99 Responses

  1. Raven
    Raven at |

    How about in a yamaha mio with 59mm engine with 28mm carbuerator wat its perfect jettings?

    1. Ray McMahon
      Ray McMahon at |

      Hi Dan I have a Suzuki ts50 with a 70cc malorossi kit a delorotto phbg 21mm race carb fmf exhaust bike has no power until 4 k when you snap throttle it bogs down and dies out engine stops if you gradually feed power in gently it will rev up but soon as you put itb into gear to pull away it just dies down and stalls if you keep Revs up over 4 k and slip clutch bike will pull away and ride normally till you haveto pull away again now there was a 50 idle jet I ordered a 45 and a 42 and still no good doing same thing there is a 98main jet also would I wan t to go Down to a 35 -40 idle jet or will I have to replace atomiser

    2. Peter
      Peter at |

      I have one of those zeda Dio Reed setup with a 21 mm carburator and I can’t figure out the main jet size I’m at sea level because I live close to the beach if you guys could help it would be great thanks.

  2. Sam
    Sam at |

    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?

  3. Simonas
    Simonas at |

    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?

  4. alex
    alex at |

    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 !

  5. Boris
    Boris at |

    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!


  6. Johnny
    Johnny at |

    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.

  7. Jamie
    Jamie at |

    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.

  8. Kenny
    Kenny at |

    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

  9. Pedheadfred
    Pedheadfred at |

    Hello iam wondering what size jet should be used on a 72cc running a 24mm flat slide Mikiuni carb ?

  10. CE
    CE at |

    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?

  11. CE
    CE at |

    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.

  12. Nathan
    Nathan at |

    I have a 140cc Konker motor with a gio 125 carb. Was wonder what maximum size jet I could put in it.

  13. renato
    renato at |

    What is the perfec jetting for 28mm carburator.my mc is yamaha 135.

  14. Husqvarna
    Husqvarna at |

    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?


  15. jay jay
    jay jay at |

    Hey daniel, would you know how to determine the size of a four barrel carb on a 350 truck engine

  16. Justin
    Justin at |

    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!

  17. Garth Pecor
    Garth Pecor at |

    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!!!

  18. carlo
    carlo at |

    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

  19. Dave
    Dave at |

    I have a 26mm mikuni on a 125cc dax, what size main jet would you suggest???
    Thanx in advance,

  20. Jayson
    Jayson at |

    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?

    1. Jayson
      Jayson at |

      Keep in mind that I’m in a coastal town right at sea level (Cape Town, South Africa).

  21. kim
    kim at |

    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?

  22. lito
    lito at |

    What is the best jetting 28mm carb 57.4mm bore 150cc for gy6 planning to bore up to 60mm

  23. steve
    steve at |

    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

  24. Asher Hedger
    Asher Hedger at |

    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)

  25. BengtM
    BengtM at |

    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?

  26. Bobby Miller
    Bobby Miller at |

    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

  27. fuzzly
    fuzzly at |

    Hi, is there any formula to determine the slowjet/pilot jet
    size? Thanks for the article. Very helpful.


  28. Jennifer Amo
    Jennifer Amo at |

    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

  29. vader
    vader at |

    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?

  30. terence buckless
    terence buckless at |

    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

  31. Rich
    Rich at |

    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?

  32. Ken Bauer
    Ken Bauer at |

    This has got to be the most straight forward , use full information I have found. Very easy to understand. And very helpful Thank You

  33. Mark Senior
    Mark Senior at |

    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?


  34. carl kennedy
    carl kennedy at |

    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

  35. dave g
    dave g at |

    what about 70cc racing engine with 36mm keihin ( Minarelli scooter)
    180 is in the carb right now but havent tested yet

  36. Bernek
    Bernek at |

    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

  37. Emilis
    Emilis at |

    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

  38. Elliott
    Elliott at |

    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

  39. Dave kozmik
    Dave kozmik at |

    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

  40. Romer Nahial
    Romer Nahial at |

    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.

  41. Paul
    Paul at |

    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

    1. Paul
      Paul at |

      Sorry, I forgot. My carb is a Dellorto 21mm.

  42. Michael Forrest
    Michael Forrest at |

    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

    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
    square root of .176 = .42 jet radius
    .42 x 2 = .84mm diameter of jet hole

  43. Luciano
    Luciano at |

    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 !

  44. Andry
    Andry at |

    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?

  45. Aidan Doyle
    Aidan Doyle at |

    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.


  46. Dave
    Dave at |

    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???

  47. Mohd Suffian
    Mohd Suffian at |

    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?

  48. Roberts
    Roberts at |

    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

  49. Roberts
    Roberts at |

    now it has got 75 main jet,but someting is not okay

  50. henry
    henry at |

    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?

  51. Yos
    Yos at |

    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 ?

  52. Northampton Customs
    Northampton Customs at |

    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 :)

  53. Kenn Godfrey
    Kenn Godfrey at |

    I have 93 trail boss 250 has been bored? 30mm carb what main jet shoul I install?

  54. Michael Keywood
    Michael Keywood at |

    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

  55. Colin
    Colin at |

    but if i’ve got in stock 17,5 dellorto karburator, which main jet shoud i put in?

  56. Ron Trader
    Ron Trader at |

    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

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    love spell at |

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    FirstLeia at |

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  59. jamie
    jamie at |

    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

  60. Non carbon based life form
    Non carbon based life form at |

    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?

  61. Earl letterboom
    Earl letterboom at |

    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.

  62. A Octavian
    A Octavian at |

    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?

  63. Toby Blower
    Toby Blower at |

    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 :)


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