80cc Mystery

MrLarkins · #1 (**permalink**) 06-17-2008, 04:39 PM

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:

MrLarkins · #2 (**permalink**) 06-17-2008, 04:40 PM

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.

frenchman159 · #3 (**permalink**) 06-17-2008, 05:02 PM

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.

MrLarkins · #4 (**permalink**) 06-17-2008, 05:07 PM

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.

Quote:

Originally Posted by frenchman159

I'm thinking piston and jug and a better oil.

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

frenchman159 · #5 (**permalink**) 06-17-2008, 05:23 PM

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.......

misteright1_99 · #6 (**permalink**) 06-17-2008, 05:23 PM

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.....
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deacon · #7 (**permalink**) 06-17-2008, 05:48 PM

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.

MrLarkins · #8 (**permalink**) 06-17-2008, 06:09 PM

i got to buy some then...got a link
what size? how do i find out?

frenchman159 · #9 (**permalink**) 06-17-2008, 07:19 PM

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...

deacon · #10 (**permalink**) 06-17-2008, 07:49 PM

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.