Got a real puzzle going on!

Got a real strange situation goin on, rebuilding a 06 yz 450 for a guy when he brought it to my shop it had no coolant and was burning oil. No big deal. Order the parts new piston, rings and gasket. Go to take the head off and this is where it gets wierd. Line the fly wheel up tdc an the cams are 180 out and yes I probed the engine to ensure tdc. The dots line up on the top an the top of the head on the inside (pointing towards eachother) instead of pointing away. Next I check the valve clearances after rotating them to proper position. The intake valves are set at .22 mm (should be exhaust) and the exhaust was set at .13. The wierd thing is with all this the bike was still running and seemed decent. So I pull the motor apart crosshatch the cylinder install the new piston an gaskets and set the valves to proper clearances an put the timing where it should be. Problem is now the bike wont run. Never seen anything like this before and my brain hurts trying to figure this one out. Why would yhe timing be set like that in the first place as well as the clearances :s ( thr guy bought it used and has no details) and why wont it run when everything is set up proper like every other engine ive built.

The four-stroke rotates twice to complete a cycle, and the cams turn at half speed.  If the cams are 180 degrees out at TDC, it simply means that the engine is on the exhaust/intake TDC, and needs to rotate one revolution farther to get to the correct compression/power stroke TDC.

Even after rotating the flywheel another 360 the bike never had the timing marks line up with the cams an flywheel. I should have taken pictures. But now I have it all set up proper and it will not fire.

And also jus to clear things up I always thought the bike sparks on every rotation so it shouldnt matter what tdc im at when I set the timing right? Jus to clarify

And also jus to clear things up I always thought the bike sparks on every rotation so it shouldnt matter what tdc im at when I set the timing right? Jus to clarify

You're thinking 2 stroke bud, 4 stroke fires every 2nd time it gets to the top.

Power stroke, exhaust stroke, intake stroke, compression stroke and repeat...

Yeah sorry wat I meant by that is it fires on exhaust and compression stroke

The four-stroke rotates twice to complete a cycle, and the cams turn at half speed. If the cams are 180 degrees out at TDC, it simply means that the engine is on the exhaust/intake TDC, and needs to rotate one revolution farther to get to the correct compression/power stroke TDC.

they were 180 out the wromg way no matter how many times I turn over the motor they never lined up on the cams yet it ran. Now that the cams are timed correctly it will not start just verified it is not a shim cocked as ive seen that happen as well as checked my clearances.

You're thinking 2 stroke bud, 4 stroke fires every 2nd time it gets to the top.

 

 

WRONG

 

Most modern 4T singles, including the YZF, fire on every rotation regardless, so it matters not which TDC you're on until you set the cam timing.  One is the same as the other up to that point.

WRONG

Most modern 4T singles, including the YZF, fire on every rotation regardless, so it matters not which TDC you're on until you set the cam timing. One is the same as the other up to that point.

Why on earth does it fire between exhaust/intake?

Because it's simpler to do it that way (triggers off the crank like anything else) and there's no downside.

they were 180 out the wromg way no matter how many times I turn over the motor they never lined up on the cams yet it ran. Now that the cams are timed correctly it will not start just verified it is not a shim cocked as ive seen that happen as well as checked my clearances.

 

This is mechanically impossible. The cams turn 180 degrees when the crank is rotated 360 degrees. So if the cams and crank are lined up at TDC, rotate the crank 360 degrees and the cams will be 180 degrees out of phase with the crank. Rotate the crank another 360 degrees and the cams will be lined up with the crank again.

 

These bikes have a wasted spark ignition, so there it does not matter how you set the crank, it is the cam timing that determines the compression stroke.

 

If it will not run, check the basics. My guess is that you are off a tooth on the intake cam timing. Did you double check the timing after the cam chain tensioner was installed? If you set the timing without any tension on the cam chain then you are most likely one tooth off on the cam timing.

 

If the cam timing is correct with tension on the cam chain, then check the other basics. Check for spark, perform a leak down test, and check that you are getting fuel.

is it possible the gears spun on the cams giving him false alignment?

is it possible the gears spun on the cams giving him false alignment?

 

That is always possible, but you would need a degree wheel to know that. The dots on the cam sprockets would still line up fine. Once I knew that I had spark, fuel, and compression I would check the cams with a degree wheel and see where they are.

I haven't seen that happen to an '06 or later yet, but theoretically, it could.  Not really too likely. 

 

What's needed is to double-check the cam timing and find out what the cause of a lack of spark is.  The first place one has to look when troubleshooting something he's worked on is whatever he came in contact with (moved , disconnected, etc.) while doing the job.  Completely random failures of otherwise uninvolved parts that coincide with the repair job are pretty rare. 

I haven't seen that happen to an '06 or later yet, but theoretically, it could.  Not really too likely. 

 

What's needed is to double-check the cam timing and find out what the cause of a lack of spark is.  The first place one has to look when troubleshooting something he's worked on is whatever he came in contact with (moved , disconnected, etc.) while doing the job.  Completely random failures of otherwise uninvolved parts that coincide with the repair job are pretty rare. 

 

I had this problem on a 2010 YZ450F. It would start, but it took a lot of kicking and ran like crap when it did start. After a few basic checks I checked the degree of the cams and one of the cam sprockets had spun on the cam.

 

Since the bike in question had no coolant in it, it is possible that the head overheated and seized a cam (causing the sprocket to slip on the cam). If this were the case though, I would expect to see significant scoring on the cam journals of the head.

 

Another possibility is that the intake valves are bent and are not sealing. This would explain the large valve clearance on the intakes. A leak down test would help determine if this is the case.

Edited by KJ790

I had this problem on a 2010 YZ450F. It would start, but it took a lot of kicking and ran like crap when it did start. After a few basic checks I checked the degree of the cams and one of the cam sprockets had spun on the cam.

 

Since the bike in question had no coolant in it, it is possible that the head overheated and seized a cam (causing the sprocket to slip on the cam). If this were the case though, I would expect to see significant scoring on the cam journals of the head.

 

Another possibility is that the intake valves are bent and are not sealing. This would explain the large valve clearance on the intakes. A leak down test would help determine if this is the case.

if the cam gear spun that could lead to valves hitting the piston and bending them so I would check both items before proceeding so it doesn't happen again.

When a cam sprocket slips, it will virtually always slip forward on the cam, which would cause the cam timing to become retarded.  Intake cams will tolerate quite a bit of advancing without too much of a performance penalty, but it doesn't take too much advance to have them run afoul of the piston.  By contrast, retarding an intake, as happens in a sprocket slip, very quickly cuts into performance, and it takes only a few degrees to make an engine stumble all over itself.  It also poses almost no threat of bending valves unless it moves so far that the engine has no chance of running at all. 

 

That, and the fact that the OP hasn't mentioned any trouble with compression is why I haven't brought up that whole question.  I'm also operating on the assumption that he checked the valves for sealing while the head was off...

 

The exhaust cam, on the other hand, carries the decomp pin.  Normally, the decomp on an '06 reseats the left exhaust valve at only about 25-30 degrees BTDC (the point where the pin rotates clear of the lifter).  If the exhaust cam were to be retarded one tooth, you'd have little to no cranking compression, and the spark might actually occur with the valve lifted slightly.  Be nearly impossible to start that way, if at all. 

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