Hi.. 1968 YG5T 80cc (really 73cc) - I have not had success getting this bike to not seize.  It was in rough shape when I got it - likely sat with significant water/ancient oil in the crank case before I owned it.

Some details..
NOS perfect working advance - timing set at 1.8mm BTDC at full advance (per spec).  Running Yamalube 2S in injector tank.  Clean tank - no sludge.

Pump (fully bled) and adjusted per instructions with carb slide circle-mark set when pump cable pulley at mark.  I've validated that it does pump, and pump significantly more at full throttle.  This bike is prior to where they were specifying an amount of oil (per 200 turns of the wheel), so not sure how to test further.

Bike produces a small amount of smoke at idle (which seems like a good sign).  Spark plug looks normal (no signs of melting or damage) - essentially a coffee color with some minor carbon.

I put a 130 jet in (std is 120) to be safe to get the mixture a bit richer/cooler for now.  Bike starts instantly when cold - no effort at all, and idles perfectly with no assist - also low idle speed.

Seizing issues:
I seized last season after fixing/setting everything (above).  After seizure, I ball honed (got crosshatch) and put new piston/rings/piston-pin/gaskets in.  Piston definitely in correct direction, clips seated, engine turns over very smoothly.  I validated all points above, timing, etc.  First ride, after 2 miles in - very mild riding - engine probably about mid-throttle - it seizes again.  Very light seize and was able to restart and further test (next day).

Just got back from second test ride - and seized again about 20 - 30 minutes in.  Riding mid throttle about 30 mph on flat stretch.  Was able to restart (after letting it cool for a few minutes) and limped it home.

A few thoughts / questions:
1. I had re-used the spark plug (D7HS).  I pulled it when I got home (coffee color - no signs of damage or overheat).  BUT - it did look like it might have been slightly leaking past the washer/gasket in one small area.  I know this could cause lean - but given the plug didn't look damaged, could this really have caused the seize issue?

2. The pump "weeps" a bit of oil, but I did confirm that it pumps as noted above.  The pump piston appears to move full stroke as expected on full throttle.  Is it likely there's something further wrong with the pump?

3. The cylinder is at the end of its bore spec - based on end gap - I don't have a bore gauge.  (Probably should bore oversize).  Could a slightly loose piston cause a seize?

4. I've read on this site that running premix on these bikes might cause under lubrication with the bottom end.  Any thoughts on this and straightforward pump disable that wont fry the pump (i.e. by running it dry)? 

Any thoughts or suggestions would be great.  I like this bike, but totally stumped.  Lubrication issue seems likely culprit to me.. but maybe something else?

Thanks,
Adam

 

Do you have any pictures of the bore and piston after a seize?

These pictures are from the original piston - (I.e. Last season's seize).  I was riding it harder at the time this seizure happened than I was for the current seizure.  That said, I don't think I was pushing harder than it should be able to take.  The piston in these pics had been seized twice before I took the top end apart.  Looks sort of like a "four corner" situation to me, but I don't really know how to interpret.  There was some scoring too.  The bore scoring mostly cleaned up on the ball hone.  Nothing that I could catch a fingernail on.  The exhaust picture has the piston pin pointing to the right.

Thanks!

  

Yep, look forward to some pics. What clearance feeler blade can you get in to the lower skirt. If you can get two thou in you'd be safe. I'd go to a 9 plug on the road but sounds like your 7 was ok, But if it seized i'd expect more evidence of heat on the plug. Very unusual situation. Ignition timing--is yours flywheel magneto like ours or do those have electric start in the US. Timing at 1.8 btdc regardless, but electric start needs to be held on advance to get your 1.8mm btdc. Piston very out of shape from multiple seizures? & is it a genuine piston.

Yes.. timing set full advance (weights held full open), crank position at 1.8mm btdc.  Electric start US bike.  Piston in pic is original for bike (Yamaha).  I replaced with Beck-Arnley Japanese made NOS kit from 60s/70s.  Parts looked just like originals in terms of quality, and everything mic'd up exactly the same as original.  The cause of this seems very likely related to cause of original seize with Yamaha original parts, so I would be very surprised if the new seize was caused by the replacement parts.

The old piston (in the pics) was really hammered in terms of scuffing and scoring - but the crown looked fine.  I didn't see what I would assume would be any signs of total lean/overheat condition - but maybe an expert would think differently.

Ok thanks, pics popped up while i'm typing. Just looks bored too tight but see what others say. All looks plenty used. Are the rings good or very scored letting blow-by down the skirt.

I had an occasion where a 100 motor had 'so much' blow-by that it built carbon down the skirt & looked like a teflon coated piston skirt i've seen on moderns & would seize on starting after a few seconds, had zero clearance, no noise just bound up then freed to do it again. So lifted cylinder & became obvious. Sorry, that's all irrelevant to your situation. Except your ring don't look good & can see blow-by of sorts.

Just--also thinking--if its electric start & the timing was set without holding advance open, you'd end up with lots of advance, maybe 3mm btdc guessing & that'd upset it.

Ok i'm a post behind sorry. I'll go back n read it all.

Update to the seizure issue.  The bottom line is: I am a fool!

Essentially, the used spark plug I installed after re-assembling the top-end (with a brand new piston, rings, pin, etc+ a hone) had an air leak and that caused the seizure.  During testing, I light seized 3 times (all testing with that same spark plug).  I put a new plug in today - nice new crush washer - and rode it for a full out test.  The thing rode great - no seizure, no problems at all. 

Really sad I did all that work and ruined it because of not replacing a 4 dollar plug.  I have a fresh piston and rings - gonna clean up the bore and put the new stuff in tomorrow.  Hoping for the best!  New plug of course.

By the way - my take away from this is: Never use a plug twice on a 2 stroke.  Plugs are cheap.  Seizures are full on depression inducers.

Thanks for all the input!

I doubt the plug was cause.

cliff