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I'm working on a 2004 CRF150F that was smoking due to the rings being wore beyond the limits (it got muddy water in it, not my bike). I pilled the top end off the motor and put new rings and a top end gasket set in it, nothing else. Put the motor back together and back in the bike. The bike started after about 4 kicks and ran great, no smoking. I let the bike idle for about 10 minutes and then took it for a short ride at low rpm to let the rings seat. I got the bike back to the shop and let it cool. The next time I started the bike it ran for about 5 seconds and jumped time, 2 teeth. I retimed it a couple times and it continued to do the same thing. The chain tensioner is not stuck, in fact it is all the way OUT and theres still slack in the chain. I pulled the motor a second time and put another head and jug gasket in it, thinking the others were to thin (my last shot in the dark). I just put the bike back together and its still doing the exact same thing, idles for about 5 seconds then jumps time 2 teeth and dies.....what am I missing??? I didn't change anything but s a set of rings and some gaskets.....

Welcome to the site !!

You did not mention the timing chain being replaced , and/or any inspection of the guides , if the timing chain guides are worn enough , the chain can't get tight enough ..... but ... This would also indicate the timing chain is worn out/stretched as well ...... I would suggest replacing the timing chain (about $30.00) and look at the chain guides for excessive wear , the tensioner should not be extended all the way , it will end up putting too much tension on the end of the tensioner shaft and it will tip downwards and then either break or jam and cause the chain to lock up or worse , it could also break off and get ground up inside the engine


Either way , I would suggest replacing the timing chain at a minimum , and possibly the chain guides ..

Hope that helped

I did take the chain guides out and inspect them, they look fine. I also thought about replacing that chain but because the bike was running fine before the top end was removed (just smoking) I never would have thought that the chain could be stretched at all, surely not enough to cause it to jump time. I’ll order a chain and see if that does the truck, that’s my last option.

You did inspect the Tensioner , correct ??? , does it spring back when compressed , and have tension on it ??

Yes, it’s good and can be installed without retracting it at all. That’s not right.

If it can be installed without retracting , then it might be the issue ... and not the chain , but usually if the tensioner goes it allows the chain to slap around and it stretches , so your best bet is to replace them both .....

Yes replace both before flayed aluminium fills the sump.
Keeps the valve timing more consistent as well so runs better.

Maybe that wasn’t the right way to say what I meant about the tensioner...the tensioner is fine, it works as it should...what I mean is, the chain has so much slack that the tensioner won’t extend enough to tighten it up. To install the tensioner you should have to retract it, install it and then release it. Not mine, it can just slide in and there’s still slack in the chain.

What I want to know is, how is it that before I put rings and a gasket set in this thing it was running fine, just smoking. Now it needs a cam chain? It’s like somehow the distance between the cam and the crank got less.

OK ...... Well , what I would say is the Tensioner was probably overextended before as I had mentioned , you just did not know it , it was probably on the verge of coming apart , then when you reassembled , it was only extended to where the detents allowed , because as you mentioned , you did not alter the height , if anything you increased it by installing the new head gaskets , but that amount would be insignificant and not be enough to affect the chain's slack enough to physically make a difference

Either way , the chain is stretched and needs replaced .... I personally would be concerned that the tensioner now has some side to side slop and will fail at some point because of overextending or binding as it must have .... so I would still suggest replacing both , if anything as a precaution to a future failure that could cause extensive damage if it does fail of valves hitting piston , etc. , (better safe than sorry as they say)

My guess is the chain did not suddenly stretch , it was already in the state it is now , but disassembling is what changed the tensioners position . thus creating the slack , again , I am suggesting the tensioner was jammed and holding the tension to some extent , now that it is not , there is a large amount of slack , and it will not take up the gap until it gets jammed again , and possibly causing real damage .....