Carb Jetting query

[chris2470]

I have recently moved form 200cc to a 225c top end, the pipe and the 30mm PWK have remained the same. I was under the impression that due to this increase in cc I would need to up jet the card to allow for this. In practice however it bogs down in the midrange when I increase the main form the original 122 up to any other size apart for a 125 when it start to run cleaner. When I did some research in the tuning guide section I found this piece of info:

"The piston diameter and carb bore diameter are closely related. The larger the ratio between the piston size and the carb size, the higher the intake velocity. That makes the jetting richer. Figure on leaning the jetting after an engine is over bored."

This would imply to me that I should have a probably left the 122 fitted as it run perfectly for 2000 miles or even fitted a smaller jet. This seems a strange concept and also goes against what I always believed, can any one of you tuning gurus conform that this is the case , or not.

[drunkmunkey6969]

Hi bud.....a full engine and carb spec (jetting etc) would help us give a more specific answer.....but in relation to your main question....im not familiar with a concept that suggest that increased CC requires a smaller jet??? Have i misunderstood?

[chris2470]

I have moved form Indian stage 4 to a Betts tuned 225 system. The carb is a PWK 30 mm with a 125 main jet, 40 pilot with a JJK on the second clip with a sterling pipe. I have found that it wants to run a basically the same level as the Indian barrel even though convention would say it needs a larger jet set up.
Thats when I stumbled upon the above statement which got me thinking. As the carb size has stayed the same but the displacement has increased then to flow the larger amount of air fuel mixture the velocity through the carb must increase thus drawing more fuel from the same jets on the way through.
That may be complete rubbish but it seems to be the scenario I have found as all the plug chops are fine.