Fix for backfire on deceleration 05 525 EXC

[Seeking Single Track]

My 2005 525 EXC backfires and pops when I close the throttle and slow down.  I also have a new issue when closing the throttle the RPMs do not drop off.  The RPMs will drop down after a brief delay.  I have had the bike to an independent KTM specific shop and they have re-worked all the jetting.  The bike ran pretty well after they re-worked the carb.  However, it continued to backfire.  Not badly, just a little bit.  I have a friend with an 07 450 EXC and his bike never backfires.  

This engine has been completely rebuilt, new cylinder, rod and bearings, new crank bearings, new piston, rings, valves.  All stock OEM KTM parts except the rod.  I have attempted to find an air leak and so far I find nothing.

I brought the bike back to the shop to replace the shift shaft and seal and asked them to look into the backfire.  They did some adjusting and it was a little better.

MY MAIN ISSUE AND QUESTION: The shop keeps telling me the reason I am having trouble is the fuel.  They tell me that if I was running race fuel it would not deteriorate the rubber bits in the carb and my tuning issues would be resolved.  Does this make sense to anyone on this forum?  Has anyone had similar issues?  What was the fix? 

My friend and his 07 450 do not have these problems and I know he runs pump gas.  

I am going to bring the bike back again and see what it takes to get it to run properly.  

Please share your experience, insight, technical diagnostics.  I want to root out the problem and make it go away for good.

Happy forth of July.  Take the time to thank a soldier for your freedom today.  We are fortunate to be able to pursue our passions.  

[7hump3r]

My 05 450EXC would backfire on deceleration. I fought with it for quite a while until I figured out it was a bad o-ring between the slide and the slide plate (#22 on the drawing). This o-ring maintains positive pressure between the plate and the carb body. If the o-ring goes bad or gets deformed, it allows air to bypass the carb throat on deceleration resulting in a lean condition and backfiring. This o-ring is available from Rocky Mountain ATV

Seriously doubt it’s bad gas.

[Seeking Single Track]

That carb is full of a bunch of funky stuff.  I have had it apart before and put it together wrong.  I'll check it out and use the exploded view and a few photos to get it together correctly and see what happens.  Do you think this will also help the run on RPM when I chop the throttle?  

Thanks for the great info!

[GOTWHEELS]

The popping and slow to come down to idle is lean. Wrong gas won't cause it.

If the jetting is correct, then there still a problem in the carb. There is a whole section of 

the carb that normally doesn't need to come apart, but after a while it needs deep cleaning.

unless you are really good with carbs, I would send it out for a complete rebuild.

Thumper racing can do this for you.

[bnr]

Air gets in at the head pipe the super high temp sealant works for a while but eventually blows out and goes back to popping.  

[MacDaddy500excf]

Do you have the air injection removed or plugged properly for sure?

[Seeking Single Track]

"Do you have the air injection removed or plugged properly for sure?"

 

Not sure.  What do I need to look for.  I want to check up on the shop that worked on the carb,

 

[7hump3r]

That carb is full of a bunch of funky stuff.  I have had it apart before and put it together wrong.  I'll check it out and use the exploded view and a few photos to get it together correctly and see what happens.  Do you think this will also help the run on RPM when I chop the throttle?  
Thanks for the great info!

If it’s going lean due to the o-ring not applying enough sealing force as I experienced with my 450, it could definitely cause run on.

[MacDaddy500excf]

"Do you have the air injection removed or plugged properly for sure?"
 
Not sure.  What do I need to look for.  I want to check up on the shop that worked on the carb,
 

Oh shit I see it’s an 05, it shouldn’t have the air injection valve I’m referring to. My bad.

[xnofriendsx]

Have you adjusted the fuel screw? My 07 450 EXC will occasionally backfire on decel and making 1/8 to 1/4 adjustments will make it better.
I usually tear down my carb twice a year and clean it thoroughly so I know all else inside is usually ok.

[Seeking Single Track]

So nobody on this thread believes that the fuel I am using eventually deteriorates the rubber bits inside the carb?

 

[xnofriendsx]

So nobody on this thread believes that the fuel I am using eventually deteriorates the rubber bits inside the carb?
 

Stop taking your bike to that shop.

[William1]

Not the fuel. Decel backfires are a lean condition. You cannot 'jet it away' because if you did, you would then be rich during normal running, it would be difficult if not impossible to hot start.

If your cab has an ACV (Air Cut Valve)  make sure it works. It is designed to help this condition. It slightly riches the mix in high vacuum situations. What happens is that when you close the throttle at high rpm, a strong vacuum condition exists. The only fuel that can get in the engine is from the pilot circuit. But because of the high vacuum, a lot of air is sucked in, thereby making things lean. No harm, just a fact of life. Things that can make it worse is a air leak on the exhaust (at low flow, it can suck air in, and cause unburnt fuel in the exhaust to 'pop'. A wet rag over teh end of the muffler with the bike idling, listen for hissing, the point of the leak. You should also ensure the packing in the muffler is intact. The muffler cannot do its' job if it is worn out. Adding a 'race pipe' makes decel popping and backfires worse. A racer never coasts. They are either on the gas or hard on the brakes.

Some race fuel will rot rubber parts and the fuel supplier data sheet will instruct you to drain the fuel at the end of the day. E85 too, can cause issues with some rubber parts (plastic too). Pump 93 (pure or up to E10) is fine. Never hurts to add a little stabilizer as well.

[xnofriendsx]

You never replied whether or not you tried to adjust the fuel screw. Buy an adjustable fuel screw if you don't have one already. Richen the mixture by backing it out 1/8th turns at a time and see if that helps. Like William said above it is caused by a lean mixture and richening it should fix it unless you have an air leak or something else happening in there. Always inspect all parts as well. But trying the fuel screw is the quickest and easiest thing to try first.

[Varget22]

On 7/12/2019 at 9:30 AM, Seeking Single Track said:

So nobody on this thread believes that the fuel I am using eventually deteriorates the rubber bits inside the carb?

 

I do. I have an 06 525 exc and I ended up replacing every rubber piece including the mid body gasket. Runs perfect now.

[Jeff aka Bolt]

On 7/9/2019 at 8:12 AM, MacDaddy500excf said:

Oh shit I see it’s an 05, it shouldn’t have the air injection valve I’m referring to. My bad.

Wrong motor...  Has a carb...  Wrong gen bike...  Wrong decade...  Was his grandpas bike.  I know cause I have one too. 

[Greekmountains]

On 7/4/2019 at 5:58 PM, 7hump3r said:

My 05 450EXC would backfire on deceleration. I fought with it for quite a while until I figured out it was a bad o-ring between the slide and the slide plate (#22 on the drawing). This o-ring maintains positive pressure between the plate and the carb body. If the o-ring goes bad or gets deformed, it allows air to bypass the carb throat on deceleration resulting in a lean condition and backfiring. This o-ring is available from Rocky Mountain ATV

Seriously doubt it’s bad gas.

Thanks for this(i know its an old thread but VERY useful) i have just bought a 525 exc which has this problem will get carb stripped hopefully next week.Lots of them have this problem-

My te450 does it as well

My crf 300l does it as well (although not much)

probably all these bikes are a little on the lean side methinks....