what is an acceptable compression of a stock motor

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biknut

biknut

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I just checked the compression on the Atomic BB's twin. It's a Flying Horse 48cc bought at the same time as mine, from the same Zoom Bicycles. It has less than 500 miles on it and is still running break in mix.

It has 62 psi. Mine with a lot more miles has 55psi. Because us this I think a 48cc probably will only have about 60 psi new.

This explains why my bike seems to run as well as new even with 55 psi.

Would be nice if more people with 50s would post their compression readings.
 
george_n_texas

george_n_texas

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I would imagine the compression would be less on the new one you just installed unless they did a burn-in which I doubt they did. When the rings set in that's when I would think you'd see the highest compression like your buddies bike has.
 
Goat Herder

Goat Herder

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Yeah you got to seat the rings first. This compression standard rings true to me with the week power out put of these motors.
 
george_n_texas

george_n_texas

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To bad it took me a hole weak two figure out my spell checker wouldn't help me with my all to often week grammar.


Too bad it took me a whole week to figure out my spell checker wouldn't help me with my all too often weak grammar.
 
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biknut

biknut

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As I recall my motor seated the rings at about 350 miles. I'm pretty sure Bruce's bike has around 350 miles or more on it.
 
george_n_texas

george_n_texas

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I'm readin over the MM web site man what a bunch of whining...talk of "No more ashtrays...No more soap dishes! While our legal team prepares to keel haul the Pirates who have chosen to steal our designs..."

Someone "stole" the design for cylinder heads?

Man if it's one thing I like about the hobby is the open source type sharing of information. I never tried to patent my trailer builds or any other idea I have shared. If someone chose to buy MM web site & biz they also bought the slow service record...man I remember that stuff goin on for months & months people getting their product.
 
Pilotgeek

Pilotgeek

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I get obscenely high compression with my 66cc - around 180psi. Then again, I modified my head quite a bit and have a thinner gasket. Are you sure the compression tester itself is working right?
 
biknut

biknut

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Pilotgeek said:
I get obscenely high compression with my 66cc - around 180psi. Then again, I modified my head quite a bit and have a thinner gasket. Are you sure the compression tester itself is working right?
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Well the tester is new. I think it's working right. From now on I'm going to check all my motors when new, and then keep an eye on them.

No one seems to be getting much compression with a 50cc.
 
Goat Herder

Goat Herder

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I ran into trouble with a Harbor Freight comp checker. Was at work it it spun my wheels on a job..I had to order my last comp checker gauge off the net.. This gauge so far has been great! Had my doubts when hitting the pay pal button but it gets used prolly three times a day in the engine shop I work in.
 
biknut

biknut

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Goat Herder said:
I ran into trouble with a Harbor Freight comp checker. Was at work it it spun my wheels on a job..I had to order my last comp checker gauge off the net.. This gauge so far has been great! Had my doubts when hitting the pay pal button but it gets used prolly three times a day in the engine shop I work in.
Click to expand...
Somehow I'll figure out a way to test it.
 
biknut

biknut

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Sep 28, 2010
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biknut said:
I just checked the compression on the Atomic BB's twin. It's a Flying Horse 48cc bought at the same time as mine, from the same Zoom Bicycles. It has less than 500 miles on it and is still running break in mix.

It has 62 psi. Mine with a lot more miles has 55psi. Because of this I think a 48cc probably will only have about 60 psi new.

This explains why my bike seems to run as well as new even with 55 psi.

Would be nice if more people with 50s would post their compression readings.
Click to expand...
Ok boys, and

lets face it there's not many girls here lol.

I just compression tested my brand new, assembled well by me, Flying Horse 80.

68 psi. It took 2 tries to get that. First try was 62 psi.

It may go up some, but I doubt by very much. It's never going to be 120 psi, I'm pretty sure. I'm really not surprised. When I checked the deck height, it looked huge, Meaning the piston wasn't even all the way to the top of the cylinder.

This confirms my belief that 55 psi isn't out of line, and that's why it still runs almost as fast as new.

The My friends 48cc, that should be in it's prime was 60 psi.

I'll keep checking this new motor.
 
biknut

biknut

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Sep 28, 2010
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biknut said:
I just compression tested my brand new, assembled well by me, Flying Horse 80.

68 psi. It took 2 tries to get that. First try was 62 psi.

The My friends 48cc, that should be in it's prime was 60 psi.

I'll keep checking this new motor.
Click to expand...
I just checked the compression, this time the motor has 51 miles on it.

76 psi.
 
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