Over-sized piston kit help!

[robcg2]

bought an over-sized (56mm) piston & ring kit, had the cylinder bored out to match. Put everything together, and checked compression... Test result was 181psi. Service manual says it should be 171psi. I can only assume the extra 10psi is from the extra 1mm bore. Runs up to 55mph like a champ... but if I run at a steady speed... say, 45mph... after a minute or two, the engine starts to sound weard( like it's bogging down or something), and seems to lock up and quit running. The first time it happened, I thought "I DID" lock the engine up, but When I hit the start button, it fired right up like nothing happened. This happened 3 times the first day after re-building it. I'm thinking, I dammaged the reeds some how , or I need to put different reeds on it alltogeather. or is my thinking all wrong. It also sounds like something is ratteling or knocking at idle inside...I'm positive it isn't the cooling shroud ratteling... alraedy had it welded back togather, and checked for rattels. PLEASE HELP...Going back to college, n need it for transportation. thanks, Rob

[mpathus]

Soft seize? Too lean? Fuel starvation? Cylinder/piston not run in properly? Recheck head bolt torque to proper specification?

Perhaps the extra compression is from the cylinder gaskets not providing enough squish, and the piston rod, once warmed up expands to 0 squish, locking the engine? I'd think that you would hear the piston hitting the head though. That's some serious compression at 181psi!

Do a plug chop to see a/f condition next time it cuts out like that again - might give a clue.

[robcg2]

thanks for the info. did some research on "soft seize". From what I found... I think the oiler isn't pumping enough for the added volume, therefore running it lean and hot. A couple of guys said "once you go to oversized kits, you have to by-pass the oiler and start pre-mixin'. I think that is what's happening... at least partly. more than likley scarred the cylinder I just had bored out. going to break it down tomorrow and see what kind of dammage I did.
P.S. plug chop result: Dark brown(almost Black) & Dry with barley a hint of moisture.

[mpathus]

Dark brown indicates a rich condition and plenty of oil (if mixed at the right ratio), and OK!

If you do a compression test to tell if engine is sealing properly you might not need to fret over the cylinder being internally scuffed.

Are you supposed to run in the new piston and cylinder before trying speed runs? That may take time so the parts bed correctly.

[harry]

i mix a 'super' oil in my tank plus the oiler is still hooked up
i put 1 oz of 100/1 amsoil saber in and it does the job nicely

oilyhans

[zeonsredcomet]

you may be loading up in addition to the other things that have been said already. It sounds like you are "loading up" though.

[harry]

whats 'loading up'??

oilyhans

[harry]

Are you supposed to run in the new piston and cylinder before trying speed runs? That may take time so the parts bed correctly.

yes its a good idea to limit the high revs for a hundred miles or so
and rings do take a bit to seat.

oilyhans