Backfire on deceleration?

[CHPBike]

I poked around a bit but couldn't find an answer in the old posts.... My 99 RT with 49K runs great. No noticible surging. But one thing bugs me and I don't know what's causing it. On quick deceleration it often gets rapid backfires until I roll the throttle back on. Is this a valve adjustment issue?

:dunno

Thanks,
Scott

[jdiaz]

Sometimes the hardware attaching the header pipes to the cylinders can be snugged down a little tighter, in case there is an air leak causing the backfire, although both my R1100's backfired and popped a little on overrun.

[MotoTrex]

I agree. It sounds as if you might be getting air leaking into the system somewhere. At least that is where I would begin the search for a cure. Start at the headers (where they bolt into the cylinder head) and work all the way back, tightening up all the connections in the exhaust system including the crossover pipe.

[username]

is there a procedure to leak test these parts? without the bike running? how big a leak would be needed ot cause a backfire? a visible one? a pin-hole?

i ask out of total ignorance and a desire to learn more.

thanks.

[MotoTrex]

My guess is that you have a slight leak at the junction of maybe the header to the cylinder or at the other end of the header and whatever collector or muffler is at that end. It may not be visible. Probably isn't. Your condition is after backing off under fairly high or moderate RPMs - when you chop off the throttle. Exhaust systems are not like a water pipe with fluid running out or not with the opening or closing of a faucet. It's more dynamic than that. Back pressure can suck air into the system and with a big motor even a slight opening anywhere where the system is connected can cause the problem.

My old Ducati had the problem now and then and it was the round header collar that held the header to the cylinder. It would loosen up a bit and gulp in some air and lean things out a bit and burp, burp burp. Every two months or so, I'd take the special tool that fit into the flanges and tighten it up.

[alien_hitchhiker]

Scott, has the bike always done this, or is it a new condition?

IF ALWAYS, then you might consider that it is simply the state of tune of your engine. Backfiring on deceleration can be a symptom of lean fuel-air mix.

I'd still start by checking the exhaust connections, as the other oilheaders have suggested. Who knows, maybe the leak has existed for such a long time that it seems like the bike has always backfired. I'd also check all the connections on the intake side. If air is leaking into the intake side it will lean out your fuel mix. Check the rubber boots where the throttle bodies mount to the manifold for cracks or deterioration. Check for cracks or loose connections along the intake tubes and the airbox. Check the evaporative cannister system. If the evap cannister has been removed, are the intake nipples on the throttle bodies plugged?

If everthing checks out tight then you are faced with trying some retuning tricks, such as switching to GS air intake tubes, 0=0 throttle sensor adjustment, or installing a Techlusion box or an aftermarket chip, etc. Just depends on how far you want to go and how much $$ you want to throw at it. The objective would be to add more fuel to the mix. I can say that retuning my engine has eliminated 99.9% of its backfiring - and it was baaad.

All of this tuning advice assumes that your valves are adjusted, your throttle bodies are synched and you have fresh plugs.

IF THE PROBLEM IS NEW, then it is very likely that you've got a leak - provided you haven't made any recent changes, such as installing a high flow air filter. If you solve the problem, post what you did. I'm always looking to learn a little more.

Oh yeah, don't forget to try cussing at it, that often seems to help.

Steve-O

[JCBR1150R]

I poked around a bit but couldn't find an answer in the old posts.... My 99 RT with 49K runs great. No noticible surging. But one thing bugs me and I don't know what's causing it. On quick deceleration it often gets rapid backfires until I roll the throttle back on. Is this a valve adjustment issue?

:dunno

Thanks,
Scott


Running rich...unspent fuel being detonated in the exhaust pipe.

[bugley]

I poked around a bit but couldn't find an answer in the old posts.... My 99 RT with 49K runs great. No noticible surging. But one thing bugs me and I don't know what's causing it. On quick deceleration it often gets rapid backfires until I roll the throttle back on. Is this a valve adjustment issue?

:dunno

Thanks,
Scott

Scott, this could be something as simple as a proper valve adjustment and throttle body sync. or air leak to something a bit more difficult to cure which is your riding style - or a combination of all the above. I have found, even with the bike running at it's mechanical best that sometimes I would leave the throttle slightly open when slowing down entering a turn where it should have been completely closed. (Motronics don't like partial throttle on deacceleration) This is the only time that I have noticed a backfire in all the proper tuned oilheads that I have ridden and there was really only one constant - ME! Tune the bike and then take it for a ride and see if you are inadvertantly not closing the throttle all the way when coming into a turn hot and note if it back fires. Try it again and make absolutely positive that the throttle is closed all the way and see what happens. Good riding and be safe. Please let us know of your findings. :)

[boofer]

Get your Oxygen sensor checked. When they go south the result is usually an over rich mixture. However, they should normally be good for 60k. A loose connection anywhere in the system ( intake or exhaust ) will also cause OX sensor to give you a richer mix.

Tim

[CHPBike]

I appreciate all the reponses and tips. It's been so rainy here in So. California (who says it doesn't rain in California?) that I haven't been able to do much riding. But.... I did find that the header pipe on the left side was just a little bit loose where it bolts to the head. I tightened it to specs. and did a warm-up and then around the neighborhood. I think it's a bit better and hopefully this weekend I'll be able to ride and test it a bit more.
Thanks again! :)

Scott