Low compression after rebuild.

[tom waite]

Well I finally got my motor together. It is a 1983 Aero 50. The piston and rings are brandy new. It was a complete rebuild.

I couldn't get it running after. I ran a compression test and got 30psi the first few times. Then I added oil to the cylinder and got 150psi once, then down to 120psi. I let the scooter sit for an hour or so and gave it another go. No I get a consistent 80psi. At first I thought the rings weren't springing open, but the oil would have fixed that.

What I THINK happened is I ported my exhaust port too much. Any way to verify this? I tried blocking the exhaust all together and the numbers didn't change. That was the only way I could think to test if it was my port work.

Suggestions? ideas?

[Wheelman-111]

Greetings:

Have you disassembled to check the rings? Unless you reamed the exhaust port up like a half-inch, you shouldn't drop by that much. Also consider gasket and seal integrity.

[motormike]

ring upside-down ??..............new piston, stock ??........any work on the bore ??.......submit more info.......................

[tom waite]

No idea how to tell if the rings are upside down or not. I got the order right. The cylinder was honed. The piston is stock size NOS off ebay. Part #s matched perfectly.

[motormike]

A stock piston with new rings wont do anything to help a worn cylinder......micrometer check of cylinder for wear should have been performed before rebuild........any chance you did that ??

[tom waite]

A stock piston with new rings wont do anything to help a worn cylinder......micrometer check of cylinder for wear should have been performed before rebuild........any chance you did that ??

No sir I didn't. It ran with that cylinder before. All I did was hone it so I assumed it was still good.

[motormike]

What was the purpose of the rebuild ??.......symptoms before ??.......were you just wanting to port it for improved flow ??

[wiguy05]

Another classic case of not knowing what you are doing before you start.

[richdavison34]

The 83'-84' Aeros are notoriously hard to find parts for. If this was not a OEM piston from Honda, I'd be forced to have some reservations about it's dimensions.

Who honed the cylinder? If it was you, what type of honing device did you use?

Starting at the cheapest thing first, did you replace all the gaskets? Are the tourque settings correct?

*Question for the Vets here*
Would it be a poor idea to install the rings on the old piston and check the compression again? Would this rule out the new piston being of improper dimension, and the possible excessive wear of teh cylinder?

[Bear45-70]

Another classic case of not knowing what you are doing before you start.

+1 and not knowing why one needs to do certain thing during a rebuild and why.

If I remember right (in non-racing situations) I only ever had one rebuild where I was able to reuse the stock bore without anything more than a hone job. A case of the wrong plug, way to long a reach plug, punching a hole in the piston.

[evilone]

No idea how to tell if the rings are upside down or not.

The tiny marks by the ring gap go up. They are pretty hard to see.

[tom waite]

The last motor had a crank bearing grenade itself. No damage to the cylinder how ever the piston was mangled badly. Fat chance I know. I had a machine shop hone it. I will pull the jug and measure. The piston is a genuine honda piston btw.

And I normally don't defend myself against the butthurt pros, but "not knowing what I am doing"? It's a scooter. If I am going to pick SOMETHING to learn on, it is this. BUT I am an avid car enthusiast. I have rebuilt several engines. From mustangs, to volvos, to lawnmowers. Mistakes happen. I'll learn from this error and move on with life having learned something else.

[bakaracer]

I would check the ring end gap clearance and piston to wall clearance. if its within spec then either you have a leak from one of the gaskets, reeds or the gauge.

[tom waite]

Ok. I spent the day trying everything. I pulled the head and measured the bore. book says no bigger than 1.574. Mine is 1.579. Sorry, I don't believe .002 is giving me enough blow by that it won't even kick. I got 30psi on the gauge WET. How ever, I did order another head and cylinder. Gaskets are all good. Everything was tight.

And I looked and looked. The rings did not have any markings for top or bottom. The service manual doesn't mention it either.

[wiguy05]

I would check the ring end gap clearance and piston to wall clearance. if its within spec then either you have a leak from one of the gaskets, reeds or the gauge.

This.