Question I have recently started to chop my bike. I installed uni filters and got size 20 and 22 jets. It was runing realy bad so I took off the muffles to let more air out. It still runs a bit off. the odd thing is that the bike had size 12 and 15 jets in it, not matching. So my question is are the jets supposed to be different size? And would I need more or less back pressure? Or is there something I have overseen?
AnswerHi Matt,
Q: "...So my question is are the jets supposed to be different size?"
A: Yes. The variation in size is designed to help cool the rear cylinder. The front cylinder has a smaller jet because it gets direct convection (air) cooling. The extra fuel added to the rear cylinder via larger jet compensates a bit for the lack of air.
Q: "...And would I need more or less back pressure?"
A: I cannot explain the arguments and science of exhaust tuning here. It's too complicated and lengthy topic.
In a nutshell, back pressure helps with cylinder scavenging depending upon design. Some argue that back pressure restricts performance. You'll have to research this on your own.
Unless you're running a drag bike, I recommend installing some type of exhaust system with some baffled restriction. In most cases, the carbs and air filters can be tuned to work with the exhaust.
Learn to read the appearance of the spark plugs to help determine what tuning needs to be made with the carbs. Visit www.ngk.com and navigate their website for more intel.
FYI... I return tech information files by email. There was no email address with your question as requested and explained in my instructions.
Respectfully,
Mark Shively
