new sound in the head

hi again lads i  just rebuilt my crf250x but not started it yet,When i turn it over with electric start it sound like the right ex velve is knocking is this normal when starting i don't have the exhaust so maybe thats why i can here it so much,  

i have had the head reconditioned all new stuff,The valves clearances  are spot on.


It may be the automatic decompressor that you are hearing. It makes a loud clicking noise. Are your valves properly lashed? Have you started the engine yet? The decompressor noise will go away once the engine fires. If it's still clicking after the motor starts, you need to dig into it and figure out what it is and fix it.

no mate not started it yet i did think it may have been the decompressor but it sound like the valve is closing too hard, I had everything done to the head by zoom factory uk shop. 

thanks mate

The auto decomp usually clunks at slow speed.  The e-start should spin the engine fast enough to where that noise is not noticeable.  The cam lobes should also me smooth enough where a smacking of the valves is not happening.  


There is a lot of movement in  there and with the exhaust off that noise will be more noticeable.  Can you take off the flywheel cover, remove the spark plug and turn it by hand?  This will give you an idea of when and where it is happening.  

I have just finished a top end rebuild on my X, did not replace valves as they where fine but did port the head which required the valves to be removed.

After I had reassembled everything, I had the same clucking sound using the manual turn over thinking it was the exhaust valves.

Turned out to only be the auto decomp.

Take your head cover off and rotate the cam manually using the TDC port on the gear box side, notice when the cluck happens paying attention to where the auto decomp rotates into play, if it makes the noise as the cam rolls through the decomp stroke then there is no problem. You will see that the sound and subsequent cluck does not happening at the exhaust valve but rather at the rocker arm where the decomp turns over.

eStart is not spinning the cam quick enough for the decomp to be complete silent, with the exhaust off reckon you would hear the sound still.

Edited by Van Wick

yep it was the decomp sounds ok now BUT I carnt get a tick over i can but only with the choke in,I cut the air filter out and opend the stock exhaust abit so i change the jetting with jd kit, i put the blue needle in with clip in forth notch 165 i think main jet thinking this would richen the mixture but there nothing on the plug and it gets very hot fast,What do i do now the carb is a shit to get at lol.

thanks lads 

What pilot are you running?  Sometimes the stock pilot can make starting a little difficult when the weather gets colder.  And when you rebuilt it, was that piston and rings, or was there any head work?


Are you the one who checked the valves or did someone else?  Also, what exactly was done to the head and what parts were replaced. 

Your new problem can be several things, lots that can go wrong with reassembling a 4 Stroke top end. 
Here is what I would check starting with the obvious causes of rough idle and heat.


You might have a air leak on your carb from assembly, check the carb fitment where the air boot meets the carb. Its a real pain sometimes to get the air boot to fit up properly, especially on the kick start side. If the air boot is not on 100% that could be the cause of your running lean (heat) and not idling properly.



Also if you took your carb apart to clean it, did you take out the slide?

On the slide there is a plate and a square cut o-ring, the plate can be fitted upside down which will cause a lean condition as well. Flip it over so that dot and cut out are facing downwards. 


Also check your Hot Start, might be binding and holding it open which will also cause a lean condition.

Without knowing what all you have done to your head, it might or might not be causing the problem, you said the lash/shimming was setup right and if your cam timing is also 100% I would start with the carb first for now. You might have bad valve stem seals dumping oil into the combustion chamber causing the bike to run so poorly, did you fit after market stem seals - any oil in the exhaust port when you remove the header?

Something else that would also cause heat related issues would be in proper torquing of the 4 Head Bolts, if the cylinder is sitting skew the added friction on the rings and piston rubbing the cylinder will cause heat. I this case the bike will still idle but it will vibrate somewhat which is a tell tale sign.

Damaged piston comp and oil rings from assembly will cause heat and rough running conditions as well, the thin oil seals snag easily and chip and incorrect gap orientation will dump oil into the combustion chamber as well, again oil in the exhaust port or any wet oil on top of the piston seen through the spark plug hole are tell tale signs of oil contamination.

With regards to your JD jetting, a #165 MJ and Blue needle is way to rich, even for the UK. Would probable run better on a #150 to #155 (max) and a 42 Pilot Jet with the mods you have made. Sure the UK is colder just not sure if the blue needle is needed - rich jetting kills performance. Just some jetting advise  :thumbsup:


When you said "Cut the air filter out", did you cut the air box open or remove the mesh screen?
If you removed the screen it would not effect your jetting or idling in any way, cutting the air box open sure maybe up one or two on the MJ but would not cause a idle problem.

Gut feel though is that it's carb related. 

not checked the mixture yet but its  ticking over fine now,I ripped the carb off again ripped it to bits cleaned it again put it back on and the bike sound fine now so i may have not fitted it properly in the first place,Also wile i was there this time i cut the breather head pipes and got rid of the big one that goes down through the swing arm,I cant see the point of that when it has a plug at the end its not doing anything,So now the pipe just goes to the air filter and sits between the engine bracket ,Is this ok by the way.

thanks lads,I still cant test the bike its pissing it down again lol.

Well, if you cut it, did you recap it?  I did the same thing to mine.  I kind of had to after I put on an FMF and it burned through the tube because I didn't secure it properly.  I think you will be good.  Mine has been fine for a few rides, but do put the cap back on that end.

The breather pipe vents air and pressure build up from the cam cover, it routes to the air box boot to prevent any dirt from getting into the vent hose and into the cam part of the motor. But don't think the air comes out here at any sort of pressure, start your bike and feel for yourself, its a breather not a exhaust vent.

From the cam cover there is a T Piece that routes another pipe down with a clear part and a cap that clips to a clamp near the front sprocket. This part of the hose breather is there to draw oily moister rich air generated from the heat inside the head that ends up condensing into a liquid in the tube away from the air boot part of the breather system.

Removing this lower part is a bad idea in my opinion as the moisture in the system has the potential to travel through the hose that connects to the air boot and can make its way into the carb air circuits, combustion chamber or back into the head and cam area.

Water and oil vapor are both heavier than air so its logical that it would drain downwards and collect in the clear tube by the sprocket. That's the reason why you have to drain the fluid that collects there every now and then.

Have seen a after market cam breather canceling kit online and a few discussion here on TT about it that suggests it improves performance as it removes the piece that goes into the air boot removing the oily air that it claims reduces performance. But this kit (basically some after market yellow tubes) still vents downwards from the cam cover so any moisture will still drain down and out.

I don't see the point of this mod as the stock system does exactly the same but with the extra protection of going into a clean filtered air boot.

My CRF450R is also setup this way and many other 4 stroke brands also route there cam breathers into the air boot, what do we know that the manufactures techs have not already figured out that justifies removing something that was put there for a reason.

Edited by Van Wick

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