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June 6th, 2017, 11:25 PM   #41
ede
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The crack surface of the decomp looks like it was caused by vibration. First the material got weakened and then it broke since it wasn't strong enough anymore.
The markings on your piston look like the exhaust valves make contact (maybe due to wrong cam timing). In that case the piston pushes against the valves. That could cause the valvetrain to stop immediately or at least to decerlate instantly. Besides the high forces on every part in the valvetrain, the decomp flyweight would accelerate in opposite direction. When the engine is running that way, it makes the decomp flywheight oscillating at full force with high frequency....

...so it's just a guess and not much helpful.

Do the clay check is bushi said and carefully check every single part! Sorry for you BLUDGEONER
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June 8th, 2017, 02:47 AM   #42
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Hi Steve, that looks very similar to an 650 engine I just bought, same type of cam with rocker cover and rockers machined for the lobes to clear, the ports have been opened up with the protruding valve guides removed but mine is a low compression piston, do you know who built it?
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June 8th, 2017, 01:14 PM   #43
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Originally Posted by bushmechanic View Post
What is the exhaust valve to piston clearance ?
Might be good to check it with clay as the Piston looks a bit odd and we don't know if that happened earlier when the valve timing was out or not
Well - not what I was expecting.... measured at - ex 2.4mm, in 1.7mm. I thought it should be round about .6mm??



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June 8th, 2017, 01:21 PM   #44
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Originally Posted by bergiefe400 View Post
Hi Steve, that looks very similar to an 650 engine I just bought, same type of cam with rocker cover and rockers machined for the lobes to clear, the ports have been opened up with the protruding valve guides removed but mine is a low compression piston, do you know who built it?
Regards
Matt
Hello Matt. I've got an idea of who might have built it, but can't be sure. I'll pm you so's not to tread on any toes!
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June 8th, 2017, 02:23 PM   #45
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Nice work !

Ok so if the exhaust valves are still straight everything should be fine

Love the pics thank you
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June 8th, 2017, 02:29 PM   #46
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Originally Posted by bushmechanic View Post
Nice work !

Ok so if the exhaust valves are still straight everything should be fine

Love the pics thank you
Thanks! Those clearances about right then?
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June 8th, 2017, 02:43 PM   #47
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Yep
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June 8th, 2017, 02:52 PM   #48
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Cool, thank you.
Thanks from bushmechanic
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June 8th, 2017, 02:57 PM   #49
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Originally Posted by ede View Post
The crack surface of the decomp looks like it was caused by vibration. First the material got weakened and then it broke since it wasn't strong enough anymore.
The markings on your piston look like the exhaust valves make contact (maybe due to wrong cam timing). In that case the piston pushes against the valves. That could cause the valvetrain to stop immediately or at least to decerlate instantly. Besides the high forces on every part in the valvetrain, the decomp flyweight would accelerate in opposite direction. When the engine is running that way, it makes the decomp flywheight oscillating at full force with high frequency....

...so it's just a guess and not much helpful.

Do the clay check is bushi said and carefully check every single part! Sorry for you BLUDGEONER
Very helpful - looks like you spot on with your assessment. I'm guessing the damage was done while I was trying to start it with the timing slightly out.
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June 9th, 2017, 12:10 AM   #50
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Originally Posted by BLUDGEONER View Post
Hello Matt. I've got an idea of who might have built it, but can't be sure. I'll pm you so's not to tread on any toes!
don't even go there!

I would never put THAT piston in a 650. it's a 470 I think and we don't touch them, they are too low.

also, it's simple now, the valve timing was retarded. what you want to do is do the same test with it delayed one tooth and then if you have too: 2 teeth. knowing how it happened will help lay down your fears.

it could be a combo of a tooth out at the bottom and another at the top etc.

don't even start bludgeoner!

Taffy

Last edited by Taffy; June 9th, 2017 at 12:23 AM.
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