Got some questions Google isn't answering.

[Marlin710]

I have a 2 stroke 80cc engine on a beach cruiser.
Issue 1: what is causing my speed fluctuation? When I'm near top speed my bike kinda rocks from front to back as if I'm playing with the throttle. Fast slow fast slow fast slow.

Issue 2: I have a fat tire bike 26x2.125 my engine chain sits very close to the tire. Is there a way to space this to make the gap bigger or do I have to get smaller tires?

 

[YetiWoodz]

The rocking back to forth is likely 4 stroking. Its when the fuel mixture is too rich and so the engine is only firing every 2 revolutions instead of every revolution, decreasing power. It sounds like a 4 stroke engine hence the name. Often it happens near top speed and it stops and goes like how you describe. The carburetors stock, run on the rich side because they put bigger jets in. If you have an nt carb, the stock jet is a #70, the stock jet for an hp carb is a #78. To fix this too rich issue, you'll need to buy a set of jets and put a smaller one in. Its super easy and quick to do. You'll notice a significant increase in power. Many people have 2.125 tires. Yes, the chain is very close to it, but its fine. As long as the chain is aligned, the worst that can happen is your tire gets dirty from slight rubbing if it even rubs.

 

[Solux]

I had the same questions too. I was told you can use a thinner chain. Someone linked me one, I'll try and find the post. And yes smaller tires will work too. Maybe some 1.75s.

Update: I was suggested kmc 710 chain.

 

[Marlin710]

The rocking back to forth is likely 4 stroking. Its when the fuel mixture is too rich and so the engine is only firing every 2 revolutions instead of every revolution, decreasing power. Usually it happens near top speed and it stops and goes like how you describe. The carburetors stock, run on the rich side because they put bigger jets in. If you have an nt carb, the stock jet is a #70, the stock jet for an hp carb is a #78. To fix this too rich issue, you need to buy a set of jets and put a smaller one in. Its super easy and quick to do. You'll notice a significant increase in power. Many people have 2.125 tires. Yes, the chain is very close to it, but its fine. As long as the chain is aligned, the worst that can happen is your tire gets dirty from slight rubbing if it even rubs.

Sounds about right. With my last motor the piston locked up because the first mixture was to light. And since this bike sat for a few months I've been doing sort of a re break in process using a 20:1 mixture. I'll try lightening up on the oil to a 25:1 mixture?.. What is a recommended mixture?

 

[YetiWoodz]

Some will recommend you to use 32:1 for break in, then switch to 40:1 after. Some say 40:1 is all you need for break in.

 

[Marlin710]

Some will recommend you to use 32:1 for break in, then switch to 40:1 after. Some say 40:1 is all you need for break in.

Dang. So my stuffs way to thick. Let me try a lighter mixture over the next day or two. Thanks guys. I'll let y'all know what happens.

 

[YetiWoodz]

Yeah, the 20:1 mixture was definitely not the cause of the piston locking up. My piston didn't even lock up when I was breaking the engine in with 100:1 mixture. Don't run the engine with 100:1 obviously , I was misinformed by the seller.

 

[Marlin710]

Yeah, the 20:1 mixture was definitely not the cause of the piston locking up. My piston didn't even lock up when I was breaking the engine in with 100:1 mixture. Don't run the engine with 100:1 obviously , I was misinformed by the seller.
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I know. With the engine that locked up I didn't measure my mixture. Just a splash of oil in the tank then ride full throttle lol. Since then I've been a little more cautious and maybe a little excessive on the oil ratio.