Kx250f No idle,lots of backfire.

[Krossipoiss]

Kx250f No idle,lots of backfire,low compression.

This problem is really killin' me.... :bang:
Without choke it runs pretty clean, maybe 1-2 backfires when playing with throttle.
When engine is warm and i pull choke back down, then bike has no idle and works only when playng with throttle and backfires A LOT...and exhaust goes scarlet with seconds.

What could be wrong ?
I have checked valve clearance and cleaned carb many times already.

Ps, bike is 04' kx250f

 

[Ol'89r]

Do a compression test. Your exhaust valves may not be sealing.

 

[Krossipoiss]

how to i do it ? I'm new with 4strokes..decompression pushes exhaust valve down while kicking ?

 

[Ol'89r]

You will have to de-activate the decompression feature. Can't remember how it works on an '04, maybe someone else does? Pull the top cover and take a look at it. You should be able to figure it out.

 

[Krossipoiss]

Without decompression, compression was only 6.5bar.

 

[Ol'89r]

If it were mine, I would remove the head and pour gasoline into the exhaust and intake ports. If the valves are not sealing, the gas will leak through the valves. This is a good way to check the seal on your valves after the head has been rebuilt. May be time for a new piston also.

 

[Krossipoiss]

If i put in steel valves do i need to buy new spring set also ?

 

[Ol'89r]

If i put in steel valves do i need to buy new spring set also ?

Yes. The steel valves are heavier than the ti valves and require a stiffer spring. IMO, the Kibblewhite SS valve and spring set is the way to go. They are a little pricy but they last.

 

[Krossipoiss]

I already brought prox set of steel valves from ebay..i'll try them firstly..
How often should i replace steel valves and piston (in houres) ?

 

[whenfoxforks-ruled]

Isn't ProX the generic side of Wiseco?

 

[Krossipoiss]

I rechecked everything and put bike together again for a test. With decomp. it has 4.5 bar and without decomp. it has 8bar compression. Its still a bit too low i think.
I tested valves with diesel and they didn't leak.Piston has only half an hour on it..
All new top end gaskets.

What shall i do ?

 

[whenfoxforks-ruled]

Take your fuel mixture screw out of the carburetor, verify that all the parts are in there. An 04, replace the o ring on the mixture screw, and the float valve and seat. Then try jetting it. Start with the ap circuit and get the squirt dialed in. Then move on to the basic routine, pilot, needle then the main jet. Having JD jetting send you an assortment of parts is always a good idea. No idle and back fire? Does this bike have 2 timing chains? What is the chances 1 is 1 tooth off? Check it 3 times. The crank thrust washer clearances and the condition of the stator are good/with in spec? You are checking the back fire with all the parts on, like the air filter and seat? Vintage Bob

 

[Ol'89r]

I rechecked everything and put bike together again for a test. With decomp. it has 4.5 bar and without decomp. it has 8bar compression. Its still a bit too low i think.
I tested valves with diesel and they didn't leak.Piston has only half an hour on it..
All new top end gaskets.

What shall i do ?

Diesel does not work the same as gasoline. Diesel is an oil and does not flow the same as gasoline. Check them with gas. You have to actually remove the head to check it. Also, when you check your compression, be sure to hold the throttle wide open.

FoxForks. Pro-X is a division of L.A. Sleeve.

 

[Krossipoiss]

Also with gas, didn't leak.
Throttle was was wide open.
FoxForks - Just brought dynojet for my bike, will try to jet when bike is all complete :)
Timing is correct, have checked it for thousand times already :)

 

[whenfoxforks-ruled]

And the crank tolerances?

 

[Ol'89r]

Check your exhaust system for air leaks. An air leak in a hot pipe can cause a backfire.

 

[whenfoxforks-ruled]

And sell the valves to your enemy.

 

[Krossipoiss]

Check your exhaust system for air leaks. An air leak in a hot pipe can cause a backfire.

First place where air leak is possible is connection between silencer and hot pipe..

And sell the valves to your enemy.

And why so ?

 

[whenfoxforks-ruled]

Because they make Kibblewhites. Do you really want to justify another problem down the road, and do this again? If you are going to use those valves, you really need the rest of the parts that go with them. And get the seats cut to match.

 

[Matt90GT]

clean your carb first. Especially the pilot jet. Then inspect the hotstart plunger and make sure it is not stuck open. Clean it, lube it up, then reinstall and make sure there is play in the cable.

If that does not help, you could have a problem with your pickup on the stator or other electrical problem.

FYI a compression test on a modern 4 stroke is a waste of time. The only spec is like 80 psi within 5 kicks. A true test of the internals without disassembly is a leakdown test.

 

[Ol'89r]

Matt is correct. If the bike won't idle, the first thing you should have done is clean your carb.

I tend to dissagree on the compression check though. While a leak-down test is a better way to determine the condition of your engine, a compression test is a quick and easy way to get a base-line reading. If your compression is low you can add oil to the cylinder. If the compression rises with the oil in the cylinder, it indicates bad rings. If the compression does not rise with the addition of the oil, you have bad valves. Not everyone has the tools necessary to do a leak-down test.

 

[Krossipoiss]

Okey, tomorrow i will take my cylinder head to vacum test...

 

[Matt90GT]

Matt is correct. If the bike won't idle, the first thing you should have done is clean your carb.

I tend to dissagree on the compression check though. While a leak-down test is a better way to determine the condition of your engine, a compression test is a quick and easy way to get a base-line reading. If your compression is low you can add oil to the cylinder. If the compression rises with the oil in the cylinder, it indicates bad rings. If the compression does not rise with the addition of the oil, you have bad valves. Not everyone has the tools necessary to do a leak-down test.

problem is that when the engine turns over the decompression system leaks out the pressure. Thus it is NOT accurate. If you deactivate it (most you cannot without pressing the mechanism out of the cams) your kick starter would break or the gears would strip out. Case in point, try the old YZ400f and 426F bikes with the manual decomp system. What a pain!

Check valve clearances, do a leak down test and that will give you a great idea of what is going in there if you know how to interpret the data

 

[Ol'89r]

problem is that when the engine turns over the decompression system leaks out the pressure. Thus it is NOT accurate. If you deactivate it (most you cannot without pressing the mechanism out of the cams) your kick starter would break or the gears would strip out.

Matt.

In my experience, most of the decompression mechanisms work on the centrifugal principal. They can be overridden by locking the the mechanism in the open position with a piece of wire or something similar. I have checked the compression on dozens of modern fourstroke engines and have never once had a kickstarter break or gears strip. Maybe I don't weigh enough. :whoa: ;)

Common sense dictates that if you can stand on your kickstarter without it going down, you don't have a low compression problem. Also, a compression gauge does not seal the cylinder like a spark plug does. It allows compressed air into the gauge instead of sealing it off completely like a spark plug.

I guess we will have to agree to disagree on this one. :cool:

 

[Krossipoiss]

Whole decompression system can be taken off with only one bolt...
Well, today i took my cylinder head to vacuum test(exhaust leaked)..Then took all apart and all 4 valves are like "round" shaped from edge....no wonder that it won't ran.. :bang: