Need a bit of help with a broken WR450F 2008

[RossG]

Went out laning a few weeks ago, bike started to run rough, would not idle, then came right, then backfired after a while started up then after about a minute died and would not restart. Took it to one of the top enduro specialists, after cleaning carb and tank, replacing spark plug.

He discovered that the bike had taken in a bit of water and some debris as well which had damaged a valve and the piston and rings, these have all been replaced with new, compression is back up to normal where previously it was apparently only getting up to about 75 psi.

Problem now is it starts first time but as soon as the engine gets hot it dies and refuses to start, the gap at the rings is apparently correct, Yamaha says there was an issue with the generator on this model, I dont see if it is getting spark it could be the generator, they are trying to eliminate a broken wire.

Has anyone else had a similar problem, or can anyone shed some light as to where to look next?

[allterra]

Just a shot in the dark. last time I had something like that happen it turned out that the float was sticking slightly. It had enough gas in the bowl to start but as it was ready to go ( motor warmed) it had used all the gas in the bowl and would stall. then after sitting( motor cooled) it would slowly refill the bowl and start nicely run a few minutes and die. It seemed as though it was the warming up of the engine but not in actuality.

[RossG]

Carbs had been cleaned and reset by the engineer, he has ruled that out. What it may seem like now is the barrel is worn, as the compression has dropped back down to 75PSI, Yamaha are saying this is more than likely the problem but until the can test with a replacement we will not know.

He has two YZ heads, does anyone know if the YZ head will fit a WR?

Edited March 6, 2013 by RossG

[Bandit9]

YZ head should fit.

[FORMA MOTO]

Hold on, are they doing a compression check....on a motor that has auto-decompression??

If you get 75ftlbs you are fine. The auto-decompressor removes the rest.

Are you sure they know what they are doing?

You can't build a motor without checking the cylinder spec to begin with, so if the cylinder is worn (which you can tell by looking at it in 15 seconds) they should not have rebuilt it in the first place.

I would get with the service manager and demand a better tech on your bike, and some more logical troubleshooting and spec testing.

There are standard tests in the manual for testing all things electric on that bike, including the generator (stator), without having to guess or 'parts swap' to troubleshoot.

[allterra]

Hold on, are they doing a compression check....on a motor that has auto-decompression??

If you get 75ftlbs you are fine. The auto-decompressor removes the rest.

Are you sure they know what they are doing?

You can't build a motor without checking the cylinder spec to begin with, so if the cylinder is worn (which you can tell by looking at it in 15 seconds) they should not have rebuilt it in the first place.

I would get with the service manager and demand a better tech on your bike, and some more logical troubleshooting and spec testing.

There are standard tests in the manual for testing all things electric on that bike, including the generator (stator), without having to guess or 'parts swap' to troubleshoot.

How do you check compression on a motor with auto decompression? I have never had to deal with an auto before I have always had a lever or spring lock button. Is there a way to disconnect with out accessing the cams?

[RossG]

Thanks for all the replies, it was a worn barrell, quite severely infact, dirt in water that had managed to find a way in and fouled a few valves, and a broken wire that goes into the connector that connects to the sparkplug cap.