4th gen mystery backfire

My '97 750 developed a backfire about a week ago. It happens at high rpm when I back off the throttle and ranges from barely perceptible to a loud pop that scares pedestrians and anyone behind me. The bike idles great, runs great, has full power etc. I'm not seeing any other symptoms - gas mileage is the same, plugs look good, no missing at idle.

Right before this started happening, I flushed and re-filled the coolant, and the bike is running pretty cold, but that is the only thing out of the ordinary that may or may not be correlated to the backfire.

I thought it might be bad gas, but I ran it down to empty and refilled and am almost through a second tank since it started. I poked around inside and can't find any obvious vacuum leaks, same with exhaust leaks. Kind of at my wits end now, my next option seems to be taking it to a mechanic.

 

leak in a exhaust gasket?

 

^ what he said, Sounds like an air leak in the exhaust system somewhere.

 

Same thing happens on my 96, however not very often. I have noticed it though. Kind of scared the crap out of me the first time, as the bike kicks a little when it does this.

 

If it is running significantly cooler, that could possibly cause it to be running slightly rich, particularly under high vacuum as you describe, and the backfiring would be the unburned fuel that has accumulated in the exhaust system igniting. This is just a guess, but it is a possibility.

How cold is it running?

 

My '96 snaps, pops, and crackles any time the throttle is closed and you are high-RPM decelerating from speed in 3rd, 2nd, and especially 1st.

No back fires, though... Just a cool popping that gets attention.

Only mod is a TBR slip-on.

 

A bit of popping and snorting on decel is not unusual, specially with a less restrictive exhaust. But a loud backfire that makes people look around for a guy with a gun is not.

Exhaust leaks will do it...specially up near the headpipes. Often they'll be more obvious if you have a friend put his foot over the end of the tailpipe while you listen up front.

Another possibility is in the carbs (think yours is carburated yet?) Most (not all though) carb banks have what's called a slow air cut valve. Visually they're mounted towards the top side of the carbs...under a quarter shaped plate. Consisting of a diaphragm, a spring, and a bunch of internal carb circuitry. Their sole job is to cut air to the slow circuit on decel...avoiding a loud backfire.

I can go into more detail, smoothbrain, but ran out of time just now. If you have other questions, please continue the thread.

 

My 86 VFR700F2 does the same thing-especially when descending a steep hill. I've had other bike that do it too. I consider it to be pretty normal, specially with and older bike.

 

mine just has the normal decel popping too, i have never experienced a backfire though. i was thinking about it today the only other thing i could think of to add to the last comments made is the one of the carbs. maybe when you are at higher rpm then go to decel maybe one of the slides in the carb is sticking causing excess fuel then get gets lit in the exhaust from the high temps, causing a backfire.

 

Bringing this back for a question. I have a 3rd gen (91) and have a popping and cracking sound at deceleration also but nothing extremely loud or anything. I have about 43xxx miles, k&n air filter, micron slip on, and mabey a jet kit ( have not been able to dig in and see if I have adjustable needles, too busy riding!). So this is considered normal? As for power it's got alot more than my previous 600 but idk if it is not producing all the power it is capable of because this is the only Vfr I have owned. So I guess my question is, is this normal to snap crackle and pop with the vfr's?