Compression PSI 25cc vs 32cc

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handy_Biker

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Jul 23, 2008
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Since I built my bike with a 25cc homelite, I have inherited a couple more ( non running. Wondering what general condition these were in I hooked up a compression gauge and I get 90 lbs ( 100 if I try really hard)..
I also Have a Mculloch 32 cc weed eater , so I took a compression reading to that came to 125 lbs easy, and that got me to wondering...

I know the homelite only has one ring, I think this is pretty standard for two strokes.

The main question I have is, are most 25cc equal about 90 psi
and do most 32 cc equal about 120 psi
 
deacon

deacon

minor bike philosopher
Jan 15, 2008
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I dont have a way to measure it so this is pure antidotel but I can feel a big difference when I turn them over. How they act on the road I don't really know since i can't get my 25cc to run just now. I don't remember it being anywhere near as powerful but that could be because I was using a different drive system. Maybe someone else can tell us how they act as bike engines.

I do know my 31cc bolens pulls the bike along very well.
 
Bikeguy Joe

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
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90 is O.K. for a two stroke, it may be getting tired....70 will run, but not well. 120 is about the highest you see on a small two stroke.

Also, some engines have a higher or lower compression ratio which will make a difference in your readings, as will how fast you spin the engine, if you are doing it "dry" or "wet" and how the engine is ported.
 
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handy_Biker

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Jul 23, 2008
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Bikeguy Joe said:
90 is O.K. for a two stroke, it may be getting tired....70 will run, but not well. 120 is about the highest you see on a small two stroke.

Also, some engines have a higher or lower compression ratio which will make a difference in your readings, as will how fast you spin the engine, if you are doing it "dry" or "wet" and how the engine is ported.
Click to expand...
I did the compression tests on three - 25cc homelites, dry, and they all read 90 psi. I added a couple of drops of regular motor oil to one of them, spun it several times, and got 100

I run one on my motored bike, and it does surprisingly well for what it is... can do ok on the flats, helps over the hiway overpasses, steep hills ( and theres lots of them here), it pretty well better to walk it than chew up the tire tryin LOL . Of course, if I weighed in at 150 or less, these lil homelites would do a lot better.



The Mac 32cc was tested dry and read 125 psi,

And as many others wonder, if using a 32 cc would be enough of an increase to make a difference.

Which now makes me wonder... how much PSI is there in one of the China kits ??
 
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