Motorcycle Repair: Yamaha 400 Seca Main Jet Question, air fuel mixture, plug colors

Question
Hi Mark, Last Month I got a 1982 Yamaha XS400RJ  Seca  My question is about re-jetting the carbs Im adding a 2 into 1 header also.  Listed in the factory repair manual, And, In the carbs which ive had apart is  42.5 pilot jets  but a 117.5 main jet in one carb and a 127.5 in the other carb ? Im Confused  why the different size main jets? I plan on buying a good foam air filter and holes in the airbox. what size jets do you think are a good starting point? 45 pilot & 130 main ?

Answer
Hi Terry,

The different size jets are for part of the design and used for the enrichment circuit (choke).

I don't have any experience with jetting these carburetors, but I recommend keeping the difference in sizes as you rejet. For street MC's I tune carbs by reading spark plug colors. You'll have to do the same. Make one change at a time and then do a plug chop. Remove the plugs and read them. Repeat for every change. Making too many changes means you won't know which change caused the effect when things don't go well and need to be dialed in.

Jetting often means on and off with the carbs many times before getting the closest air-fuel mixture (correct jetting).

Purchase the service manual for your MC.

Email me if you need my carb tuning files. mshively1@woh.rr.com

Respectfully,
Mark Shively