QuestionQUESTION: I have a '82 Yamaha Virago 750 which runs well going down the road but backfires through exhaust upon deceleration.
Of coursre it does it more at higher rpm's but does it at any rpm. It has good acceleration and idles good. Any sugggestions would be much appreciated.
ANSWER: Popping on decel is usually due to too much unburnt gas getting into the exhaust. Maybe check the air filter and if that looks good you may need to check the operation of the carbs.
The carbs will boost the mixture if the float valves are leaking or if the sliders are getting hung up.
Does you bike have a vacuum operated petcock? Sometimes that will leak too and suck extra gas down through the vacuum hose.
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QUESTION: Yes, It does have vacuum operated petcocks and one is slightly leaking. Also, if I hold the throttle at say 2,000rpm's the tach will flucuate and the rpm's will not stay at one place. Could this also be caused by the leaking vacuum operated petcock? Please explain. Thanks!
AnswerThe tach is an indication of RPM but it sort of averages what the motor is doing. When you hold the throttle and the tach is jumping do you hear the motor mis-firing? If the motor is smooth and the tach is jumping you may need to remove the tach cable, clean it, and apply grease.
To test if the petcock is leaking you should set it in the "ON" position with the bike off. You then remove the fuel line from the petcock nozzle. You might have to wipe a few drops of fuel off with a rag. Now watch for any drops of fuel. There should be none. Next you need a length of hose (about 1-2 feet long) attached to the petcock fuel nozzle and emptying into a clean container. You will now remove the vacuum hose from petcock and attach another length of hose (again 2 feet long) to the vacuum nozzle on the petcock. You now take the vaccum hose and suck gently on it. You should see fuel flowing into your container. YOU SHOULD HAVE NO FUEL COMING DOWN THE VACUUM HOSE. If you even taste a little fuel, you need to rebuild or replace the petcock.
What happens sometimes is that the rubber diaphragm in the petcock will get old and crack. This will cause a leak under vacuum. They sell this diaphragm (along with a few other parts) as a petcock rebuild kit for about $30. They also sell new petcocks for $80.
Fuel running down the vacuum hose gets pulled into the motor, making it run rich on the cylinder which provides the vacuum. There is extra vacuum when you decelerate causing extra fuel to enter the motor with almost no air. This unburnt fuel enters the exhaust were it finds a little air and pops.
The other place where too much fuel can be entering the motor is via the float valve. This is a tiny valve in the carbs which controls how much fuel is available in the float bowl. This little valve can get dirty and there's usually an o-ring around it which can shrink. The result is that fuel will leak past the valve and too much will get sucked into the motor. This is usually not a problem since the motor can consume more fuel than is being leaked. So I'm thinking this is not you problem.
