 which is essentially a Yamaha ybr I have a straight pipe with little restriction and have a free flow air filter could you tell me what the optimum jet would be
Rich
Jun 14, 2016
Let's say for example starting w/a stock 50cc 2-stroke air cooled and you upgrade to a 70cc piston,cylinder,head kit how would you determine carb size at that point?
Ken Bauer
Jun 15, 2016
This has got to be the most straight forward , use full information I have found. Very easy to understand. And very helpful Thank You
Mark Senior
Jun 22, 2016
I have a 50cc yamaha aerox with a 21mm molossi carb and a polini highspeed variator and a decent exhaust, if i follow the carb size times 5 that gives me a 105 jet, this seems way too much for a 50s, what am i doing wrong or is this correct?
thanks
thanks
carl kennedy
Jun 26, 2016
hi i have a dellorto carb 17.5m main jet 83 idal 38 on my speedfight 100cc scooter i have put a new exhaust on a felix open air filter and new inlet and reed now a got the dellorto 21mm car wot wpold be the best jets for a set up as the bikes revs really good but when trying to ride it no power or gain on speed the new 21m dellorto atm is main 100 and idal 40 is this close as i have tryed a cupple things going a bit lean and going a bit rich
dave g
Jun 28, 2016
what about 70cc racing engine with 36mm keihin ( Minarelli scooter)
180 is in the carb right now but havent tested yet
180 is in the carb right now but havent tested yet
Bernek
Aug 27, 2016
I have a question regarding the main jet I should use on this setup:
AM6 engine with:
- Airsal 50cc Sport cylinder kit
- Polini CP 17.5 (defaul has 80 main jet)
- Polini Air filter Sport
- Gianelli exhaust
AM6 engine with:
- Airsal 50cc Sport cylinder kit
- Polini CP 17.5 (defaul has 80 main jet)
- Polini Air filter Sport
- Gianelli exhaust
Emilis
Sep 1, 2016
Hey i have stock 50cc with tecnigas next r , multivar and PHBN 16mm carb. What jet, needle, and other things should i use? Aprilia sr stealth minarelli
Elliott
Sep 2, 2016
I got a Yamaha aerox put a streetrace 70 kit on it and polini high speed veratior and malossi exhaust and replay air filter what jet would it be been trying jets could you help asap
Dave kozmik
Sep 9, 2016
Hi, your formula is correct, interesting the sizes everybody wants advice on, i would stick to the jet the bike came with plus or minus ten percaent, i had an h100sd stock it would do 65mph, the dealer gave me tenpercent up on jet, standard exhaust, smoothbored the carb, clylinder ports, and exhaust port, man it flew, 72mph 7mph overstock, i left the airbox alone as i wanted longevity, 2 pistons 1 rebore and topend rebuild when noisy, 72,000miles pity it was stolen
Romer Nahial
Sep 15, 2016
Sir, I have 68mm bore 48.8stroke single cylinder with IV 25mm and OV 22mm and UMA Carb of 32mm with 130/135 mainjet & 40 slowjet....please advice if i have the correct jetting for this set-up.....thanks.
Paul
Sep 22, 2016
Hi! I'm having a hard time accepting the figures I'm getting from your formula. I have a 66 cc with good high compression and a squish band, and large flow area through the transfers and exhaust and also a tuned pipe. The carb I was using was a Kheihin 24mm with an 89mm main jet. Though the intake tube was only at 22mm in inner diameter. I actually thought the carb was a 22mm, as it was marked on the package. Turned out, the packaging was wrong. So with a 24mm carb and an 89 main jet, I am surprised that your formula gives my new Dellorto carb a 100 main jet. I also have a large reed setup too. But a 100 main jet seems huge after using an 89 in a significantly larger carb. Anyway, I'd like to hear your two cents worth. Best Regards, Paul
Paul
Sep 22, 2016
Sorry, I forgot. My carb is a Dellorto 21mm.
Daniel
Mar 4, 2017
Hi Paul,
sorry for the late reply. The thing is, the numbers used for the main jets are really randomly assigned by the manufacturer, so a PWK carb and Delorto Carb might both have the numer 100 on the mainjet, but the actual hole diameter is different. This is why my formula only works for dellorto carbs. Keihin PWKs are way different i'm afraid!
sorry for the late reply. The thing is, the numbers used for the main jets are really randomly assigned by the manufacturer, so a PWK carb and Delorto Carb might both have the numer 100 on the mainjet, but the actual hole diameter is different. This is why my formula only works for dellorto carbs. Keihin PWKs are way different i'm afraid!
