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I just bought a 2002 400ex. The guy said he bored it one over, spent $600 on a top end rebuild and now it won't start. It will only sputter. I take it home and start looking at it. It seems to have good compression my compression gauge won't fit it. I look at the timing chain and put the motor at TDC. The cam has marks arent lining up. So I jump the timing chain one tooth on the cam gear. It still isnt quite lining up but it is a little closer. I put it back together and still will only sputter (little more consistant this time though) Do I need a new timing chain? Is it common for these chains to stretch? The timing marks on the cam gear are only off like 2mm. Should it still idle but rough? Any help will be greatly appreciated. Thanks!
 

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It's hard to tell without actually looking at it, but the timing may not be your problem. Is it firing and getting fuel? I might not be understanding you, but are you saying that the timing looks like it's off about half a tooth one way and then when you move it over one tooth, it looks like it's out about a half tooth the other way? If that's what you are saying, I believe it should run like this, at least one of the ways that you had it. You may have a different problem.
 

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Discussion Starter #3
Yes. The timing is off half a tooth either way. I am spraying carb cleaner in the carb so I know it's getting fuel. I think you are right about the timing. It should atleast run with the timing off half a tooth.
 

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With these unicams, you would be surprised what half a tooth will do.
 

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Yeah it could still be the timing.
 

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Discussion Starter #6
I'm going to go to the honda dealership tomorrow to pick up a CRF 450 timing chain for $50. I heard those are the same exact chains but a bit beefier. Hopefully it fixes my problem.
 

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Timing chain is stretched or the guide is worn past it's service limits.


And for the record UNI means one 400ex's are only equipped with one cam
Honda TRX 400 EX RFVC (99-08) SOHC 4v
 

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Did you try a new spark plug????
Good calll!! sometimes we spend entirely too much time trying to over anilyze
an issue and overlook the simplest of things.
 

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The spark plug is brand new. Everywhere I've read said the CRF 450 will fit the 400EX. I even called the dealership and they said they both have the same exact number of links. I believe it was 120.
 

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just buy one for a 400ex unless you already have the other but i strongly suggest replacing the timing chain guide as well
 

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quote above... "for the record" :

WRONG!!!!

actually Unicam is a registered trademark of Honda

the heart and soul of the CRF450R engine centers on the innovative Unicam single-overhead-camshaft (SOHC) design, which incorporates a carburized single camshaft that directly actuates two 36mm titanium intake valves. The camshaft's single exhaust lobe actuates two steel 30mm exhaust valves via an unique forked, low-friction, roller rocker arm.

On the other hand.. this one is correct:

sometimes we spend entirely too much time trying to over analyze an issue
 

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quote above... "for the record" :

WRONG!!!!

actually Unicam is a registered trademark of Honda

the heart and soul of the CRF450R engine centers on the innovative Unicam single-overhead-camshaft (SOHC) design, which incorporates a carburized single camshaft that directly actuates two 36mm titanium intake valves. The camshaft's single exhaust lobe actuates two steel 30mm exhaust valves via an unique forked, low-friction, roller rocker arm.

On the other hand.. this one is correct:

sometimes we spend entirely too much time trying to over analyze an issue
So what did you say that was different than what I said? It is one cam shaft that controls the intake and the exhaust valves. Just cause Honda trademarked the term Unicam doesn't mean anything.Just like a unicycle, it means one wheel.
 

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hey im having the same exact issue one tooth it lines up on F the next tooth it lines up after T and its just not staying timed =/ im gonna pick up guides and a chain i think just wondering if you ever found out what it was?
 

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quote above... "for the record" :

WRONG!!!!

actually Unicam is a registered trademark of Honda

the heart and soul of the CRF450R engine centers on the innovative Unicam single-overhead-camshaft (SOHC) design, which incorporates a carburized single camshaft that directly actuates two 36mm titanium intake valves. The camshaft's single exhaust lobe actuates two steel 30mm exhaust valves via an unique forked, low-friction, roller rocker arm.

On the other hand.. this one is correct:

sometimes we spend entirely too much time trying to over analyze an issue
you are absolutely wrong don't confuse people making them think something doesn't work when it does i personally run a crf450 cam chain in my 400ex,i know many members that run them and i know people personally that have used them in their 400exs,it is a direct drop in no modifications and its stronger than the stock 400ex cam chain which is why people use it.
 

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grrr

im having the SAME EXACT issue i already replaced the beat to shit timing chain and guides still exactly the same issue... it just wont line up on the T no matter what i do i checked the piston and it is infact at TDC on the t mark so i know the flywheel did not move and i also checked the tensioner and it appears to be working fine my next guess is that lower sprocket on the crank other then that i dont have a freekin clue what is going on and im getting pissed :icon_ cry:
 

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So when you installed the timing chain, you made sure the slack of the chain was at the tensioner side? and the tight side of the chain was near the exhaust?
Then you put the cam sprocket back in with one tooth leaning towards the exhaust side?

when you do it this way, and put the tensioner back into place, it will pull the slack of the chain away from the cam sprocket down into the motor, aligning those marks where they need to be.

Its the same concept as the 22r yota motor.

Cheers
 
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