Poor idle and jerkink under acceleration.

[Lyndon]

Hi, I'm Lyndon.
I have a 2001 GZ250 that seems to take an age to drop back to idle speed when the grip is released. It also jerks as I am accelating through the gears. Engine speed seems generaly slightly unstable. I've checked the throttle cable operation and it looks ok.
I'm hoping that someone can point me to a solution or al least somewhere to look. I'm thinking fuel mixture in the carb and I'm planing to strip and clean it, but I'm really only guessing. Any assistance would be appreciated.

[jonathan180iq]

How many miles are on your bike? As a 2001, I would assume that, with a high amount of miles, that it may also be time for a complete service.

A complete servicing would entail:

New Spark plug
Valve Adjustment
Oil change
Carb clean/flush

If the bike has been sitting for a long time, the gas that was in the float bowl would have turned to a gooey substance and needs to be removed. On a simple bike like ours, those 4 maintenance procedures can make a world of difference in the operation of the bike.

PS: I wouldn't adjust the carb mixture until I've verified that the problem can't be corrected with some routine check-up stuff.
The last thing that you want to do is take a properly tuned carb and throw the mixture out-of-whack and not know how to re-set it properly.

[Easy Rider]

I have a 2001 GZ250 that seems to take an age to drop back to idle speed when the grip is released.

While you are probably due for some carb. cleaning, it might not be necessary to tear things apart to do it.

Your symptoms could be caused by a vacume leak. Most common places are the hose going to the petcock and the boot between carb. and engine.

A good dose of Gumout or Berryman's B12 in the tank might clean things up without dis-assembly.

[Jaime]

Slow rpm drop to idle = lean mixture (we're all day talking about mixtures, haha). As Easy Rider said, check for any vacuum leak, as it opens the fuel tank petcock. There's a tube connected behind it.

http://img.photobucket.com/albums/v248/hellcracker/carb/carb31.jpg

Also, any carburetor jets could be obstructed, specially the small pilot jet.

http://img.photobucket.com/albums/v248/hellcracker/carb/carb0.png

Jonathan is right, don't tune up anything before an overhaul.

[jonathan180iq]

I forgot out that little hose that runs behind the carb and under the tank. My bike is running well, so I guess I hooked it back up. I'm glad that it doesn't have a clamp on it, or it would be a bitch trying to get the carb off.

A quick note:

Generic rubber hose should be a perfectly acceptable replacement for anyone who finds a leak or something.
The only exception, that I have seen, is the crankcase breather tube that runs from the crankcase into the air filter housing. That is a tightly shaped "S" tube that cannot be replaced with generic tubing. Believe me. I tried.

[Lyndon]

Hello Jaime,
Yes, I have since discovered your other post about Power Increases and I did see you're statement about the idle screw. This would suggest that my bike is lean at idle. However another part of your post suggests that my GZ is running too rich throught the gears. Am I reading your words correctly ?

Hi Jonathan, My bike has done 19000KM so you're probably right. I'll do all of those things over the next week or two. Not sure about valve adjustment on these but I'll read up on it first. Thanks.

Hello ER,
I'm quite keen to get my fingers dirty and I'd like to pull the bike apart a bit and have a good look around ( I only bought the bike a month ago). I'll pay attention to gasket sealing when I reassemble but will check the Petcock first. Those products you mentioned may not be available in New Zealand but there may be a local substitute.

If/when I find the problem I'll post the result. Thanks.

[Easy Rider]

I'll pay attention to gasket sealing when I reassemble but will check the Petcock first. Those products you mentioned may not be available in New Zealand but there may be a local substitute.


If you are going to take anything apart where there is a gasket, you should have replacement gaskets on hand before you start or know where you can get them quickly OR be willing to have the re-assembly delayed !!!

The boot from carb to engine is just that, a rubber sleeve (boot) and not a gasket.
I mention those two things because folks have done rather extensive rebuilds only to find the bike still runs like crap......because of a cracked intake boot or cracked (or loose) vacume line.

