KX250F backfire

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3/19/2018 6:14 AM

My kx250f is backfiring as the rpms fall. Head, valves, and cams are new & in spec. Motor has stage 1 hot cams. I run cam2 110oct gas. Carb was taken to a dealer to be cleaned. Running condition did not improve. Tried going from a 40 to 42 pilot jet & backing the fuel screw out & it still backfires. Sprayed carb cleaner around the boots and it made no difference. The bike is running lean obviously. Below is a video of what mine is doing(not my bike however). Mines also not quite this bad. Additionally theres a hanging idle. Forcing the throttle forwards seems to help that issue.



Not sure where to go with this. Sadly my local kawasaki dealership has zero clue where to go with this issue either.
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3/19/2018 7:00 AM

The exhaust seal at the pipe and head can leak causing this issue.
Also check for air leaks at the carb to head manifold.
Also remember that running too high an octane fuel can cause issues as the fuel burns too slow for the engine application. I can see no reason for the use of 110 octane, but here again I don't have all the engine spec's either.

Paw Paw

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3/19/2018 7:07 AM
Edited Date/Time: 3/19/2018 7:11 AM

Engine specs:
Wiseco Racers choice piston
Stage 1 hot cams
Prox SS valves & spring kits

Exhaust seal is new & the bolts are torqued to spec. Should i put anything like anti seize or copper silicone around the header where it goes into the head? The joint to the muffler seems ok, i cant feel any air coming out.

Also the dealership mechanic sprayed carb cleaner all over the air boot on both sides of the motor & it did not affect the idle. Im almost lead to believe the choke or hot start is leaking - i have a brand new pluger, seal, and oring for the hotstart.

As far as the gas, im assuming your logic is its not burning fully, thus being expelled into the pipe and burning off from the heat?

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3/19/2018 8:01 AM

If the octane is too high the combustion will be slow enough to allow some of the combusted fuel to still be burning when the exhaust valves open and thus it will enter the exhaust while doing so.
You may want to try and reduce the octane and see how that does.

Paw Paw

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3/20/2018 8:26 AM

I would first try running a lower octane and see if that get rid of the popping

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3/20/2018 11:22 AM

Does high octane cause a hanging idle too? Its starting to sound like race gas doesnt provide the benefits to a carbed 250 like it does with a efi 450 with similar engine packages

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3/20/2018 3:50 PM

Again, too much octane is not a good thing. You may have other issues, but that is where I would start as it is the lowest cost option at this time.

Paw Paw

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3/22/2018 6:10 AM

Tried premium pump gas yesterday. Made no difference. Tried spraying more carb cleaner around the boot again and nothing happened.

When i first got the bike, the hot start was seized in the hole. I pulled it out and replaced it. Is there a chance air is leaking through it even tho its new with a new spring & seal? If the plunger isnt seating fully with that allow air past the seal?

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3/22/2018 6:41 AM

kb228 wrote:

Tried premium pump gas yesterday. Made no difference. Tried spraying more carb cleaner around the boot again and nothing ...more

If it's not seating fully then I would assume that is an issue in general I'm pretty sure the hit start creates a lean issue which would cause the bike to run like shit , I'm not am expert though

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3/22/2018 7:26 AM

kb228 wrote:

Tried premium pump gas yesterday. Made no difference. Tried spraying more carb cleaner around the boot again and nothing ...more

suzukirmz250 wrote:

If it's not seating fully then I would assume that is an issue in general I'm pretty sure the hit start creates a lean issue ...more

I appreciate the info regardless. Carbs are where i dont know much about when they go wrong. We have a local “paw paw” thats a straight up wizard with these things so i might break down and take it to him.

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3/22/2018 7:26 AM
Edited Date/Time: 3/22/2018 7:28 AM

Yes, if the hot start plunger is leaking it will cause a lean condition. With the engine running spray starter fluid or carb cleaner around the plunger an see if the engine speed changes.

Paw Paw

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3/24/2018 5:32 AM

Got the backfiring to settle down. Hanging idle is still an issue. Once again sprayed carb cleaner everywhere and no change. I also cleaned out the hot start.
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11/18/2019 12:16 AM

I had the same issue on my 2009 kx 250f and I found it to be a mixture of problems. The backfiring was caused by the hot start on mine for sure, it make the bike hard to start at times and it never seemed to idle the same on a different day. My hanging idle was no doubt my float bowl height Once I set my float bowl to the correct height I never had issues with the idle hanging again. I know this thread old but I just went through the problem recently.


My recommendation
Replace hot start
Set float bowl height
Check proper jetting
Reset idle
Set accelerator pump timing

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11/18/2019 1:03 PM

There is a plate on the slide of the carburetor. That plate has a beveled o-ring seal on it. I've seen these go bad and cause all sorts of headaches. Its an easy item to overlook. Just because the carb was cleaned by DLR doesn't mean they replaced this o-ring seal.

Photo

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11/18/2019 1:06 PM

Also one more thing to replace. The carb slide oring.

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11/18/2019 1:48 PM

BrianMnMoto wrote:

There is a plate on the slide of the carburetor. That plate has a beveled o-ring seal on it. I've seen these go bad and cause ...more

I never thought about that o ring when I mentioned this or when I dealt with these problems I'm glad this wasnt my issue 🤣🤣🤣

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11/18/2019 3:44 PM
Edited Date/Time: 11/18/2019 3:45 PM

Was the exhaust pipe seal at the head replaced when the pipe was off? If not it will cause the backfire as will leak causing a lean condition in the pipe.

Paw Paw

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11/19/2019 12:40 AM

Thanks guys but i fixed the issue. Took it to a guy who is better with carbs than i am. All he had to do was rejet it i guess.

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