[Swift-Rider]

So I just want to get this out of the way first. I am an inexperienced dirt bike engine rebuilder. The reason why I wanted to try and rebuild a dirt bike engine was because I was off school for the summer, and since Covid happened it felt like the perfect time to try something new out.

Ok so back to the actual problem. Me and my dad bought a blown up 2007 CRF 250r that was bored out to a 270. After taking a peek at everything inside the engine, it seemed ok except for the crank which was blown to pieces and the piston that had hundreds of little tiny nicks on the bottom side of it. We replace the crank and the piston and assembled the whole engine while following the steps of the service manual. When we started the bike for the first time we heard a clicking type sound. I recognized the clicking sound from when we first turned the engine by hand outside of the frame. We have a very strong feeling that it is the decompression weight (I wish I could post a example picture but I have no idea on how to do that) snapping back every time the engine is hitting top dead center. It does not stop even with the engine running. We have no idea on what could cause such a thing. Could it be because of how much compression there is? We checked the timing and it is spot on. We checked the valve clearance (even though I'm a rookie and might not have done it right) but even then I couldn't see on how that would cause such a thing. We also dont know if it is a aftermarket cam and if that could cause the problem.

Its been months since we embarked on this project and I just want the sweet satisfaction of it being completed. Any type of input that could possibly help us figure out this problem would be greatly appreciated. Thank you and have a very nice day!

[JimDirt]

Sorry for the late reply , did not notice this post till now ....

From your description it does sound like the (I am assuming and hoping you replaced the Timing Chain) Decompressor , they do break sometimes , and that could be the tick you are hearing , if the weight on the end is not moving properly then that would be my guess , otherwise it could just be a stuck weight (it should snap back into place when you flick it with your finger) and make sure the spring is attached correctly and everything is in place , as if it is not aligned correctly , it will not work correctly ..... So I would look at the weight itself and make sure it is moving freely and the spring has tension and is installed correctly , then if that checks out look at the shaft itself and see if it is stuck or broken ....... The weight should only move at lower RPM's , once the engine is running at idle speeds and above , the weight should not be moving , if it is , I would guess it is to do with the spring system .......... Report back with findings ....

[Back2-2]

Is it a clicking - like a tap, tap, tap or a rattle sound as in a chain dragging across something? With the amount of damage you described the cam chain should have been replaced. Have you checked the cam chain tensioner? They can go back also causing the chain to drag and slap.

[Swift-Rider]

Is it a clicking - like a tap, tap, tap or a rattle sound as in a chain dragging across something? With the amount of damage you described the cam chain should have been replaced. Have you checked the cam chain tensioner? They can go back also causing the chain to drag and slap.

The sound I hear it making is a "tap tap tap tap" sound and yes the cam chain has been replaced.

[Swift-Rider]

Sorry for the late reply , did not notice this post till now ....

From your description it does sound like the (I am assuming and hoping you replaced the Timing Chain) Decompressor , they do break sometimes , and that could be the tick you are hearing , if the weight on the end is not moving properly then that would be my guess , otherwise it could just be a stuck weight (it should snap back into place when you flick it with your finger) and make sure the spring is attached correctly and everything is in place , as if it is not aligned correctly , it will not work correctly ..... So I would look at the weight itself and make sure it is moving freely and the spring has tension and is installed correctly , then if that checks out look at the shaft itself and see if it is stuck or broken ....... The weight should only move at lower RPM's , once the engine is running at idle speeds and above , the weight should not be moving , if it is , I would guess it is to do with the spring system .......... Report back with findings ....

Yes the Weight does snap back when I move it back with my finger. Should I replace the spring system and see if that clears it up?

[JimDirt]

I would also suggest doing what Neil mentioned and check the Cam Chain Tensioner , if it was ever over extended from the prior catastrophe , it may have been damaged , there are several things it could be , so start with the tensioner and make sure everything is where it supposed to be , also go back over the decompressor , as you can assemble it wrong and it will still function , but not work correctly , so go thru every step in the manual , and also just physically make sure it is doing what it is supposed to ..... check the shaft , I would think it has a decompressor pin/plunger that is serviceable , but I did not see one mentioned in the Service Manual I have ... , but something has to open the valve slightly and usually it is a pin/plunger , unless it has the decompressor arm on the rocker that is adjustable , which it should not , so make sure (with the cam out if possible) that the decompressor is fully functioning and the pin/plunger is moving in/out (it should be retracted normally)

If , as you said the chain was replaced .... were the Chain Guides replaced ?? ...... No matter what , it sounds from your description that the issue is in the cam or chain area , so focus on going over each item and eliminating it as a cause ..... you might hear the clicking when turning the engine over with the normal function of the decompressor , but you should not hear it once the engine starts , unless you got the lash too loose and it is the tapping noise you hear , so triple check valve clearance just to rule it out ....

[Back2-2]

I agree with Jim, I suspect something askew in the cam chain system.
Side note - Be absolutely certain it is not an exhaust leak you may be hearing. I know it sounds crazy but I have run into that before trouble shooting.

[Swift-Rider]

Here are some videos that might help you hear the kind of sound it is making.

Video 1:

Video 2:

In Video 2, you can hear the knock like sound a lot better. I have ruled out that it is not the decompressor because it doesn't make that deep of a sound when flicked.

Here are some things I have checked:

-The Cam
-Cam Chain
-The Cam Chain Tensioner
-The Valves
-The Valve Clearances
-The Timing

Pretty much every part that has to do with the head I've checked. When the cylinder head was off the bike I kicked it over a few times and it didn't make a sound. Like I've said before it only makes the sound at top dead center. Where should I go from here now? I want to try and avoid redoing the head because of the cost. If there are anymore Ideas yall have please let me know. Thank You!

[Aussiecrf230]

The piston isn't just kissing a valve or the head?
Just throwing it out there.

Piston is around the right way?