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oldblue

No compression when warm

28 posts in this topic

I have an 03 YZ450F. When cold the bike starts like a champ with 1-2 kicks. The problem starts when the bike is hot and I try to start it (especially after I inadvertantly kill it). It feels like there is little to no compression and only after multiple attempts to start from TDC will it start.

I tore it apart this evening and the valves all appear to be in spec. I manually open the decompressor and it feels like it sticks ever so slightly before it closes. For the most part it moves freely but do you think this could be the problem?

Any help would be appreciated.

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Sounds like a plan. Racking my brain this evening is not helping. :applause:

Maybe the solution is an 06 YZ450!

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Everyone is a comedian.

I'm going to pull the exhaust cam and see if I can tell if there is any issues. I've heard that carbon build can be an issue so I'm going to see if my dealer has Yamaha Ring Free Fuel Additive. I've heard good things about it and I'm willing to try anything.

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It's your decompression acutator on the exhaust cam. The spring & little shaft can get gummed up causing the bike to have no compression. My 04 450 would do it once in a while & my 06 has done it once so far...you go to start it & the kick starter falls right through the stroke...just like my 02 426 did when pulling in the de-compression lever. You can take the valve cover off & relax the tension on the cam chain to remove the exhaust cam then spray contact cleaner on the spring & mechanism to free it up. Dirty motor oil can cause this to happen alot...so keep fresh oil in her.

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It's your decompression acutator on the exhaust cam. The spring & little shaft can get gummed up causing the bike to have no compression. My 04 450 would do it once in a while & my 06 has done it once so far...you go to start it & the kick starter falls right through the stroke...just like my 02 426 did when pulling in the de-compression lever. You can take the valve cover off & relax the tension on the cam chain to remove the exhaust cam then spray contact cleaner on the spring & mechanism to free it up. Dirty motor oil can cause this to happen alot...so keep fresh oil in her.

Thanks for the info. I'll pull the cam tonight and give it a good cleaning. Does contact cleaner leave any residue?

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It's your decompression acutator on the exhaust cam. The spring & little shaft can get gummed up causing the bike to have no compression. My 04 450 would do it once in a while & my 06 has done it once so far...you go to start it & the kick starter falls right through the stroke...just like my 02 426 did when pulling in the de-compression lever. You can take the valve cover off & relax the tension on the cam chain to remove the exhaust cam then spray contact cleaner on the spring & mechanism to free it up. Dirty motor oil can cause this to happen alot...so keep fresh oil in her.

Hmmm. Sounds familiar. :applause:

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It's your decompression acutator on the exhaust cam. The spring & little shaft can get gummed up causing the bike to have no compression. My 04 450 would do it once in a while & my 06 has done it once so far...you go to start it & the kick starter falls right through the stroke...just like my 02 426 did when pulling in the de-compression lever. You can take the valve cover off & relax the tension on the cam chain to remove the exhaust cam then spray contact cleaner on the spring & mechanism to free it up. Dirty motor oil can cause this to happen alot...so keep fresh oil in her.
Sorry, that won't hold up to close inspection. The decomp is always active when the engine is turning slower than about 500 rpm. Since this is true, you are getting a compression stroke reduced by the decomp every time you kick the engine, hot or cold. Compression lower than this is often due to carbon on the valve stems, or tight exhaust valves. The force that disengages the decompressor (the centrifugal force on the flyweight) greatly exceeds the force applied by the return spring, so if the unit tends to stick, it generally will stick in the inactive position, and you'll have higher than normal cranking compression much like that of a 426 when not using the manual lever. There is no clearance adjustment to get out of whack as on a CRF, either, so that's out too.

You're barking up the wrong tree.

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Sorry, that won't hold up to close inspection. The decomp is always active when the engine is turning slower than about 500 rpm. Since this is true, you are getting a compression stroke reduced by the decomp every time you kick the engine, hot or cold. Compression lower than this is often due to carbon on the valve stems, or tight exhaust valves. The force that disengages the decompressor (the centrifugal force on the flyweight) greatly exceeds the force applied by the return spring, so if the unit tends to stick, it generally will stick in the inactive position, and you'll have higher than normal cranking compression much like that of a 426 when not using the manual lever. There is no clearance adjustment to get out of whack as on a CRF, either, so that's out too.

