Backfiring

[Ryan]

I have a 99 GZ 250. It back fires on deceleration. I have tried two sizes larger and one smaller than stock for the jet. I have adjusted the air flow screw. I have cleaned the carb, I have also replaced the exhaust gaskets. It still does it. I dont know what else it could be? Help Please

[blaine]

Welcome to the forum.How did you adjust the idle mixture? I noticed you are from California.Does your bike have smog equipment on it?The best way to adjust you idle mixture is with the bike running,warmed up.Turn mixture screw out (rich) till the idle starts to roll.Than turn screw in (lean) till idle starts to slow.Than back out to were the bike idles best.Go slow so the idle has time to react to the changes.It helps to have a fan blowing air over the engine to prevent overheating.
:cool: :roll:

[Ryan]

I have tried it any which way I can. I have started in- out - 2 to 2.5 out like it should be and it ildles fine. Just slight back fires on decelleration when reved quickly or slowing down in gear.

[Ryan]

I dont think it has any special equipment for smog. It is a 99 and it is stock except for the pilot jet is one size bigger right now from trying that. Runs great though...

[blaine]

Have you tried going up a couple of sizes on the main jet? The pilot jet only affects idle to about 1/3 throttle. A little backfire will not hurt anything,but is usually caused by a lean mixture.
:cool: :)

[Ryan]

it had a 120 i put in a 125, 130, and for fun a 115. none made any significant correction. I left the 125 in

[blaine]

About the only thing left,is to shim the needle.Have you checked your plug to make sure your not running too lean? :??: :)

[jonathan180iq]

Have you checked all hoses for vacuum leaks and made sure your exhaust is securely tightened against the head. Did you drill holes in your muffler? ;)

Sounds more like you have an air leak than a mixture issue. And if there is a leak, adjusting your jets and stuff won't really do any good. Eliminate the vacuum leak variable first.

[Water Warrior 2]

What Johnathan said. The bike is old enough that there could easily be vacuum leaks developing in any rubber parts. Just a thought, did you notice if the airbox was modified by a previous owner ??

[Ryan]

I have not checked the plug. I have replaced the gaskets on the header. it is also tight. I have not drilled any holes in the exhaust. the exhaust is stock. I am now however running a k&n air filter and a 130 main jet instead of 120 to compensate for the better air flow. Still sputtering on decel though. I dont think it is in the carb anywhere

[Ryan]

Aside from the new filter airbox is stock too

[alantf]

Have you checked the valves? Seem to remember that on older vehicles (many moons ago) that valve timing could affect this. Just a guess, at best. :??:

[Ryan]

Have not checked the valves it only has 1700 miles. I dont think that would be the problem.

[alantf]

Maybe not, but Suzuki recommend a check at the 1000km (620mile) service. Just sayin'. :2tup:

[jonathan180iq]

I have not checked the plug. I have replaced the gaskets on the header. it is also tight. I have not drilled any holes in the exhaust. the exhaust is stock. I am now however running a k&n air filter and a 130 main jet instead of 120 to compensate for the better air flow. Still sputtering on decel though. I dont think it is in the carb anywhere

So then you've adjusted the main jet. How about idle jet and needle? That's where the decel would be affected.
Main jet is mostly only for WOT.

[sarahrose1420]

Hello my name is Sarah.
I have a 2004 gz250 that backfires as well. I have checked everthing Ryn has mentioned and still it backfires. The only thing it has that is different is, the last owner did drill holes in the end of the exhaust.? Any help would be appreciated.
Hugs and Kisses,
Sarah Rose.

[geezer]

ohh hugs and kisses? yaya! how bad is the backfire? my bike does have a little put put noise when i get off the throttle but its nothing to concern with. you have a drilled muffler so im sure its gonna mke some noise, but that also means your bike might want more fuel to compensate form the increase capacity for airflow. try putting a 20 pilot in there and adjusting your idle mixure like blain says, warning i did this today and it was a major pain in the ass, hot engine burning my hands ouch.

[Water Warrior 2]

Or you could just plug the extra holes in the exhaust and see if it is to your liking.

