Backfiring

Ryan · 03-14-2011, 11:54 PM

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 · 03-15-2011, 12:18 AM

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 · 03-15-2011, 01:01 AM

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 · 03-15-2011, 01:04 AM

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 · 03-15-2011, 01:27 AM

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 · 03-15-2011, 01:51 AM

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 · 03-15-2011, 02:01 AM

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

**jonathan180iq** · 03-15-2011, 09:26 AM

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 · 03-15-2011, 02:11 PM

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 · 03-18-2011, 08:21 PM

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

03-14-2011, 11:54 PM	#1
Ryan Junior Member Join Date: Mar 2011 Location: burbank, ca Posts: 7	Backfiring 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 Login or Register to Remove Ads
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03-15-2011, 12:18 AM	#2
blaine Senior Member Join Date: Feb 2010 Location: P.E.I. Canada Posts: 3,784	Re: Backfiring 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: __________________ If at first you do succeed, try not to look surprised.
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03-15-2011, 01:01 AM	#3
Ryan Junior Member Join Date: Mar 2011 Location: burbank, ca Posts: 7	Re: Backfiring 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. Login or Register to Remove Ads
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03-15-2011, 01:04 AM	#4
Ryan Junior Member Join Date: Mar 2011 Location: burbank, ca Posts: 7	Re: Backfiring 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...
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03-15-2011, 01:27 AM	#5
blaine Senior Member Join Date: Feb 2010 Location: P.E.I. Canada Posts: 3,784	Re: Backfiring 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: __________________ If at first you do succeed, try not to look surprised. Login or Register to Remove Ads
	0

03-15-2011, 01:51 AM	#6
Ryan Junior Member Join Date: Mar 2011 Location: burbank, ca Posts: 7	Re: Backfiring 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
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03-15-2011, 02:01 AM	#7
blaine Senior Member Join Date: Feb 2010 Location: P.E.I. Canada Posts: 3,784	Re: Backfiring About the only thing left,is to shim the needle.Have you checked your plug to make sure your not running too lean? :??: __________________ If at first you do succeed, try not to look surprised.
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03-15-2011, 09:26 AM	#8
jonathan180iq Super Moderator Join Date: Jan 2007 Location: Dalton, GA Posts: 3,995	Re: Backfiring 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.
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03-15-2011, 02:11 PM	#9
Water Warrior 2 Senior Member Join Date: Dec 2006 Location: Squamish B.C Canada Posts: 11,409	Re: Backfiring 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 ??
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03-18-2011, 08:21 PM	#10
Ryan Junior Member Join Date: Mar 2011 Location: burbank, ca Posts: 7	Re: Backfiring 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
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