Hmm... Bad choice on my part... Obviously was.

Looked around for a post relating to this (since I don't really like posting) and couldn't find anything, so here goes.

So I just recently finished my break-in period on a nice 66cc grubee skyhawk. I just changed the fuel oil ratio to 4 oz per gallon after my last gallon was with 5.5 oz oil and I had 8 oz per gallon during the 2 gallon break in (more like 2 month break-in...) and had noticed a nice change in the power. I also slapped on a SBK expansion chamber and shift kit as well as a nicer air filter on the carb.

Now here is where my stupidity kicked in. I was riding to the campus with my gf in her car behind me and I was running at about 25 mph in my second gear accelerating to about 28 mph or so and got used to hearing the high pitch loud noises from the exhaust pipe. Then I felt it, a nice jerk and a sudden decrease in power. Obviously I shouldn't have rev'd it so hard but I forgot to shift up and had gotten used to the loud noise. (I usually shift once I hear the high pitch noises but recently got a muffler to cut that down, since I no longer heard the high pitch noise then I found I could push the bike throttle further with no ill effects and more power. Then I lost the muffler when i vibrated off (didn't secure it well enough). So from there I was used to pushing the engine and figured that the high noise of over revving was just the noise that used to be canceled out by the muffler.)

So long story short, I think I burned out the engine and don't really know where to go from there. A friend of mine in my class (rides a 1000cc crotch rocket - forgot what make) educated me on the fact that I could have broken a gasket or valve or something and am losing pressure.

If it's just a gasket problem I think I could fix it myself, but if it's a valve then I don't know, or worse a crack... Perhaps some JB weld on a crack would work?

Is there any way to fix this or will I have to ditch the engine and get a new one?

-One more note, I have noticed black leakage at the bottom of my engine, but I noticed that about a week ago when it dripped on the floor. I figured that it might have been from the chain as I lube it pretty good.-

Thanks

does it still run?

Yea, it pulls really well in the first gear like nothing happened, but once i reach 2nd gear going 23mph it seems to lose all power and dies back down to 17mph instantly. I'll downshift and start again.

Check for air leaks... These engines are notorious for this. Check the oil seals in the crank. If it's black oil, it's mostly probable that your oil seals popped out, or damaged.

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That sounds scary, but if it still runs and pulls OK until a certain speed/rpm, then I wouldn't think something is "blown".
I don't have experience with this motor or shift kits. My motor doesn't have a float bowl. Motors that do can sometimes run fine until they get up to a certain rpm and fuel demand. Then they bog down for a bit; then rev back up and repeat. My big leaf-vac (pulled behind a riding mower) was doing this. The needle seat had a tiny rubber gasket that the gas had made swollen over time.

I hear some setups can get fuel-foaming problems from vibration at a certain rpm. Probably not your case, but something to think of.

Black leakage from a 2-stroke is not unheard of. You run those Chinese motors rich (oil-wise), so you gotta expect some oily exhaust and maybe some drips on the garage floor. Get the motor as clean as you can and try to find the source of any leaks.

Maybe just a bearing?

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are you sure that the motor isnt 4 stroking when you hit the higher rpms...i had trouble with this till i found a thread here that got me straghtened out...the high rpms dont worry me here...these things got such a short stroke and a light crank where they can really be tacked high...does the sound of the motor change when it falls on its face???and oh yeah im sure you didnt burn a valve since you dont have any....hehehe....just messing with you

Thanks for all the replies.

Now that I think about it, it did bog down and once I was around 18 mph, i reved it up again and then it repeated. Don't know if that's relevant though. I haven't been having any oil dripping from the exhaust, it was from the bottom of the engine, but my theory is that it's just the throw off from the chain, I find that black soot all over the place after I lube the chain and seeing as the engine is at an angle in the frame, I don't see why it wouldn't build up on the bottom.

By 4 stroking does that mean a change in the sound? I've heard changes in the sound when it bogs down. Kinda sounds like there are extra rotations going on or something. I guess that's what would be a four stroke.

If there is an air leak how can that be fixed? I read a blog somewhere about pressure testing and then using windex to on the engine to look for bubbles where a leak might be. But once a lead is found, how to seal it? Just some epoxy?

I actually did another stupid thing and took off the head to check out the insides myself. Now I can't get it back on since those piston rings are getting in the way.


Edit: Oh and thanks for the reassurance on the valves thing. I noticed that I didn't have any once I tore the head apart but was unsure if there were any below the cylinder head.

Like noco said, and Bikeguy Joe 4 stroking at high revs, it could be 4-stroking. I'm no expert, in fact, I'm probably the last person in the world you should take any advise from on any subject. But when my motor does that, I screw in the left carb screw an hour (1-o-clock position to 2-o-clock position).

Hmm, Hopefully it would be that easy.

I just spent the past 2 hours unmounting the engine and finally figured out how to put the head cylinder back on without breaking piston rings. So lets say that it is just 4 stroking, that would most likely cause my symptoms? Engine bogging down at around 23 mph then reving back up at 18ish or so mph.

So I guess I'll just reseal what I've taken apart. I'll have to wait for a new piston ring to come in the mail since I shot that one... unless it's not necessary, but I'm sure it is. I also tore a base gasket taking it out by mistake. Then I'll try that. Where is this left carb screw you're talking about a-dam? That's not the idle screw is it?