qt50 top-end post mortem

Rob LaFreniere /
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To any and all,

What do you think were the causes for the following piston/ring damage?


1984 Qt50 with 3600 miles

New oem PW50 head

New oem PW50 cylinder with mildly chamfered ports.

New oem NOS QT50 - standard size

New oem NOS QT50 rings - standard size

New oem base and head gaskets.

New oem reed valve gaskets

Boysen reeds

standard plug, whatever the manual calls for.

rebuilt Mikuni carb with new gaskets and 70 main jet.

MLM Peoples pipe

Uni Pod air filter.

Oil pump still intact and connected, presumed working

Using 50-1 premix

Replaced both crankshaft seals

compression at 110 psi

a leak down test held 7 psi for 6-7 minutes.

While, playing with jets and needle settings, there were occasions where it was running lean, which I determined by noticing a surge in power when applying the choke.

I was going 35mph on a level road, 30 going up a slight incline. Starting from a stop on a incline was painfully slow.

Temps where usually between 340-360. I saw it once go up to 378. I might have a soft seize at that point, the engine died but I didn't feel anything lock up. not sure what a seize would feel like. The bike started and ran a minute later.

There are perhaps a total of one and a half hours running on this top-end rebuild.

My guess is that it was running lean partly because of running premix with the oil injection still working in combination with poor carb settings.

I would like to know if it is possible that previously existing debris inside the crankcase could get inside the cylinder and cause this kind of damage.

I was about to install a malossi kit this weekend until I saw all this.

More photos to come.



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