QuestionHey I recently put a set of MotoGP exhaust pipes on my bike and of course it cuts out and backfires a little bit, now I put two shims to raise the needle in each carb and that helped a lot with cutting out but it still does a bit. I am wondering what size the stock jets are so I can try one or two sizes up from that, I've heard they are 115s but i'm not sure.
Any help will be much appreciated
Thanks
Answer
Hi Torry,
The main jets affect mostly the 3/4 to full throttle range.
The stock jets for 84 are likely #115 but in 1982 they used
#118 on the SC models so if you go up you will likely need #120-125.
The backfires result from less exhaust backpressure for the
most part but any air leaks on the header gaskets
or anywhere along the pipe can also cause fresh
air to be drawn into the pipe igniting spent exhaust fumes.
Seal any exhaust leaks to prevent this.
The fuel mixture should be checked by doing a quick run with
clean spark plugs and shutting the engine down quickly.
The jets should be adjusted to get a light brown plug electrode.
Black is too rich and light colors are too lean which
means too much heat.
http://verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm
The idle mixture can also be adjusted to the best throttle response
and the least backfiring or popping.
The popping is due to fuel mixtures being too lean to burn
in the combustion chambers.
It then passes into the exhaust pipe where it mixes with outside
air and ignites. Oxygen always aids combustion.
Keep the mixture rich enough to burn well and keep the exhaust
air tight and that should aid the problem but may not
totally eliminate it without the stock exhaust backpressure.
That of course defeats the purpose of adding the new pipes
which is to get rid of the stock system.
Good luck!
Wayne S.
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