Look for a product that specifically says Carb. cleaner......NOT fuel injector cleaner and not a combination product like cleaner, conditioner, and stabilizer, etc.

Good luck!

P.S. with the kind of mileage you have, inspect the chain/sprockets closely.

[Lyndon]

I checked the hose to the petcock - I think it looks ok but when I removed the Pilot Screw I found a spring and an O ring in there. According to exploded diagrams I have seen there should be a tiny washer in there as well. I don't think it fell out while I was unscrewing the Pilot Screw. How important is this washer to the operation of this part of the carb. I assume that it sits under the O ring ?

[jonathan180iq]

Hmmm...

That's why we suggested other methods before you took the carb apart.

I can't tell you from experience whether that washer should be there or not. Sorry.

Before you removed the pilot screw did you try adjusting your throttle mixture knob?

[Lyndon]

I counted the number of turns on the Pilot Screw so that I could reset it. I did find some sort of white dry residue on the spring and the body of the Pilot Screw which came off with spray type carb cleaner and some scrubbing.

I think the washer could be important in that it may be there to keep the O ring 'square' in the bore.

Not sure what you mean by "throttle miture knob". I did try running with the 'choke out' to varying degrees to see if it would have an effect on the jerking. It didn't.

After cleaning the Pilot Screw, there did seem to be an improvement in the idle. If I blip the grip, it did seem to drop back to idle more positively. Perhaps a small adjustment would improve matters further but I don't have a short enough screw driver to get in there. I'll get one today. Many thanks again for you're interest.

[Jaime]

Well, there was no need to remove the pilot screw, as it isn't so affected by dirt. The washer is there for a proper spring support; the pilot screw is attached to the carburetor body. Maybe the diagram doesn't correspond exactly to your GZ250 version, so don't worry about it.

I suppose that engine-carb and carb-air filter nozzles are fine, as it could be air leaks there too.

About lean and rich mixtures, sometimes is hard to distinguish. A rich mixture is felt like the bike is accelerating but sometimes it jerks a bit like the engine is being drowned in fuel... On the other hand, a lean mixture is more like a lack of power: smooth if it's not too lean, and heavy (like running out of fuel) when it's so lean (this case could be missmatched with a rich mixture). I'm now able to feel the difference, but in some cases the symptoms are very similar.
Do you get jerks all around the rpm range or only at low-mid rpm? To rule out the possibility of a rich mixture anywhere, clean the air filter, but sure you have a lean mixture at idle.

You should clean the pilot jet, with compressed air or a carb cleaning product. No gaskets need to be replaced, unless it's worn out or broken.
Following my how-to, in the step 5:
http://img.photobucket.com/albums/v248/hellcracker/carb6B.jpg

I had your same idle problem when I put a smaller pilot jet -thinking it was bigger-. You'll may not compensate it properly with a pilot screw "enrichment".

[jonathan180iq]

Not sure what you mean by "throttle miture knob".



My fault. I meant to say "idle mixture knob".


http://www.postimage.org/Pq48wgCA.jpg (http://www.postimage.org/image.php?v=Pq48wgCA)

I've had to play with this several times.

[Lyndon]

Many thanks to everyone for all of the help with this but I've decided it's a bit beyond my comfort zone and have booked the bike into a Suzuki dealer to get it sorted along with a full service and an apraisl of the overall condition of the motor. Will post the result if it's interesting.

[Lyndon]

Conclusion - The bike is running well now. I decided to put it in to a dealer to sort. Just before I took it there, I decided to give the clamp on the rubber carb mount to the motor on more turn. To my amazement, the unstable idle was no more.
The second thing was the lurching at low speed. This was cured by replacing the unevenly worn (very worn) chain. The sprokets were also relaced at the same time.
All of the problems with the bike were resolved by carb cleaning, carb mounting boot clamp tightening, chain/sprocket replacement and a service.
Many thanks again for all of the input. Lyndon.