In regards to carbs, what causes backfiring?

[JimmyR]

Sorry for such a basic question, but I don't know! I am working on tuning some RS36 carbs for my GS1100 and as I get closer to getting it right I get less backfiring. Is backfiring a sign that the carbs are running rich?

Just curious in case it helps in diagnosing my tuning. Thanks!

[rapidray]

Backfiring on DECELLERATION is lean on the needles most of the time. Are you sure all the float heights are correct? Also, have you VACUUM synced them yet? Ray.

[daturat100r]

Most likely culprit is a leaking exhaust or inlet manifold especially if it occurs on the overrun

[old_skool]

backfire in the intake, like rapidray said, most likely the needles, make sure you don't have one broken off in the carb body, lean condition. If your talking about exhaust backfire, could be leaky exhaust, or possibly ignition related, not burning all the fuel, out of time...

[spyug]

I'm in agreement with both points of view. It can be lean settings in the carbs but it also can be air infiltration on the exhaust side. I've experienced both but not on the same bike. One thing I did notice that may help diagnosing is backfiring on the exhaust side is often very loud and usually happens once or possibly twice. With lean carbs it appears as a series of muted pops...mchine gun fashion.

Check the pipes for a good fit to the head and look for any pinhole or larger holes down the pipes. On one bike I had a dime sized hole on a pipe going in to the collectors. If that doesn't show anything then I'm guessing its carbs.

Good luck,
spyug

[mrbill5491]

My bike was doing the backfire thing when I got off the throttle, turned out it was my pipe. Where the megaphone connected to the collector the opening and somewhat enlarged. I took a exhaust expanded to the collector and enlarged it just a hair so it would fit back tight. No more backfires.

[JimmyR]

Wow - never knew it was so complicated! Thanks for all the info guys. I will check for exhaust leaks. That is a definite possibility even though it is a new exhaust. When I got the exhaust custom made it came from the other side of the country - 4000km away - and the guy who built it put wide flanges on the headers instead of the stock narrow flanges and I have found it tricky to seal the pipes at the engine. I thought I had it though.

But I am getting a lot less slide rattle noise and backfiring since I put smaller main jets in. I had 127.5 mains and swapped them for 120 mains as the pipes are a more stock style. I am still waiting for some smaller pilot jets as per Rapid Ray's advice.

So the carbs are brand new, the intake boots are brand new and have new O-rings so I am confident that the intake is not leaky. I haven't synched them yet because I'm still waiting for some more parts. I can't remember exactly when the back firing is happening but seem to remember it would happen on idle. I will take better note once I have replaced the pilots.

Thanks guys!

[1_v8_merc]

Popping is usually lean
Flames from tailpipe is rich(actually flooding rich)

[JimmyR]

Haven't had flames from the tailpipe yet. Something to look forward to.

[Crazy_Russian]

it looks very cool, you should try it