what is an acceptable compression of a stock motor

These motors will drop in psi too when warmed up. So in short what Ibedayank is saying I feel and concur is about right . For a good cold reading. As in if you warm up a engine that had a cold reading that is too low well..
 
Ibedayank said:
I take it you where having problems with this motor?
On all the motors I have tested under 100psi it was rebuild time.
last motor was a 100cc hodaka dualsport it tested 105 psi and the boss says... rebuild it
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Well, this is the first time I've ever tested the compression on a cg. I don't really know if there's anyting wrong with it or not.

I'd have to say it runs great, and is as fast as it ever was. The only thing it does that might indicate a problem is, doesn't return to idle right away after full thrittle running, when it's warmed up.

Other then that, it runs great. It does 30 mph with a 40T. Running 25 mph it gets 145 mpg. When warn it starts on the first pop. When it's cold, it usually takes several tries before I can keep it running. I almost never use the choke. I roll the throttle to get it started. It's jetted leaner than stock with a 68 jet.
 
I know if you work on something enough you will eventually break it LOL If it's workin I don't fix it generally especially with a disposable HT engine.
 
well some carb spray would show any intake leaks cigarette smoke would show anything blowin out I would think only other leak I could imagine would be a hole in the piston head not sure how to check that one.
 
pull the head look and see what the inside of the cylinder looks like...
only gasket that matters for compression is the head gasket.

prolly safe to say that topend has had it
 
george_n_texas said:
well some carb spray would show any intake leaks cigarette smoke would show anything blowin out I would think only other leak I could imagine would be a hole in the piston head not sure how to check that one.
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Ibedayank said:
pull the head look and see what the inside of the cylinder looks like...
only gasket that matters for compression is the head gasket.

prolly safe to say that topend has had it
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I looked closer for a leak, and found the intake manifold leaking. I installed a new gasket, and now the idle seems consistant again.

So back to the compression. I agree 55 psi sounds pretty low. Right now all I have to go on is one guy says he had 80 psi, one guy has 60 psi, and I have 55 psi (just talking about 50s). Seems like it should be higher to me, but it's never been any faster than it is now. If I didn't know better I'd think it was running good.


What kind of performance should I expect from a run down 50cc china girl motor? I was just riding back from the post office on it, and I was doing 33 mph twice in 2 miles. I'll do 30 on a flat, and I see 34 mph out of it regularly on slight downhill grades.

I think it's days are short though, because I've been looking for an excuse to replace this motor with an 80 for months.
 
I did some google research last night about 2 stroke compression. There's a lot youtubes showing compression tests on 50s.

Although it depends a lot on your compression ratio, most people agree above 100 psi is ok but not great. Some engines show about 175 psi.

Nobody thinks 55 psi would be good. They say that low compression mainly effects bottom end power. Low compression causes hard starting.
 
with compression that low its going to have a had time sucking the fuel through, hence the hard starting. a trick i use on outboards is to put a little oil in the cylinder before you start it. If it starts right up you know its rooted.

make sure the O-ring on your gauge is properly seated. do you have any other motors to test the gauge on?
 
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Peter16 said:
with compression that low its going to have a had time sucking the fuel through, hence the hard starting. a trick i use on outboards is to put a little oil in the cylinder before you start it. If it starts right up you know its rooted.

make sure the O-ring on your gauge is properly seated. do you have any other motors to test the gauge on?
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What gives you the idea it's hard starting? It's actually pretty easy starting. Like I said, I never use the choke. When warm it starts first pop.

It has a twin that belongs to my friend. I built it for him right after I made mine. His has very few miles on it. It's probably barely broken in. I plan to test it as soon as I can, and I'll post the results.
 
Peter16 said:
I am speaking generally as you stated, one symptom of low compression is hard starting.
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I guess the only thing I could say is the bottom end, and midrange is gutless, but that's the same impression I had 2 years ago lol.

Seriously though, when I saw this thread I wasn't thinking there was anything wrong with my motor, I just decided it would be nice to check it's compression so I could report it. The issue only came up after we found out it only has 55psi.

Even with such low compression my motor seems to run better than a lot of other 50cc motors that are brand new, based of what I've read on this forum. I'm kind of a motorhead though. I could start the box the motor came in.

lworider1
 
Peter16 said:
sounds like you should just run it into the ground my friend.
ignorance is bliss.
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And that's exactly what I've been doing, but it's days are short, because I have 2 new Flying Horse 80 motors on the shelf. They're calling to me lol. I don't know how much longer I can resist.
 
Because Motorkote bonds to the metal I put a capfull in a tank of fuel and check compression after spent. The stuff is $25 a quart off Amazon shipped to your door and treats 2 cars. Probably $5 worth would lube every chain and cable as well as the engine on a MB...I mix a little with my grease as well. The stuff works your engine will be quieter. I saw my turbo's temp go down using this stuff on a '98 Peterbuilt (they have a pyrometer). I wouldn't use it till the engine is broke in but I'd say yours is past the break-in LOL
 
george_n_texas said:
Because Motorkote bonds to the metal I put a capfull in a tank of fuel and check compression after spent. The stuff is $25 a quart off Amazon shipped to your door and treats 2 cars. Probably $5 worth would lube every chain and cable as well as the engine on a MB...I mix a little with my grease as well. The stuff works your engine will be quieter. I saw my turbo's temp go down using this stuff on a '98 Peterbuilt (they have a pyrometer). I wouldn't use it till the engine is broke in but I'd say yours is past the break-in LOL
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What are you saying George? That I might improve my compression and then I wouldn't have an excuse to install my new Flying Horse motor? Damnit man, it's wore out lol.
 
I'm just curious what the stuff would do if ya tried OTOH you could donate the worn out engine to me LOL I can then torture it till it self distructs LOL!
 
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