YZ450 too much compression

Bike was fine 2 weeks ago when finished my last ride. Today when trying to start it, the compression stroke is very hard! Managed to bump start but it didn't have much power and wanted to die the whole time.

I removed the sparkplug and the engine turns easy, as soon as I put the spark back in it's hard to turn over again.

Any ideas?

Either the exhaust cam has jumped a tooth, or the auto decompression system is stuck. Cam timing is by far the more likely to be the problem.

If you find the cams out of time, you will probably also find the chain has a few tight links in it. Change the chain and the tensioner, and don't try running it again until you do.


Both bad crank bearings and a skipped tooth on the cam show these symptoms, how can you diferentiate the two by how the bike is running? I'm sure they would both run poor, but I suppose that the crank gears would probably die right away if it was an issue?

...bad crank bearings ...(will) show these symptoms, ...
Really? Explain that.

Well when the crank bearings are shot I'm sure there are times when they are rough, making the bike overall hard to kick over and impossible to start? I swore I had read that in this forum.

Maybe, but the crank bearings are not likely to suddenly feel nice and smooth when you take the plug out.

Didnt read the plug part....

Thanks for the input grayracer513 and brentn. I have done the trick of rolling the engine backwards to release the Autodecompression with no luck. I am convinced that the Exhaust Cam has jumped a tooth (Speculation as I have't taken the top off yet).

Everything has been running very smooth so I wouldn't think bearing would suddenly go.

I have also read another TT forum with a very same issue and the same diagnosis.

The cam is a bit beyond my experiance so I will take the bike into mechanic to look at the cam and tentioner (and replace) along with Valve clearance.

I read that the cam chain should be replaced every 60hrs or so, I have never changed this since I have had the bike (07 model)

Once more question? Do the valves overlap with piston (YZ450F 07)... I am worried that if the cam can jump a tooth, the engine will blow!?

If you mean can the valves contact the piston if out of time? Very definitely yes, they can and they do. It all depends on which cam moves which way, and how much.

In your case, I'd say you were probably OK, as the bike still has good compression. If the valves had bent, you'd have very little, and your clearances would be looser than normal.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now