Backfire "Pop" When Engine Shuts Off - Normal?

[Badbob]

It doesn't have a backfire sensor. I've had ever electrical component on my bike out and identified every one of them. No backfire sensor.

There also is no revlimiter. I've had motorcycle mechanics tell me that it did.

[bigfour]

I think it's because compression will sometimes push the cylinder in the opposite direction on the final rev, causing the startor motor/clutch to catch.
P. Henry desribes the sound's origin on the yahoo forums:

"I think what happens is sometimes when you shut off the GZ the
compression of the engine kicks it backwards slightly turning it in
the wrong direction. When this happens you hear a clunk and a whirr
as the starter motor catches and spins over slightly. It's nothing to
worry about! It's better for the engine and starter if it dies
quietly and doesn't kick bacwards. But mine will make this sound
about 50% of the time"

[birdmove]

That backfire on shutting off the engine is really common on the Suzuki S40/Savage too. I think its due to the lean carb.
jon

[Dirt_McGert]

Well that's good. Yeah I'd say it does it about 50% of the time too. I'll have time to work on it pretty soon here. I'll do the needle shim mod and richen her up, see what happens. I eventually wanna change out the stock exhaust too - it sounds kinda sally-ish (like a beefy moped.)

Changed the oil today too, for all I know it was probably factory oil that came out of it hahahah!!!

[jonathan180iq]

Originally Posted by Dirt_McGert

It doesn't exactly "over run" (or "diesel") when the motor shuts off.... when I turn the key to off, the motor stops cranking like normal, then at the end lets out a whisp of air (possibly the exhaust valve opening and letting out the compressed air? - sounds like "pshhhh"), and then a half-second or less later, the muffler lets out a nice little pop.

It's not backfire. It's after-fire caused by running RICH

It's unburned fuel igniting in the exhaust.

It may also be time for a valve adjustment.


Take care,
Jonathan

[Dirt_McGert]

Actually guys, I think I fixed it. In automotive class today I turned the idle down (it was idling really high ever since I got it) to where it should be, and every time that I've shut it off since then (probably about 4 or 5 times) it hasn't backfired, or afterfired.

I'll keep you posted if it keeps doing it.

Someone mentioned a valve adjustment? I highly doubt the valves need to be readjusted since there's only 900 miles on the motor and bike. And I doubt the bike would be running rich, unless someone messed with the carb before they sold it to me. Not to be "rude" or anything against your knowledge, I'm just saying that that's probably not the issue at hand. Again I'll let you guys know if it keeps doing it now that the idle is set properly

[jonathan180iq]

I mentioned a valve adjustment because you said this:

"possibly the exhaust valve opening and letting out the compressed air"

David gives a better explanation below.

[davidc83]

Actually, the service manual states the first valve adjustment check should be done at 600 miles, then every at 3000, 5000, 9500 miles, etc.

The Suzuki gz250 could be running lean or rich from the factory. Depending on how much adjustment (one way or another) they need to do to meet EPA standards. Mine is running a little lean, but a friend of mine with a new S40 is/was rich from the factory.

If you do a search on this forum, you can find and download the service manual. The maintenance chart is on page 38.

[jonathan180iq]

With all the use of the word backrife, I'd like to add that this term is commonly misused.

A backfire, on a motorcycle, is very startling because it's a very loud echo-ing BANG that comes back through the air box.

Afterfire is the scenario that you are describing.

"There are two types of backfire. One is an explosion in the engine's exhaust manifold or exhaust system, this is most correctly referred to as afterfire."
-Property of Wikipedia.com

[Dirt_McGert]

Yep, then it was afterfiring through the exhaust pipe. Wasn't backfiring out of the carb or anything.

But, the problem IS fixed! The high-idle speed was the cause. Before I adjusted it, it would idle normally at the speed it does now with the choke on, maybe even higher!