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OT : CRF250R Backfire

csharp

Stainless
Joined
Sep 24, 2009
Location
PA
Hello,

I know this is WAY OT. Does anyone know of a good resourse or forum that could help me diagnose a backfire on a CRF250R?

I tried the thumper talk forum but never got a response.

This is a backfire under load or acceleration NOT deceleration. It works fine at idling along. Once you quickly crack the throttle to accelerate it will backfire.

I have checked and shimmed the valves. Checked and double checked.

Could vavles not be sealing? Weak valve springs? Intake leak?

Moderator if this is too OT please delete.

Thank you
 

Vancbiker

Diamond
Joined
Jan 5, 2014
Location
Vancouver, WA. USA
Does it just pop a time or two after you open the throttle of continuously? If just a time or two then look for an exhaust leak. If continuous, then I’m out of ideas.
 

barbter

Diamond
Joined
Oct 27, 2007
Location
On Tour...
I'd be thinking electrical....new plug first as I've had the exact same thing happen after I fitted new.
Cap, lead, coil earthing - but it sounds more electrical to me breaking down under load.
 

csharp

Stainless
Joined
Sep 24, 2009
Location
PA
Does it just pop a time or two after you open the throttle of continuously? If just a time or two then look for an exhaust leak. If continuous, then I’m out of ideas.

Thank you. I would not call it continuous but it happens once then a delay then again.

Thank you. The exhaust seemed to be cocked at the studs where it attacehs. It has a copper gasket. Will look to see if it is bent from not being installed correctly.
 

csharp

Stainless
Joined
Sep 24, 2009
Location
PA
I'd be thinking electrical....new plug first as I've had the exact same thing happen after I fitted new.
Cap, lead, coil earthing - but it sounds more electrical to me breaking down under load.

We did install a new plug. Wow those are a little salty to buy. $30-$40 each depending on where you get them.

This things has throttle position sensors and crap like that. I bought he service manual that tells how to test the throttle posiiton with an ohm meter.
 

wheelieking71

Diamond
Joined
Jan 2, 2013
Location
Gilbert, AZ
We did install a new plug. Wow those are a little salty to buy. $30-$40 each depending on where you get them.

This things has throttle position sensors and crap like that. I bought he service manual that tells how to test the throttle posiiton with an ohm meter.

Cady, is this bike old enough to be carbureted? Or is it fuel injected? That later carb'ed CRFs had TP sensors for the ignition curve.
If it is carb'ed? I would think this is a lean-pop. Check accelerator-pump first. Then pilot-jet could have something to do with it?
But, as soon as you wick the throttle you should come off the pilot and be in to the needle and main.
Regardless, if it is popping, it is either lean (way lean), or you have an ignition problem. I would tend to investigate the carb before the ignition.
Todays pump fuel is very hard on carburetors! The best practice you can get in the habit of doing (again, if it is carb'ed) is:
closing the petcock, and letting it idle till the float bowl runs dry before you park it for any period of time.
If it has sat for a while with pump-gas in the carb already? The carb definitely needs to come apart. You will be appalled at the white schmoo/trash in the bowl!
 

csharp

Stainless
Joined
Sep 24, 2009
Location
PA
Cady, is this bike old enough to be carbureted? Or is it fuel injected? That later carb'ed CRFs had TP sensors for the ignition curve.
If it is carb'ed? I would think this is a lean-pop. Check accelerator-pump first. Then pilot-jet could have something to do with it?
But, as soon as you wick the throttle you should come off the pilot and be in to the needle and main.
Regardless, if it is popping, it is either lean (way lean), or you have an ignition problem. I would tend to investigate the carb before the ignition.
Todays pump fuel is very hard on carburetors! The best practice you can get in the habit of doing (again, if it is carb'ed) is:
closing the petcock, and letting it idle till the float bowl runs dry before you park it for any period of time.
If it has sat for a while with pump-gas in the carb already? The carb definitely needs to come apart. You will be appalled at the white schmoo/trash in the bowl!


Thanks Wheelie!

It is wha they list as PGM-FI

They call it dual timing fuel injection. I wonder if injectors could be dirty?
 

barbter

Diamond
Joined
Oct 27, 2007
Location
On Tour...
Surely...if it's way lean, you can see this with the colour of your exhaust pipe (outlet)?
If there's throttle sensors and gubbins - that may have gone AWOL?

If you are gradual on the throttle - does it ride correct and not misfire?
Under load issues - always was a sign of electrical breaking down....
 

Turbowerks

Cast Iron
Joined
Nov 9, 2018
Location
Windom
Did you find the valves were way off and then reshimmed them ? Usually they start like crap when the valves are tight. On the hondas (at least a few years ago) i seen a lot of valve issues. They would cup and you could re shim them but they weren’t sealing because of the cup. Usually they would start a little better though. A leakdown test would help you find a leaky valve.
Good luck


When I find it I don’t need it
When I need it I can’t find it!
 

Hodge

Stainless
Joined
Jul 30, 2008
Location
spartanburg sc
Cady, is this bike old enough to be carbureted? Or is it fuel injected? That later carb'ed CRFs had TP sensors for the ignition curve.
If it is carb'ed? I would think this is a lean-pop. Check accelerator-pump first. Then pilot-jet could have something to do with it?
But, as soon as you wick the throttle you should come off the pilot and be in to the needle and main.
Regardless, if it is popping, it is either lean (way lean), or you have an ignition problem. I would tend to investigate the carb before the ignition.
Todays pump fuel is very hard on carburetors! The best practice you can get in the habit of doing (again, if it is carb'ed) is:
closing the petcock, and letting it idle till the float bowl runs dry before you park it for any period of time.
If it has sat for a while with pump-gas in the carb already? The carb definitely needs to come apart. You will be appalled at the white schmoo/trash in the bowl!


Spot on wheelie, you didn't specifically mention it but I like to run non- ethanol (providing it meets octane requirements) in all my carbureted equipment. More so if run infrequently.
 

Freedommachine

Hot Rolled
Joined
May 13, 2020
Does it have an aftermarket pipe on it? I'm not familiar with any of the EFI systems on bikes. On engines with a carburetor, they always need jetted when installing a different pipe or they will run lean.

Is this a recent issue on a bike you've owned for awhile? Or did it come to you with this issue?

Edit: I've also seen a lean condition show up after repacking a muffler. This was on a 2 stroke 250 though.

Edit again... didn't realize this thread was so old, my bad. OP, what was the issue?
 








 
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