80cc Mystery

MrLarkins

MrLarkins

HS Math Teacher
67cc Mystery

I have an 80cc (chinese) motor mounted on my mountain bike. It initially ran great, for about 50 miles. I got a max of 36.1 MPH. Now on mile 120, I can't get over 20 MPH. I have tried a bunch of different things.

I have replaced the spark plug.
I have checked for an air leak.
I have replaced the plug cap.
I have cleaned the carborater and verified proper working order.
I have replace gas/oil mix with 30:1 blend.
I have checked that all screws are tight (mounting, carb, chain,etc.).

It's a mystery.

I have noticed there is a 'squeek'. With engine off and clutch engaged (free rolling), I hear a 'squeek' in the motor casing.

At 3/4 throttle, it gets the shakes and bogs the motor down. I no longer get full rpm's.

Here is a pic of the engine, opened up. I tried to wipe off the top of the piston with a rag, but some build-up is still there. And now that I've taken the top off, I'll have to replace the gasket, right?

Now some pics:
broken008.jpg


broken011.jpg


broken012.jpg


broken015.jpg
 
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also, once this is fixed, i'd like to change my 44 tooth sprocket with a 36 tooth sprocket. if you know where i can buy one for my bike, PM me please.
 
Looks like it's been run hot and lean.......... I'm thinking piston and jug and a better oil. do you live in a very hot part of the country ? Others will surely have more to input.
 
it might have been run lean for just a bit...i noticed some extremely light scoring on the piston, below the two metal rings.

I live in central arkansas, temps are 90 to 95 right now. very humid.

frenchman159 said:
I'm thinking piston and jug and a better oil.
Click to expand...
what does that mean? i'm a math teacher, not a mechanic.

my fuel mixture (now) is 30:1 per the man I bought it from
 
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What i'm thinking is If the engine was new when you got it , the mix should have been closer to 16/1 for at least the first tank. Should be no scuffing at 120 miles. you should get a jug and piston along with a gasket set.......
 
I think you are running it too lean for breakin, prob too late now but I would have started at somewhere around 20:1 as the instructions with my kit stated, with a quality 2 stroke oil. I would have to agree that it probably needs a new piston, rings, and cylinder, but you could try new rings first. One of the vendors to the left has all the items.....
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
 
Putting in new rings is a pretty easy task and since you have it apart, I would try that at a minimum. Of course I'm not as well versed as some of the other here.
 
Thats a good question, I would also love to know. As much as I've been tinkering with these things i have yet to have to replace a piston.......We'll way for a more seasoned mechanic to answer...
 
I bought mine from the dealer who sold me the motor... Im pretty sure you can get them from most any off the dealers. Probably dax for sure. His adv is on the left of this page.
 
If I may throw another question out there while we're on the subject.I just removed the head on my 70cc boy go fast with about 1100 mi. on it.Rings and cylinder wall look new,but top of cylinder and head have about 1/64" or less burnt crud on them.Too rich or normal? How do you clean that off.Thanks,Ron
 
ran49 said:
If I may throw another question out there while we're on the subject.I just removed the head on my 70cc boy go fast with about 1100 mi. on it.Rings and cylinder wall look new,but top of cylinder and head have about 1/64" or less burnt crud on them.Too rich or normal? How do you clean that off.Thanks,Ron
Click to expand...

Normal buildup of burned oil Ron. Carb cleaner and a wire brush will clean it.
 
Ok, this is kinda crazy for me, since the most mechanical thing I've every done is change the oil in my car...but,

I took the piston apart. (rings, head, bearings, etc.) and i showed them to my friend, who has actually seen a bad piston before. He looked at the head, rings, and cylinder and said all were perfectly fine. The cylinder itself is flawless. the piston head has no scratches, just some light rubbing near the bottom. nothing that changed the shape. and the rings are still in tact, spring'y' and looking good he said.

broken027.jpg


broken029.jpg


broken030.jpg


broken031.jpg
 
Yes.. They look good in the pictures. What does your head gasket look like? Also.. Running a new motor at such high speeds will burn it up. Mix 20:1 and keep speed under 20mph for the first 200 miles or so. Vary speed up and down. Run engine under load for 15-20 min and shut down for 10. Do this for the first 50 miles. Once you get 500 miles or so, you should be broken in. All the while checking for air leaks. An Air lead is certain death for any 2 stroke. Once you get broken in, switch over to AMSoil synthetic oil 30:1. You should last a long time if you observe these few simple rules. Oh.. Also.. There is no such thing as an 80cc out of china. This is your chance to find out, measure your bore and stroke and then do the math. You will then know that who ever sold you an 80cc motor is either unknowing or lying. Both are not good . Let me know.. Enjoy the ride..
 
ok, so maybe there is no problem? I am on mile 120 right now. I can sometimes get up to 27mph, highest was 36. avg about 20 now. I ordered new gaskets all around. I will also mix a new back of fuel, 20:1 (real quick, how many gallons of gas is that to 8 oz or oil?)

what brand of synthetic should I be using? best?
 
You are definatly lean... Look at the pic of the spark plug, Might want to raise the needle in the carby (lower the clip) one slot......Parts all look ok though. Also when you put the piston back in the cylinder make sure you line the opening of the rings up with the little pins in the grooves...

8oz @ 20:1 = 160oz of gasoline 128oz = 1 Gallon So 8oz of oil to 1 Gal and 1qt of Gasoline

Any QUALITY oil..... I use Amsoil Sabre....
 
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rgr, i'll look for Amsoil Sabre. and I'll remix my fuel. I've seen the pins, thanks for the heads-up on that.

now, what is this suggestion?
misteright1_99 said:
Might want to raise the needle in the carby (lower the clip) one slot
Click to expand...

this is my carborator
carb.jpg
 
You might not find Amsoil locally I order it from Pablo, a member here. Just get a good oil, not weedeater oil, lol, maybe Pennzoil, or a good name brand 2 stroke oil. But if you can find Sabre locally it seems to be very good IMO.

In this post Norman shows you the internals of the carb. You need to read it an familiarize yourself with it then you might want to try moving the clip one notch and riding your bike and see what you think. And then maybe try another slot and test ride etc. Looking at that spark plug I think you are running lean so you should start with moving the clip DOWN 1 slot to increase fuel to air mixture....

http://motorbicycling.com/f4/carburetor-pictures-how-install-rebuild-195.html
 
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hmm, well I thought I had found the solution...i replaced the gaskets on the intake, exhaust, head/cylinder, and lower block. my bike ran great for about 20 miles...then it started acting up again, same as before.

i am beginning to think that something is loose on the inside, that got tightened when I was putting it back together after the new gaskets, and now it has come loose again. any ideas?

i also moved the 'c' clamp to the lowest position in the carborator...isn't that suppose to give the most oxygen in the mix? i don't think i'm getting maximum combustion...lots of oily black on plug...and i mixed 4oz 2 cycle oil with 1 gallon gas, that's the right mixture, right?
 
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MrLarkins said:
i also moved the 'c' clamp to the lowest position in the carborator...isn't that suppose to give the most oxygen in the mix? i don't think i'm getting maximum combustion...lots of oily black on plug...and i mixed 4oz 2 cycle oil with 1 gallon gas, that's the right mixture, right?
Click to expand...

Lowering the clip is raisng the needle giving you more fuel LESS air. I would move the clip to 2nd from top as a starting point...

If your engine is already broken in 4oz would be ok thats 32:1
 
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