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Backfiring when starting, wont start...
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Posted by: 450racers.com

I recently put a new top end on my YFZ due to a blown head gasket.

To start you off with the right information: The quad was running great at the end of last summer. I put it away for several months due to winter setting in. On a warm day in January I tried to ride it but it would not start. It backfired very loudly when I'd hit the start button. Finally, when it did start white smoke poured out the pipe. Ok so now I replaced the head gasket along with a new piston and rings, but now its just more of the same.

The problem is the quad is VERY hard to start. It will either backfire or give one "pop" like its going to start, but then not.

I have checked the timing several times. One thing I did notice is that the TDC marks on the magneto rotor/cover does not quite match true TDC at the cylinder. Possible sheared keyway?

Along with a factory service manual and websites such as this one, I do feel that I have the valves and timing right. Any info would be greatly appreciated!

My guess is whatever was wrong before the head gasket blew is still wrong and the new top end did nothing to fix it.

Posted by: scrimmy

do you have it in the right stroke? you may be on the exhaust stroke. your timing should be dead nutz.. try turning the motor over again to tdc and try reinstalling the cams and see where you are at. i would bet you are on the wrong stroke

Posted by: ktmbill

Check your valves clearance you may have to set the valve clearance, as the motor gets more hours on it the valves seat in the valve seat more you have to re shim for clearance.
Oh by the way you if you are at tdc and you have the cams set . Your good check the clearance

Posted by: scrimmy

i still say your on the wrong stroke. try pull starting it, if it starts you need to shim the valves. if not your on the wrong stroke

Posted by: 450racers.com

Thanks for the input. I will try the suggestions and post my results.

Posted by: Ride1Rob

I wouldn't try to pull start it if you think the timing is off. That could lead to bent valves if they touch that piston. That would then lead to catostrophic failure to major parts of your motor soon after. It's obvious that you don't have a battery issue so why pull start it? Like he said, check your valve clearance 1st, if they're in spec check your timing marks 2nd, if they're good check to see if your key is sheared. If you think this is the problem, take a pic of the top of the piston at TDC in conjuction with the flywheel marks. Post the pics and let me take a look. There's a gray area where the piston is on the downstroke a tadbit as you cross the midline mark of the flywheel.

Posted by: 450racers.com

quote:
Originally posted by Ride1Rob
I wouldn't try to pull start it if you think the timing is off. That could lead to bent valves if they touch that piston. That would then lead to catostrophic failure to major parts of your motor soon after. It's obvious that you don't have a battery issue so why pull start it? Like he said, check your valve clearance 1st, if they're in spec check your timing marks 2nd, if they're good check to see if your key is sheared. If you think this is the problem, take a pic of the top of the piston at TDC in conjuction with the flywheel marks. Post the pics and let me take a look. There's a gray area where the piston is on the downstroke a tadbit as you cross the midline mark of the flywheel.

Yeah I can tell you right off the bat that when I have the TDC mark on the flywheel lined up it seems like the piston is not quite TDC...

Posted by: ktmbill

Their is two marks on the fly wheel. Take your spark plug out stick a screw driver in turn your piston to TDC set your cams or take the fly wheel off check the key.

Posted by: jrspawn

Sheared flywheel key.

Posted by: 450racers.com

Upon rechecking the TDC (with another person helping) I found that the mark on the flywheel was true TDC. Also checked keyway, it was good.

Problem was both cams off one tooth (dots to low below valve cover mating surface), also both intake valves were out by .10mm and 1 exhaust valve was out of tolerance by .10mm

After correcting those errors, it starts flawlessly.

Thanks to all that answered my post!