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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 5The 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 ExampleLet’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 = 60The 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 ExampleAnother 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 x 0.95 = 83.125 - 84Again, 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 DeterminationSurely 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 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!
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?.
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. @ 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:-). 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.
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!. 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.
How to install a hotfix in safe mode. Anyway, I’d like to hear your two cents worth. Best Regards, Paul.
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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 – 1998Engine: 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!.
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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.