New 2-stroke engine won't start

Dood Surfston

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Joined
Oct 31, 2021
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17
I'm having trouble starting my brand new 80cc 2 stroke bicycle engine. When I pedal up to speed and release the clutch I can hear the engine firing, and it fires as long as I keep pedalling, but dies as soon as I pull the clutch back in. I've tried again and again but the engine dies each time.

I'm following all the correct starting procedures as far as I can tell (20:1 gas/oil mixture, pump a bit of gas into carburetor, choke closed). The spark plug is definitely working, and I've double checked the wiring.

The problem might be low compression. I don't have a compression gauge, but I did the thumb-over-spark-plug hole compression test, and didn't feel much pressure. Even with the spark plug screwed in tight, I can push the bike forward and turn the motor over with a bit of effort.

Any suggestions on how to solve the issue?
 

Dood Surfston

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Oct 31, 2021
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I couldn't see anything wrong with them.

I took the head and jug off, both gaskets looked fine and the side of the cylinder wasn't scored. Only thing that seemed odd was that it was very easy to put the piston back back into the jug. I just had to squeeze the piston rings a little and it went right in. But it's my first time taking apart an engine, so I'm not sure what to look for.
 

Karl Snarl

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Sep 20, 2019
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3,397
Piston in the right way? ring retainers toward the intake? how'd the base gasket look? any signs of creep by?
 

Dood Surfston

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Oct 31, 2021
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Piston's definitely in the right way, with the retainers toward the intake.

The base gasket had a lot of oil soaked in it, but was otherwise intact. What's creep by?
 

Chainlube

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Jan 5, 2020
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4,849
The problem might be low compression. I don't have a compression gauge, but I did the thumb-over-spark-plug hole compression test, and didn't feel much pressure. Even with the spark plug screwed in tight, I can push the bike forward and turn the motor over with a bit of effort.

Any suggestions on how to solve the issue?
Your clutch is too tight, if you push the bike when the clutch is adjusted correctly, the back tire will just skid. Take the right side cover off and in the center is the flower nut, remove the little retaining screw and back out the flower nut a quart turn, or enough to get the little screw back in. Then try rolling the bike again, repeat until there is good resistance.
 

Dood Surfston

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Oct 31, 2021
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I don't think it's a problem with the clutch. I tried adjusting it like you suggest and the clutch got looser and looser until it wasn't biting, so I put it back to where it was.

I'm able to push the bike forward with the clutch fully engaged, and the motor turns over, so I'm pretty sure it's a compression issue. I listened closely I and I could hear air being pushed out when the piston goes up.

On closer inspection the head does look a bit rough, so I'm going to sand it flat on a pane of glass. I've attached a picture of the inner surface of the head. There's a thin ring of metal sticking up from the surface that has a few scratches and gouges. Is it safe to sand the ring down flush with the rest of the head?
 

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YetiWoodz

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Dec 19, 2019
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Is it safe to sand the ring down flush with the rest of the head?
That gouge is pretty deep. Looks like you'd have to sand a lot and into the combustion chamber. That's a lot of material to take off and it may not work because the piston could hit the head or have too little squish clearance. I would spend the $10 and get a new head honestly.
 

YetiWoodz

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Dec 19, 2019
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1,257
Its a process to sand the head down that much and get it right. Measuring squish, keeping it flat on a belt sander, and finishing on a perfectly flat surface like glass. Doing that much work is more worth it on a cnc head.
 
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