Backfire "Pop" When Engine Shuts Off - Normal?

[Dirt_McGert]

Whenever I get done riding and turn the key off, the motor slows, and a half a second after everything seems to stop, a fairly loud "pop" comes out of the exhaust. It's not extremely loud, but it has a nice "pop" sound to it.

What is it, and how come it happens? Is it normal? If not, how can I fix it?

Thanks!!

[Dupo]

Sounds like advanced timing .... just like on a car. Do you have the service manual for the bike? If not, download it from this forum (check the stickies in general)

[Dirt_McGert]

Yep, I just downloaded it earlier today. I'll check it out when I have time, it's not a big deal, just kinda annoying when I turn it off.

[Dupo]

I'd go right to the section on timing. That should fix the problem right up

[Dirt_McGert]

Hmmm, I didn't find a specific section on the timing, just that it's controlled by the ignitor.

[Badbob]

I could be wrong?

I don't think the timing is adjustable.

The fully transistorized ignition system consists of the following components: a generator, ignitor, igni-tion coil and spark plug. The ignition timing is programmed and stored in the ignitor. The pick-up coil is mounted in the generator. The induced signal in the pick-up coil is sent to the wave-form arrangement circuit and the CPU receives this signal and calculates the best ignition timing. The CPU outputs the signal to the transistor of the ignition coil output circuit which is connected to the primary windings of the ignition coil which is turned "off" and "on" accordingly. Thus, it induces the secondary current in the ignition coil's secondary winding and produces the spark between the spark plug gap.

[Dirt_McGert]

Yeah, that's what I found....

[Dupo]

Hm, well i guess its not adjustable then. Maybe the ignitor then??

[Dirt_McGert]

Just to add to the advanced timing possibility, when I rev the engine pretty high and then let off the throttle, it sometimes lets out a few minor pops, but they aren't very noticible. This is probably normal, and it's not a big deal to me, unless something mechanically is off.

[Dupo]

See your other post that i answered, might be crapped up in the carbs from sitting.

I think you might want to get it checked for the over running its doing when shut off, unless the seafoam i suggested in the other post clears it up.

[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.

[Dupo]

Well, thats odd lol. Hopefully the sea foam might clear it up. Let us know!

You sure you didnt buy it off Uncle Buck? :lol: Remember his car... dieseling, pssshhh then BOOM!! haha.

[Dirt_McGert]

Hahahaha I love that movie.

But no, it doesn't diesel. It shuts off normally, then a quick little "pshh" and then "pop".

I took her for a spin about an hour ago. I started it with a cold engine (about 70 degrees out) and it started right up after the first or second "whine" of the starting motor (took about a second to start.) I shut it off, then I put about 2/3 of a can of SeaFoam in the gas tank, gave it a little extra for good measure. Took the bike for a spin to get the SeaFoam throughout the fuel system.

It says on the back that I can pour SeaFoam directly into the crankcase.... is this recommended for this bike??? This might be good to clear away some of the gunk in the engine, since the bike sat quite a while.

[Dupo]

Yea you can put it in the crankcase if you want, but i doubt it would be necessary. Pull the dipstick, if its clear yet, then dont bother. They really dont get sludged up in there anyways.

Be careful, dont wanna put too much sea foam in there :)

Let us know how it turns out.

[Dirt_McGert]

Starts right up just fine now (don't think I fully opened the choke lever all the way haha.)

Still lets out a little pop when it shuts off though, not a big deal personally.

[Dirt_McGert]

I asked one of my teachers about it, and he said something like the backfire sensor regulator controller thing (not exactly what he said) could be broke...... to my knowledge, I'm pretty sure these bikes don't have any type of backfire sensors haha.

[Dupo]

Yea they do, bystanders that say 'hey, your bike is backfiring' :lol:

[Dirt_McGert]

Haha, yeah today shut it off and it didn't backfire, so who knows. I'll do the shim mod in about a week or so once I get time. Is this recommended since my bike only has 850 miles on it? I read in the manual that it isn't fully broken in till about 1000 miles, so maybe I'll wait till then...

[Dupo]

Doesnt matter when you do it. All it does is make it run a little more rich than factory. Wont make a difference in the break in period.

[Dirt_McGert]

actually he said a backfire solenoid, but then he checked it out and said it looked like it didn't have one :roll:

[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]

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!

[Dirt_McGert]

The "popping" is fixed, hasn't done it since I adjusted the idle. Plan on doing some checks/maintenance pretty soon (check the valves, chain, etc)

- Is Quaker State 10w-40 "peak performance" alright to use? It's API "SL", and does NOT have energy conserving/friction modifiers in it. The clutch isn't slipping, so I assume everything is fine and dandy... I've noticed it shifts smoother and easier now with the fresh oil.

One of my buddies let me ride his 2000~ GSX-R 600 around the block a couple times today..... WHOA, HUUUUUGE difference in power and feel. I have no plans to buy a sports bike any time soon (I've always wanted to though, but know that starting out with something that fast can get me killed REAL quick), but eventually after I become confident with riding motorcycles, I may have to own one in a few years. Kind of uncomfortable though... after a few minutes, my hands were sore from leaning forward, but I suppose you get used to it. That's why I love the 250 though - small, light, relaxed feeling, and good for getting around town.

[davidc83]

We just had a street racer killed yesterday on his new bike here in Louisville. New street racing bike, new rider (had his permit for one week), speed (police states witnesses state him and his 5 riding buddies were 'showing' off, and he hit a car head on. DOA at the scene. Be careful out there, especially riding bikes you are not familiar with riding.

[Dirt_McGert]

Yep, that's exactly why I didn't buy a sports bike as my first motorcycle. I only took it around the block a couple times - only got it up to like 35ish, even though it took about a second hahaha.