Info Jetting up a MC22 for improved performance

Damus,
You are starting with a flawed premise. That being "big jets = more power". There is an optimum air/fuel ratio. Adding more fuel will only make your bike more sluggish and destroy your engine faster. I select the main jet by starting with what I think will be rich and going down until the engine just stutters at wide open throttle (not in top gear) and then going back up one size, with no regard to what the standard jets are. In other words, I will use jets that are smaller than standard if that yields the best result. I am not suggesting that it is my way or the highway, it is just what I do and it is this technique that I used to tune racing machines that were winners.
Here is a quote from a person with a similar view:
"Everyone wants to put bigger jets in a carb because they have a hot rod and think more fuel equates to more power. The exact opposite is true. Lean is mean. The average carb is engineered on the rich side to save you, so fine tuning usually involves removing fuel from a motor, not adding it." Read more: http://www.hotrod.com/how-to/engine/0802phr-how-carburetors-work/#ixzz3fp6dOGNB

Now that may be referring to a car engine but the principle remains the same.

You need to understand that there is a pulse running back and forth through the inlet tract. The amount of fuel that can be drawn into the inlet tract is dependant on the pressure of that pulse and the jet opening size. If the engine was tuned to pump air more efficiently than before (air cleaner and muffler hardly qualify as tuning) then it can draw more fuel simply because of the greater negative pressure as a result of more air passing over the emulsion tube. If you can pass more air over that tube then it will draw more fuel by itself. It does not require you to put in bigger jets (think of a paint spray gun, it cannot spray much paint at 5psi). Consider that my FZR400 3TJ1 uses Mikuni BDST32 carburettors and the standard main jet sizes are smaller than an FZR250 3LN1. A Honda CBR600 F2 uses 102 and 110 main jets. Does your 250 produce more power than a CBR600? No, of course not.

Other members are also trying to give you good advice based on their experience, including @Mclaren who has gone to the trouble of having some objective testing done. He used a dyno, something that I do not have access to, and has given you proven information. The changes that you are making, air cleaner element and muffler, are minuscule in terms of engine performance and I would expect that they may have a slight effect on the power curve but very little on total output. These four cylinder 250cc engines are already in a very high state of tune, 160bhp per litre in 1990 being a staggering amount of power. Any attempt to gain more is going to be hellishly expensive.

cheers
Blair

 

These posts imply that you believe that going up in jet size will yield more power.
Once upon a time I raced my Ducati at Bathurst. I went down in main jet size from 155 to 145 after fitting the factory racing cams. This was done during practice (the jet changing not the cam fitting). Each time I went down my top speed increased. My top speed was clocked by radar at 146mph, not bad for a 1975 twin. I am sure that 140 would have been the correct size but I had no more testing time. The standard jets for that bike were 152. Why did it use smaller jets? Because the engine is a more efficient air pump at maximum rpm with those camshafts fitted. I can guarantee you that nine out of ten motorcycle owners would not even try smaller jets because of their mindset.
Now it can be argued that Bathurst being some 600 metres above seal level may be the reason for the smaller jets, but the reality is that due to the much colder temperature, the air density was actually greater in Bathurst. I usually raced in Brisbane and when factoring in both the 9° difference in temperature and the elevation, the air density in Bathurst was approximately 1.179 kg/m³ vs 1.134 for Brisbane.
cheers

 

Sorry @MCiN7 for hijacking your thread.

 

A1 Road and Race in Arncliffe specialises on 250cc bikes, specially MC22.