Michael Forrest
Sep 23, 2016
What matters is area, both of carb and jet, not diameter.
That is because if you graph carb area per carb diameter it won't be a linear graph.
I think a good estimation is to have the main jet area .16% of the carb area.
area = 3.14 x (diameter/2)squared
example:
21mm carb divided by 2 =10.5
10.5 squared is 110.2
110.2 x 3.14 = 346 square millimeters (sm)
346 x .0016 = .554 sm
.554/3.14=.176
square root of .176 = .42 jet radius
.42 x 2 = .84mm diameter of jet hole
That is because if you graph carb area per carb diameter it won't be a linear graph.
I think a good estimation is to have the main jet area .16% of the carb area.
area = 3.14 x (diameter/2)squared
example:
21mm carb divided by 2 =10.5
10.5 squared is 110.2
110.2 x 3.14 = 346 square millimeters (sm)
346 x .0016 = .554 sm
.554/3.14=.176
square root of .176 = .42 jet radius
.42 x 2 = .84mm diameter of jet hole
Daniel
Mar 4, 2017
Hi Michael,
Thanks for your comment. Actually the jet numbering system isn't always related to any mathmatical value. This is also why my calculation is only valid for delorto carbs up to 21mm. Other carbs have a different numbering system, so i guess your formula would work better there.
Thanks for your comment. Actually the jet numbering system isn't always related to any mathmatical value. This is also why my calculation is only valid for delorto carbs up to 21mm. Other carbs have a different numbering system, so i guess your formula would work better there.
Luciano
Oct 8, 2016
I really need help with the following setup ! Can't decide if the components are "tuned" or not ... so I can't calculate exactly :)
Aprilia RX50 - 1998
Engine: Minarelli AM3 (3 gears)
Cylinder kit: Top Performance 80cc Pink version (cylinder head 2 parts Aluminium; block is cast iron)
Carb: Polini CP 17.5
Exhaust: Gianelli (black with a flame on it - quite old)
Silencer: Stock Aprilia
Spark plug: NGK BR9ES / NGK BR10EV (I want to use the later one but can't read it)
I would say 89 for main or even 87 but I get quite good results with 91 anything above it starts to lose power and above 95 it sputters and won't rev.
The carburetor came with 80 main jet but I think that is too low !
Should I try 85 ? can I damage the cylinder kit ? (was quite expensive).
Thanks !
Aprilia RX50 - 1998
Engine: Minarelli AM3 (3 gears)
Cylinder kit: Top Performance 80cc Pink version (cylinder head 2 parts Aluminium; block is cast iron)
Carb: Polini CP 17.5
Exhaust: Gianelli (black with a flame on it - quite old)
Silencer: Stock Aprilia
Spark plug: NGK BR9ES / NGK BR10EV (I want to use the later one but can't read it)
I would say 89 for main or even 87 but I get quite good results with 91 anything above it starts to lose power and above 95 it sputters and won't rev.
The carburetor came with 80 main jet but I think that is too low !
Should I try 85 ? can I damage the cylinder kit ? (was quite expensive).
Thanks !
Daniel
Mar 4, 2017
Hi Luciano,
sorry really late reply! If 91 works well for you, then I would stick with it. Check the engine temperature after long full throttle/high rev rides. If the jet is too small the engine overheats quickly. You should be able to tell by the water temp!
Cheers,
Daniel
sorry really late reply! If 91 works well for you, then I would stick with it. Check the engine temperature after long full throttle/high rev rides. If the jet is too small the engine overheats quickly. You should be able to tell by the water temp!
Cheers,
Daniel
Andry
Nov 10, 2016
Hey Daniel, I'm Andry from Indonesia. I have 250cc engine and I use carburetor keihin PWK 28 but I get some problems with that, the engine can't stationer in lower rpm and unstabel when middle rpm. Would you tell me how to determine the size of a main jet and slow jet for carb on a 250 engine?
Aidan Doyle
Jan 7, 2017
Hi Daniel,
Great guide, you know your stuff and seem genuinely passionate! You might be able to advise me (I'm hoping!)
