HEADS UP!!!

Site upgrade in progress... Core site functions are working, but some non-critical features/functions will be temporarily unavailable while we work to restore them over the next couple of weeks.

Please post any bugs you encounter, but before you do, check to see if it's already listed.

Thanks for your patience while we work to improve the community.

Guest Guest

BACKFIRING WHEN LETTING OFF THE GAS?

13 posts in this topic

Does anyone have any ideas as to why my '98 WR400 backfires out the stock pipe (No Baffle) when I let off the throttle? It doesn't do it all the time, but enough to be annoying and I wonder if something is being damaged. I don't remember having this problem when I first got the bike.

The carb has totally stock settings with the exception of a leaner 165 Main Jet (Stock is 168).

As my previous letter indicated I too have experienced the dreaded Carb Slide plate failure as well. I hope that this was not related to the backfiring.

If anyone out there can tell me how to eliminate the backfiring problem I would surely appreciate it.

Thanks

Share this post


Link to post
Share on other sites

Make sure your headpipe is tight at the head. If not, your bike can make a popping sound on decel.

Share this post


Link to post
Share on other sites

If your headpipe is not leaking, you might look next at a lean condition at the idle circuit. My 426 popped terribly on decel and I couldn't tune it out with the fuel screw. The culprit was the intake boot. This is the little manifold that the carb bolts to. I discovered that when I was reinstalling my airbox and put pressure to slip it on the carb throat, the twisting motion caused the manifold to move away from the head leaving a gap where raw dirty air was being sucked into the engine creating a lean condition. If you look closely at this manifold, you will notice that it is held on by two Allen head bolts that go thru bushings press fitted into the manifold. The head of the Allen bolts are only large enough to contact the bushings only and the manifold slid over the bolts. I corrected this problem by adding a washer under the Allen bolts that were large enough to overlap the bushings and hold the manifold itself against the head. The popping completely disappeared and my carburetion is now perfect. Hope this helps.

Share this post


Link to post
Share on other sites

If no air leaks on exhaust or hot start stuck open etc, then I guess idle mixture is a bit lean. Turn out the fuel/mixture screw so its 2 turns out from closed. Located on bottom of carby. Would depend on what altitude etc you are at. May need to turn out more if at a higher altitude. This change means you might need to use hot start button occasionaly.

Share this post


Link to post
Share on other sites

I second Damians, turn out the screw 1/4 at a time and retry it, the back-fire will let the place to a constant popping, which seems normal.

Share this post


Link to post
Share on other sites

HI,

I have the same exhaust and same problem as you I have cure it with a bigger pilot and about the slide thing It will keep breaking I think it due to back pressure and I intend to machine out a stainless steel version of it. :)

1 person likes this

Share this post


Link to post
Share on other sites

Thanks for all the suggestions. I just got back in town and will try them all. Appreciate the help!

As for the carb slide, does anyone make an aftermarket slide plate that is more bulletproof?

1 person likes this

Share this post


Link to post
Share on other sites

Does anyone have any ideas as to why my '98 WR400 backfires out the stock pipe (No Baffle) when I let off the throttle? It doesn't do it all the time, but enough to be annoying and I wonder if something is being damaged. I don't remember having this problem when I first got the bike.

The carb has totally stock settings with the exception of a leaner 165 Main Jet (Stock is 168).

As my previous letter indicated I too have experienced the dreaded Carb Slide plate failure as well. I hope that this was not related to the backfiring.

If anyone out there can tell me how to eliminate the backfiring problem I would surely appreciate it.

Thanks

 

 

You have a lean condition at closed throttle

The most common reasons:

 

Leaking header crush washer

Leaking pipe junction

Lack of muffler packing

Leaking hot start plunger (usually from corrosion)

Improperly adjusted fuel screw <<<<<<< the most common reason

Bad fuel screw oring

Pilot jet too small (over 2 turns out for max idle speed)

Missing slide plate seal

Worn out carb body/ slide no longer seals well

 

Your motor NEEDS a muffler and packing to run well

 

ALL carb'd race bikes should decel pop a little bit, all the time, or you are too rich on the fuel screw adjustment, causing a flat spot off idle

Share this post


Link to post
Share on other sites

You have a lean condition at closed throttle

The most common reasons:

 

Leaking header crush washer

Leaking pipe junction

Lack of muffler packing

Leaking hot start plunger (usually from corrosion)

Improperly adjusted fuel screw <<<<<<< the most common reason

Bad fuel screw oring

Pilot jet too small (over 2 turns out for max idle speed)

Missing slide plate seal

Worn out carb body/ slide no longer seals well

 

Your motor NEEDS a muffler and packing to run well

 

ALL carb'd race bikes should decel pop a little bit, all the time, or you are too rich on the fuel screw adjustment, causing a flat spot off idle

Coffee, Bill. That post is from 2000.

1 person likes this

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now