Motorcycle Repair: 1982 yamaha 920 virago, air leaks, rear cylinders

Question
QUESTION: back fires at 4000rpm sitting still, will not rev higher, idles ok,carbs clean, fresh gas, new plugs and air filter. Was not ridden for three years. flushed tank, new (TEMP) fuel filter to catch anything from tank. What do you think?", Thanks for your help....Dan

ANSWER: Dan, your best bet is to go to the Yamaha Virago Experts at http://viragotechforum.com/viewforum.php?f=11&sid=ef5ded99c464a881fc0877a2681201

You said "carbs clean" which means:
1. Full disassembly and overhaul repair kits and diaphragms.
2. Carb float bowls removed and available jets removed and cleaned.
3. Carbs sprayed off with carb cleaner in a can.

In general, if the engine doesn't rev up, the main symptoms are fuel lean/starvation, insufficient spark advance, air leaks on carb diaphragms or intake manifold air leaks.

Bill Silver


---------- FOLLOW-UP ----------

QUESTION: Bill, 1) I put new plugs in, started bike, ran very good for a minute and died. Pulled plugs, rear cylinder plug,"black" and NO spark. Cleaned plug,dried, tried again, same thing,no spark on rear cylinder, front plug clean and dry, perfect.rear Black after running a minute and it dies.     2) Ordered starter shim kit, installed per instructions, starter still sounds like a box of rocks shaking around. I will attack the starter again, but my main question is the " no spark" or only temp. spark until it warms up and dies??? what do you think?,thanks, Dan

Answer
Dan, I really don't specialize in this make/model. You could try swapping coils between front and rear cylinders.

I went to the forum I mentioned to you before and found this:

http://viragotechforum.com/search.php?keywords=no+spark&fid%5B0%5D=75

which may or may not solve the problem. Sign up there and start your own forum subject with experts on this bike.

If it started up and ran on both cylinders momentarily, until the plug fouled, then it sounds like a carburetor issue. You didn't mention the extent of your carb clean comments before. You could have a leaking float valve, missing o-rings, carb enrichener not shutting off, etc.

Bill Silver