You're barking up the wrong tree.

So...that being said, sounds like you're thinking that carbon is the issue. Would Yamaha Ring Free be a possible solution? Do I need to take the head off and clean the valves?

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Sorry, that won't hold up to close inspection. The decomp is always active when the engine is turning slower than about 500 rpm. Since this is true, you are getting a compression stroke reduced by the decomp every time you kick the engine, hot or cold. Compression lower than this is often due to carbon on the valve stems, or tight exhaust valves. The force that disengages the decompressor (the centrifugal force on the flyweight) greatly exceeds the force applied by the return spring, so if the unit tends to stick, it generally will stick in the inactive position, and you'll have higher than normal cranking compression much like that of a 426 when not using the manual lever. There is no clearance adjustment to get out of whack as on a CRF, either, so that's out too.

You're barking up the wrong tree.

It can stick....past the active position.The flyweight can get stuck or hung up to where it sticks down further than it should below 500 rpm causing the exhaust valve to open up just a little further than it normally would at low rpm. It feels just like the 426 did when you pulled in the decomression lever...very little or no compression..then the next kick it frees up & all is good.

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Good luck on this one Oldblue. I'm more red than blue now, so I'm stumped.

While it's open I think it's best to try it all...scrub the decompressor clean, Yamaha Ring Free in the fuel and an all night prayer session by a high voodoo priest :D .

If one of the three doesn't work then the 06 is starting to sound like the best solution. :applause:

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It can stick....past the active position.The flyweight can get stuck or hung up to where it sticks down further than it should below 500 rpm causing the exhaust valve to open up just a little further than it normally would at low rpm. It feels just like the 426 did when you pulled in the decomression lever...very little or no compression..then the next kick it frees up & all is good.
No, it really cannot. The flyweight can only be retracted by the spring to the point where it is drawn all the way back against the hub of the sprocket, for one thing, and and at that point, the cam arm that extends the pin is centered (which is the only way that it has the ability to lift the valve. The spring simply can't do it by itself). Moving the flyweight father to the center of the sprocket would result in the pin being retracted, not further extended.

For the other thing, the mechanism's function does not depend on how far the valve is lifted, but when. The extended pin rotates onto the valve lifter near BDC, lifting the left exhaust valve off its seat and releasing compression from the first part of the compression stroke. When a point about 40-60 degree BTDC is reached, the cam has rotated far enough that the pin moves off of the lifter, the valve seats, and you get the top 50% or so of the compression stroke. This gives you something more along the lines of 110 psi of compression as opposed to what would be more like 200 psi without the decompressor.

The decomp system in a YZF is just too damn simple to cause this problem. It's usually sticking valves.

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So...that being said, sounds like you're thinking that carbon is the issue. Would Yamaha Ring Free be a possible solution? Do I need to take the head off and clean the valves?
The additive may work. If not, something you can try short of pulling the head is this:

Get the engine good and warm. Put some of the undiluted carbon cleaner additive in a squeeze or spray bottle capable of delivering a controllable stream of fluid. Pop the air filter out of place, run the engine at a fast idle, and shoot a stream of the carbon cleaner into the engine just fast enough that the engine staggers a bit because of it, then shut it down and let it soak in it for 30 minutes. DO NOT get carried away with this, or you'll hydro-lock the engine, which is bad, OK? Put it back together and run it hard for a while. With a little luck this will get rid of the problem, but you might still have to disassemble the head.

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grayracer513

I read that these fuel additives clean the intake valves but not the exhaust valves. Since the problem lies in the exhaust valves I'm concerned that this will not do anything.

Can I remove the header and spray some Yamaha Ring Free directly on to the exhaust valves?

Thanks for the assistance.

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