Backfiring when cold

TwistedKestrel

TwistedKestrel

King of GTAM
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I have had very little experience with bikes backfiring, so asking GTAM's opinion here.

*PSA: Backfiring means combustion charge is flowing out the wrong port in the engine - in this case, through the carburetors. Does not refer to general popping noises.

My wee GS500 backfires at low RPM and/or when it's cold (but not at cruising RPM). It's very irregular, and I didn't even realize that was what it was doing until this morning, when it stalled itself with a backfire and I could see a faint amount of smoke from the airbox area. Backfiring when it's warm is very rare, usually I only notice it a couple times in stop and go traffic when the bike is trundling along in 1st gear at or near idle. Should I even be concerned about this? Why is this happening? It generally runs well otherwise. Jetting was modified at one point, but supposedly has been set back to stock by previous owner.
 
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My friend had this issue. Have you tried higher grade of gas? Have you done carb synch? Hows your plugs? Minimal backfiring is okay at cold start.

edit. Changing the exhaust or the intake from stock means readjusting your carbs as youve altered the back pressure from the exhaust and the flow of air from the airbox.
 
Bike runs on 87 ... switching to a higher octane is not the solution to the problem, if this even is a problem.
Plugs are new, I just got Denso Iridiums for cheap.
Exhaust is stock, airbox is intact and is using the OEM paper filter (which is new).

I have not done a carb sync on it yet, but the only way I could see that as a cause if it was WAY off ... the carbs have never been separated so I don't think that will be the case.
 
Have you adjusted idle mixture?

Backfiring is a sympom of running lean. The GS500 lean/rich site says:

http://cgi.stanford.edu/~sanjayd/gs500/Upgrades/Rejetting-LeanOrRich
"Popping or spitting through the carb occurs when the throttle is opened. Or popping and spitting occurs through the pipe on deceleration with a closed throttle."

Stock GS jets are lean from factory, and the dense air at these cold temperatures make it worse. If you just changed to a new, clean air filter that would increase flow and make it worse again. I think if your idle mixture is too lean it could make the low end of the rev range worse than other spots.

I rejetted my GS a few months back and it runs way better now. With stock jets it used to pop like crazy when it was this cold out, but now it's a lot less common. I also felt a noticable increase in performance.
 
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The other thing you could try is a carb cleaning. If some jets are partially clogged and flow is restricted, could do something similar. I'm not expert, just a thought.

I've had mine out and apart a few times now, it's relatively easy to do.

[video=youtube;AJh1hxNf7BA]http://www.youtube.com/watch?v=AJh1hxNf7BA[/video]
 
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Lean lean lean... I have a similar issue when its cold, and I blip the throttle a little. Only once and a while, though.. but I've ordered 22.5 pilots to see if it helps. I'll be putting them in over the winter for next season
 
I have a bag of misc. gs500 jets from when I had one 8 bikes ago. If you let me know what sizes you need, they're yours for free (assuming I have it).

Cheers,
Adam
 
luffy said:
I have a bag of misc. gs500 jets from when I had one 8 bikes ago. If you let me know what sizes you need, they're yours for free (assuming I have it).

Cheers,
Adam
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I have a Dynojet kit on hand, have just been waiting for a good day to install it ... but thanks
 
Paging Dr. Brian...
 
TwistedKestrel said:
I have a Dynojet kit on hand, have just been waiting for a good day to install it ... but thanks
Click to expand...

As raydowe said, your carbs need tuning. They are lean from stock to pass emissions

Sent from my tablet using my paws
 
TwistedKestrel said:
I have a Dynojet kit on hand, have just been waiting for a good day to install it ... but thanks
Click to expand...

I would highly recommend taking lots of photos and marking vacuum tubes attached to the carbs before you start pulling them off. By far the hardest part was putting them all back where they belong.
 
I've been having the same problem for the past two weeks.

My 99 GS500 finally gave up yesterday morning and wouldn't turn over. I'm charging the battery now to make sure that's not the problem before I get my hands dirty..
 
The GS's are known for that, see if you leave the choke on it resolves the issue.

The air cooled motor comming off the hwy will do the same thing, or high speed decel.
 
frekeyguy said:
The GS's are known for that, see if you leave the choke on it resolves the issue.

The air cooled motor comming off the hwy will do the same thing, or high speed decel.
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The other option I've heard is to partially cover the air filter opening with a piece of duct tape. It reduces airflow and creates a enriching effect. If the bike runs better, it's lean.
 
frekeyguy said:
The GS's are known for that, see if you leave the choke on it resolves the issue.

The air cooled motor comming off the hwy will do the same thing, or high speed decel.
Click to expand...

See, the way you're describing it though, that almost sounds like an air injection thing ... which mine doesn't have (I think they put it on the next year or something - the spots for it are present on my engine block, but they aren't machined out).

I was iffy on the general jetting up until recently - it has gotten REALLY anemic at low RPM in the cold. The first time turning on to the road after starting it is always a bit hairy... it's acceptable once it warms up (as much as it can in this weather) but definitely could be better ... and will be, once I rejet.
 
Temps have gone down, the air is more dense so you're going to see a more lean mixture.

What you have to be careful of when jetting in this weather is that it's going to be to fat in the mid summer heat, be sure to make notes so you can go back to where you started.
 
On cold startups, my Ninja 250r will "backfire". Really loud pop when starter is first pushed. Starts up no problem on the second try. Only happens every 5 or 6 mornings so I don't really mind (but my neighbourhood might.. XD)

Doesn't bother me, it seems like a kind of normal thing after reading some Kawi forums.
 
reciprocity said:
Temps have gone down, the air is more dense so you're going to see a more lean mixture.

What you have to be careful of when jetting in this weather is that it's going to be to fat in the mid summer heat, be sure to make notes so you can go back to where you started.
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Yup, good advice
 
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