[Bc40ton]

Okay, im working an a honda 300ex. It came to me barely running, loss of power, hard to start. While cranking or idling gas is being blown back out of carb throat every compression stroke. A constant in and out of vapor. Pulled valve cover, timing chain was tight but approx 90° out. Pulled head and replaced bent valves. Put everything back together according to specs. Cranked up and ran fine for 10 seconds. I shut it off to finish connecting odds and ends. Tried to start, won't start. Some minor blowback through carb again. Pulled valve cover at TDC and cam was out 12 teeth. Reset cam to align marks with head surface. Put it together. It started right up. I shut it off with a smile. Came back 2 minutes later and it won't start. Pulled valve cover again, cam off again approx 12 teeth. Guys, what am I doing wrong. Chain tensioner seems to be working fine. 3 times I reset the cam timing and it went out of time by 12 teeth each time when at TDC. Chain is tight, all teeth look great. Any ideas on why or how this is jumping time? It a neighborhood kids quad, help! Thanks in advance.
Bruce

 

[SlammedRanger]

My best guess is a sheered flywheel key. And alot of coincidences. Theres no way the chain can slip on the cam gear. Could also be a bad lower timing gear on the crank end. Or maybe you are not getting the chain on the lower gear? And it just so happens to be 12 teeth off when it shuts off everytime? Either way it is lottery winning caliber luck you havent bent more valves. I would highly suggest not starting it till you find the issue with 100% certainty. You may not get that lucky again. My .02

 

[fishfiles]

What about the sprocket on the crank being worn out

 

[Bc40ton]

I'll be pulling the side cover today I suppose and report back.
So timing mark at Tl, chain adj screwed in to provide slack, slip cam and gear into place lobes down, right side of chain tight while positioning timing marks level to head surface, release chain adjuster. Everything tightens up with minor movement.

 

[fishfiles]

I was just looking at a 300Ex crankshaft print and it looks from the pic that the chain sprocket is made onto the crankshaft , I just went thru almost the same thing as you with my 95 TRX300FW , except I would get a little longer than minutes , more like 30 hours of ran time and the chain was so loose from being worn out from misalignment of the crank sprocket teeth and chain rollers

 

[Bc40ton]

Every time I pull the valve cover the chain is tight and no signs of slippage like metal particles or rounded sprocket teeth. And yes, I believe you would need to replace the half crank but I'm going to look closer for wear anyway. And also pull the tensioner and push on the stub against the work bench to see if it collapses. Other than that I am out of ideas. Unless I pull a last ditch effort and try to put it together approx 12 teeth off and see if it changes to a near normal alignment. I know, I'm grabbing for straws. Lol

 

[Bc40ton]

Fixed, so here is what I found. Crank and cam gears look fine. We all know it's a pretty foolproof design so it got me thinking about the auto chain adjuster. The chain is stretched some I'll admit and the adjuster is pretty much at the end of its travel but still has the chain tight enough. I pulled the adjuster out, went over to the bench and leaned my 200lbs of weight against it. It held firm. Did it 3,4,5 times and it held firm. By now I'm thinking what else it could be but something said to tap it against the bench. Bingo. It collapsed. I lean on it and it holds, I push it suddenly from 2" away and it collapses. So my thought was the chain coming around and slapping against the adjuster collapsed it and it skipped time. So I made it into a solid non-adjuster, put it back in and she cranked right up and held timing. Stopped and started it 5 times and she kicked right off. So I'm 98% sure I got it licked. It's 11 degrees outside so I came inside for the day. I want to thank all that replied and hope this might help someone one day.
Thanks, Bruce.