Recently got a new Malaguit Firefox (LC Mineralli engine) everything stock, I'm planning to do some tuning so have purchased a few parts:
-Stage 6 Pro Sport Replica Exhaust
-Stage 6 Pro Sport Variator, belt, clutch and medium clutch/torsion spring
I've held off on getting a bigger carb for the moment, is this essential or do I need to upgrade the carb, could I get away with a bigger main jet on the existing? If I do need a larger carb would you go for a 17.5mm or 19mm? (I've tuned a 17.5mm before, took some time to get right).
Any advise would be much appreciated.
Cheers.
Aido
Great guide, you know your stuff and seem genuinely passionate! You might be able to advise me (I'm hoping!)
Recently got a new Malaguit Firefox (LC Mineralli engine) everything stock, I'm planning to do some tuning so have purchased a few parts:
-Stage 6 Pro Sport Replica Exhaust
-Stage 6 Pro Sport Variator, belt, clutch and medium clutch/torsion spring
I've held off on getting a bigger carb for the moment, is this essential or do I need to upgrade the carb, could I get away with a bigger main jet on the existing? If I do need a larger carb would you go for a 17.5mm or 19mm? (I've tuned a 17.5mm before, took some time to get right).
Any advise would be much appreciated.
Cheers.
Aido
Daniel
Mar 4, 2017
Hey Aido,
sorry for the late reply. With stock cylinder I wouldn't go higher than 17.5mm, however you are also ok with the stock 12mm carb. If you want to upjet 5-10% is enough with the stock carb, but keeping the original (presumably 55) jet is also fine with your setup. Only if you at a sports airfilter (which i don't recommend on stock cylinder kits) or a 70cc you really need to upjet.
Hope that helps!
sorry for the late reply. With stock cylinder I wouldn't go higher than 17.5mm, however you are also ok with the stock 12mm carb. If you want to upjet 5-10% is enough with the stock carb, but keeping the original (presumably 55) jet is also fine with your setup. Only if you at a sports airfilter (which i don't recommend on stock cylinder kits) or a 70cc you really need to upjet.
Hope that helps!
Dave
Feb 15, 2017
Hey I got a stock tomos stock carb just has a biturbo pipe on it and just found out about the upjettin what size jet should I move up to???
Daniel
Mar 4, 2017
Hey Dave,
If at all no more than 5%-10 from the current size usually. I guess currently you run something like 55, so 60 would be ok, but 55 is probably also going well!
If at all no more than 5%-10 from the current size usually. I guess currently you run something like 55, so 60 would be ok, but 55 is probably also going well!
Mohd Suffian
Mar 3, 2017
Hi daniel..im from malaysia
im using yamaha nuovo s 110cc
installed with
-uma racing 28mm carb
-racing exhaust n open filter
just now im using pilot jet 32.. 0 to 1/2 throttle,its working well,but 1/2 to full throttle,my main jet seem not working..is that possible to use 140?
im using yamaha nuovo s 110cc
installed with
-uma racing 28mm carb
-racing exhaust n open filter
just now im using pilot jet 32.. 0 to 1/2 throttle,its working well,but 1/2 to full throttle,my main jet seem not working..is that possible to use 140?
Daniel
Mar 4, 2017
Hi Mohd, I'm afraid the size calculations are only for delorto carbs up to 21mm. Whats happening 1/2 to full throttle. is it start bogging and about to go off?
Roberts
Mar 5, 2017
hi,so i have got gilera stalker
my setup:
yasuni c16.07
dr evo 70cc
17,5 dellorto
air box-stock(big old gilera runner)
mallosi valve
my setup:
yasuni c16.07
dr evo 70cc
17,5 dellorto
air box-stock(big old gilera runner)
mallosi valve
Roberts
Mar 5, 2017
now it has got 75 main jet,but someting is not okay
henry
Apr 2, 2017
hi, I've got aprilia sx 50 2015
its got a gianelli full exhaust and I'm going to fit 101 octane air filter foam to it, the car is 17.5 mm standard jetting, what size jet should i use?
its got a gianelli full exhaust and I'm going to fit 101 octane air filter foam to it, the car is 17.5 mm standard jetting, what size jet should i use?
Yos
May 20, 2017
Hi ,
I have piaggio typhoon 2001
With polini corsa 70cc + 5mm longer crankshaft. makes around 80cc
Yasuni city 16
Dellorto 19mm and main jet 92
Polini variator 6.3g rolls + stage 6 clutch
Ist take to him time to going higher rmp from start .