[sarahrose1420]

I will try pluging the holes. But where is the mixture screw located,? I can't find it. All the holes on the side of the carb on my bike seem to be pluged off, is it under the float bowl somewhere.?
Thanks,
Hugs and KIsses,
Sarah Rose.

[DAN]

Check the decelaration circut in the carb. It involves the smaller side diaphram and the associated jets. after 10 years or so the diaphram gets stiff and cracked (new ones at bike bandit dot com). The jet behind the small diaphram could be blocked and the passage under the spring loaded check ball could get a shot of carb cleaner and a shot of wd40 (stick the pray straw into the hole and depress the ball under the small diaphram plunger). This circut enriches the fuel mixture on decelleration with the throttle closed. --Dan--

[blaine]

I will try pluging the holes. But where is the mixture screw located,? I can't find it. All the holes on the side of the carb on my bike seem to be pluged off, is it under the float bowl somewhere.?
Thanks,
Hugs and KIsses,
Sarah Rose.
Welcome to the family.The mixture screw is plugged from the factor.You need the carb off the engine to drill out the plug.It is in front of the float bowl.Pages 4-6 to 4-8 in the manual explain how to remove the plug.Good Luck.
:) :cool:

[DAN]

The plug over the idle mixture adjusting screw/needle is made from brass. Make sure you identify the correct plug to drill. To drill it out use care not to go in too far. Stop the drill just as it goes through the plug. Screw in an appropiate size long machne screw (force the threads into the brass), attach a vise grip plyiers or whatever to the machine screw and tap out the plug by hitting the vise grips with a wood mallet. After the carb cleaning and run in adjustment of the idle screw, the plug hole can be filled with a new brass plug (from dealer or bike bandit) or with an appropriat size arch punch and a piece of neoprene gasket 1/8" to 3/16" thick, just make a neoprene plug to keep the dust out of the hole. --Dan--

[sarahrose1420]

Hello again, my bike is still backfiring. I haven't messed with the mixture screw yet because I want to check the ignition timing first, the bike run's far too hot and wont idle and does not want to start again after a long run, I was thinking maybe the timing is too far retarded.? Can anyone tell me if or where can I get to the ignition timing and can it be adjusted.? Does anyone have a link to a workshop manual or some information with some data on about adjusting the timing.? Unfortunately my bike did not come with the basic owners manual so I dont even have basic maintenance data.
Thanks for any help.
Hugs and Kisses,
Sarah Rose.

[blaine]

The timing is not adjustable and is set by the ignition box.Here is the link to download the service manual.
viewtopic.php?f=3&t=27 (http://www.gz250bike.com/forum/viewtopic.php?f=3&t=27)

Also have you checked your air filter to see if it is plugged.
:) :lol:

[sarahrose1420]

Hey, thanks so much for that information Blaine, yes we checked the air filter and it was clean. I am pritty sure it is something to do with the ignition box you mentioned because it is behaving just like the timing is way too far retarded, it wont idle, it backfires, runs way too hot and wont restart after a long run, that say's to me it is running either very lean or to far retarded. Is the price of this ignition box going to bankrupt me.? I am out of work, filing for disability and living on borrowed time in a house that is in forclosure, so as you can imagine I can't afford a huge bill for the ignition box.
Well thanks again for that information.
Hugs and Kisses,
Sarah Rose.

[Water Warrior 2]

Sara, I would be more suspicious of the carb than timing. With the extra holes in the muffler plugged you should have normal air flow as designed to meet the environmental concerns. Your bike just might need to have some carb tinkering to make it happy. The air idle screw will help a lot if/when you get at it. You should be able to get er running virtually cost free with some patience and research on the forum.
I personally have not touched a carb since 1989 when my riding buddy had a bike that was very lean, hot and gutless. I fixed it in a few minutes and he thought I was a genius. Little did he know I made the time to research the problem and the most direct route there. Sara your best info is this forum and the members who have been into and around GZ carbs to make them run right.

[blaine]

I agree with W.W.I have never heard off a ignition box failing in this manner.When they fail,they just stop producing spark.
:) :cool: