Compression Loss? Engine Will Not Start

Sikk_29

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Apr 14, 2019
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I recently upgraded to an aftermarket high compression head on my Amazon-purchased (Seeutek) 66cc two-stroke engine. I had been having issues maintaining an idle after the bike had been ridden for 10 minutes or so, which I assumed to be head warpage. I removed the cylinder body to replace a failed (split) base gasket, and, after reassembling with new head and copper head gasket, the bike would not start or even fire at any throttle or choke setting. I lapped the head gasket mating surface on the cylinder (several times now) to no avail. The shape of the head's compression chamber prohibits me from doing this to the head. There is an extremely small discrepancy between head and cylinder when placed together; the head can rock back and forth almost imperceptibly, though the head gasket prevents it when placed between.

The bike can be pushed with the clutch engaged, with some effort, and about the same as when it was in running condition. The engine makes a very small squelching sound as the piston moves, though you cannot feel any air movement on the outside of the cylinder. The clutch is not slipping (the drive gear rotates). I can feel quite a bit of pressure when turning over the cylinder with my thumb over the spark plug hole. When attempting to start the bike you can hear the piston compressing air and the bike drags to a stop as if stalling.

I have checked for spark (maybe slightly weak?) and magneto coil passes a visual inspection (though it has gotten wet before; after drying it ran fine). I have also tried the original head, which did not work in exactly the same way. Fuel seems to be getting to chamber, I cleaned the main jet and the float valve and choke are working. I replaced the spark plug to see if a new crush washer would do the trick (it didn't). I need to replace exhaust and base gaskets again, but this shouldn't prevent it from starting at all?

Any advice or speculation appreciated. Bike has never looked better - but doesn't actually run
 

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Greg58

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May 1, 2011
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Welcome to the forum, I would remove the cylinder and check what went wrong. Did you make sure the ring ends were at the pins in the grooves? I've found that if I place a piece of wood across the block to rest the piston on its easier to install the cylinder, finger pressure is all that's needed to compress the rings one at a time, after both rings are in the cylinder I remove the wood.
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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I agree with Greg58... Since you've taken off the cylinder (for the base gasket), you might have broken a piston ring putting the jug back on?

Another thing to try is installing a new spark plug (NGK B6HS) ~ gapped at .025", & make sure your choke lever isn't up.
 

Sikk_29

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Apr 14, 2019
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The piston rings are intact, and I've been careful to keep the opening matching with the index pins. Is it possible I've deformed/scratched them while removing and replacing the jug - several times now? I have had some difficulty getting the piston rings compressed to get the jug back on, so I'll try your suggestion next time. Piston moves very smoothly in cylinder. That was the plug I changed to recently and the approximate gap it came with. And I've checked the choke isn't stuck.

Would copper Permatex gasket sealer be appropriate to try on head, to rule out head compression issues? I've seen a divide on this forum as to whether using it on the head mating surfaces is a good idea.

Thanks for the prompt replies
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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Los Angeles, CA.
Compression is easy to check... If you roll the bike forward (with the clutch lever out), you should be able to feel the compression in the engine every few feet.

Next, do a 'spark check'... remove the spark plug, attach the wire to the plug, touch the plug threads to the head to ground it & have someone spin the back wheel & see if you have a spark jumping the gap in the plug.
 

Sikk_29

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Apr 14, 2019
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There is considerable resistance when pushing the bike with clutch engaged, though I can still move it. Spark is there but perhaps slightly weak? It's difficult to see in daylight
 

Greg58

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May 1, 2011
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Does the spark plug ever get wet when using the choke? I hesitate to suggest starting fluid but a 1/2 second squirt in the filter will tell you if it is firing, don’t use much, just enough to make it pop.
 

Sikk_29

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Apr 14, 2019
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Thanks for the help all. Unfortunately, we aren't likely to learn what the problem was since the bike was stolen last night from my balcony. I had just finished finals yesterday and was going to start working on it again today, infuriatingly enough. I will likely be building another at a later date, once I have saved up for a while