You think I need to pot higher main jet ?
I have piaggio typhoon 2001
With polini corsa 70cc + 5mm longer crankshaft. makes around 80cc
Yasuni city 16
Dellorto 19mm and main jet 92
Polini variator 6.3g rolls + stage 6 clutch
Ist take to him time to going higher rmp from start .
You think I need to pot higher main jet ?
Northampton Customs
May 21, 2017
thanks for posting this - actually that's pretty much bang on with my experience so far. My 50cc scooter came with a huuuge jet (110) and wouldn't run - I've gone down to 64 (the smallest I have currently) and now it runs but only on lots of throttle. I've ordered smaller jets and according to your formula I think a 60 should be about right. Thank you :)
Kenn Godfrey
Jun 4, 2017
I have 93 trail boss 250 has been bored? 30mm carb what main jet shoul I install?
Michael Keywood
Jun 11, 2017
Hi daniel I have a piaggio zip 50 2 stroke air cooled with a pm 360 pipe and a metal mesh air filter have tried loads of jets and none work
Colin
Jul 20, 2017
but if i've got in stock 17,5 dellorto karburator, which main jet shoud i put in?
Ron Trader
Sep 30, 2017
Hey Daniel,
1989 Yamaha Zuma 2. Just put on a Polini big bore 70cc on my was 49cc. Also got Delorto Stage 6 19mm black edition. So my question is, it came with a 92 main 50 pilot. The big bore came with an 88 to install. What would be a good break in jetting and after jetting? Pretty mechanical just an idiot when it comes to carb jetting... your formula says 95???
thx in advance...rt
1989 Yamaha Zuma 2. Just put on a Polini big bore 70cc on my was 49cc. Also got Delorto Stage 6 19mm black edition. So my question is, it came with a 92 main 50 pilot. The big bore came with an 88 to install. What would be a good break in jetting and after jetting? Pretty mechanical just an idiot when it comes to carb jetting... your formula says 95???
thx in advance...rt
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Oct 18, 2017
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jamie
Aug 21, 2018
my bike stock main jet is 97.5 and i've put a 105 in. ive got a new exhaust and air filter. bogs out only at 6.5k rpm and pops abit. do i need to increase the main jet size? its a 125 4 stroke
Non carbon based life form
Jan 18, 2019
Hi Daniel, I am the captain of a Klingon Bird of prey Warship and although its running fine at impulse power at warp drive we have been lately detecting some 4 stroking.
Can you recommend a ball park jet size or some good drugs?
Can you recommend a ball park jet size or some good drugs?
Earl letterboom
Apr 8, 2019
Hi Daniel, A question here from Surinam.
I own a 2011 Keeway F act evo 50 scooter.
My scooter has a Malossi 70 cc cilinder kit
Jog 90 vario.
Stock clutch with yello springs.
carb and cilinder are stock. I don't know what the stock mainjet and pilotjet is.
With mainjet can I use, cause the bike runs like a 50 cc.
I own a 2011 Keeway F act evo 50 scooter.
My scooter has a Malossi 70 cc cilinder kit
Jog 90 vario.
Stock clutch with yello springs.
carb and cilinder are stock. I don't know what the stock mainjet and pilotjet is.
With mainjet can I use, cause the bike runs like a 50 cc.
A Octavian
Sep 20, 2019
hy , i have a yamaha tzr 50 with orig set 50cmc , air filter sport , exhaust just final 125cm and dellorto 21 , what main jet and pilot jet can i use ,<95?
Toby Blower
Oct 21, 2019
Hello, I'm not sure you still use this but if you help I'd greatly appreciate it. I have a 1996 Piaggio zip. It has a 70cc kit and a giannelli performance exhaust. I've just brought a 21mm carb. Your formula suggests a 100 main jet, does that sound correct to you. Thank you, Toby :)
This guide is really just applicable for the Dell'orto carbs up to 21mm. The PWK carbs use a different numbering system which is close but apparently not the same. If it runs well at 138 that great, tho if you have some overheating issues, it helps to increase the jet size!
almost all my parts are stage6.
But it had a moto force sports foam air filter and I was ringing it and it came off and I lost it so I pushed my bike home a baught a same style euro air filter and now it takes about 50seconds to start then if I let it die it won't start again for another 2 hours or something ?
Is this coz I need the same air filter on the carb do u think ?
Thanks