Ulstermx

Member
May 26, 2012
1
0
Hello folks, I recently bought an 82 yz490 to race in the classic grasstrack over here in Northern Ireland. The bike went well apart from being quite a pig to start but now it seems she wont sart or run. The spark is strong and so is compression. I lifted the head off and noticed alot of detonation around the outer edges of the piston crown, the cylinder dome also shows signs of detonation. I've heard these motors are prone to detonation even when jetted correctly so I can only assume the ignition timing has slipped out of sync with the crank. Has anyone any thought? i havent yet pulled the flywheel off. I cant seem to find much info on timing these motors so any help would be greatly apreciated.

cheers

Dee
 

RonD

Member
May 20, 2012
4
0
Ulstermx said:
Hello folks, I recently bought an 82 yz490 to race in the classic grasstrack over here in Northern Ireland. The bike went well apart from being quite a pig to start but now it seems she wont sart or run. The spark is strong and so is compression. I lifted the head off and noticed alot of detonation around the outer edges of the piston crown, the cylinder dome also shows signs of detonation. I've heard these motors are prone to detonation even when jetted correctly so I can only assume the ignition timing has slipped out of sync with the crank. Has anyone any thought? i havent yet pulled the flywheel off. I cant seem to find much info on timing these motors so any help would be greatly apreciated.

cheers

Dee

So I assume you don't have a manual . . . perhaps someone who DOES have a shop manual can tell you where timing should be set.

Also, you should be able to pick up a manual on-line for your bike, although you probably don't want to wait that long to get it started. . :whoa:
 
Dec 17, 2005
35
0
I checked my manual and it says with the engine running, check with a timing light. The two marks, crankcase and flywheel, should be aligned. With your engine not running, you can still check it. Remove the spark plug, spin the engine with a socket on a drill at the flywheel, check with timing light. I doubt it is a timing problem though. I would put a new plug in and try it. If it still doesn't crank, pour a little gas into the sparkplug hole. If it cranks then its a fuel problem.
 

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