Compression test How-to (?)

junglepig

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#1
I'm new to 2-strokes. When I test compression on any four stroke engine, I open the intake/throttle so it can quickly and easily get a full air charge into the engine.

I was reading through the Grubee Skyhawk manual, and the procedure given there for a compression test says to seal both the intake and exhaust.
This makes sense for a leakdown test of course, but it says the same for doing the compression test.

I guess this makes sense. We're just working with whatever volume of air is in the cylinder after the rings close the transfers I guess.

Without me thinking too hard about this tonight, is this how you usually test compression on a two stroke?
How different will the numbers be if I leave the exhaust and intake open to the atmosphere when testing?
I don't have plates made up to seal the ports, so I was just curious if I tried to test compression the way I usually would on a 4-stroke would my numbers be useful and comparable...
Thanks!
 


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#3
O'Reilly auto parts store will let you borrow their compression test long as you use in the front window.

Take out your spark plug connect 2 parts screw it into spark plug hole left the bike turn the wheel by hand which is a b*tch keep spinning tire till gauge won't go any higher should be about 112psi which is difficult to tell cuz it makes long readings.

I bought motor from California motors its about 60psi which is half what it should be the bike scares me cuz I have tried to start motors with bad cylinders it feels the same as those no heavy jerks on dropping clutch it just pud, pud, puds, then takes off except instead rearing back on throttle got to dive down slowly pull back bike scares the hell out of me I live in rural area miles to go to find help today it just died halfway in 4 mile stretch of road no reason for it, but I got it to run again what a worry it is.
 

junglepig

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#5
I started a thread on compression tests not long ago, some good info---> https://motoredbikes.com/threads/compression-test.52855/
Thanks. Good thread. I commented on the death of one of your CDIs. That experience will remind me to protect my cdi when doing a compression test. I suggested grounding the spark lead. Another option is to disconnect the blue wire from the magneto to the CDI while spinning it over.
 

Tyler6357

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#6
Thanks. Good thread. I commented on the death of one of your CDIs. That experience will remind me to protect my cdi when doing a compression test. I suggested grounding the spark lead. Another option is to disconnect the blue wire from the magneto to the CDI while spinning it over.
Yeah, I still don't quite understand what caused it to fail, meh, stock CDI's are cheap, but I can't see how the compression test could have caused it to fail.
 

junglepig

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#7
Yeah, I still don't quite understand what caused it to fail, meh, stock CDI's are cheap, but I can't see how the compression test could have caused it to fail.
Well, like I said in the other thread, if you don't have a normal path to ground (across the spark gap) for the high voltage generated in the secondary, it is likely to arc within the CDI box and take out components there. That's my theory anyway. So that's why I'd suggest grounding the plug wire or disconnecting the blue wire from the CDI so that can't happen.
I'd bet a box of doughnuts that if you get her going down the road and yank the plug wire off you'll fry the CDI almost every time. (Or shock yourself and end up in the ditch! ) That box is so small that the insulation can't hold up and the easiest path to ground will be right through a PN junction in, say, an SCR within the box.
 

Tyler6357

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#8
Well, like I said in the other thread, if you don't have a normal path to ground (across the spark gap) for the high voltage generated in the secondary, it is likely to arc within the CDI box and take out components there. That's my theory anyway. So that's why I'd suggest grounding the plug wire or disconnecting the blue wire from the CDI so that can't happen.
I'd bet a box of doughnuts that if you get her going down the road and yank the plug wire off you'll fry the CDI almost every time. (Or shock yourself and end up in the ditch! ) That box is so small that the insulation can't hold up and the easiest path to ground will be right through a PN junction in, say, an SCR within the box.
Yeah, that's probably what happened, it was working good before I did the test. I did put new boots on my CDIs, I don't use those plastic ones that require you to remove the spark plug cap.
 
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#9
O'Reilly auto parts store will let you borrow their compression test long as you use in the front window.

Take out your spark plug connect 2 parts screw it into spark plug hole left the bike turn the wheel by hand which is a b*tch keep spinning tire till gauge won't go any higher should be about 112psi which is difficult to tell cuz it makes long readings.

I bought motor from California motors its about 60psi which is half what it should be the bike scares me cuz I have tried to start motors with bad cylinders it feels the same as those no heavy jerks on dropping clutch it just pud, pud, puds, then takes off except instead rearing back on throttle got to dive down slowly pull back bike scares the hell out of me I live in rural area miles to go to find help today it just died halfway in 4 mile stretch of road no reason for it, but I got it to run again what a worry it is.
damn. i bought a compression tester for 20 dollars. aaaaahhh
 

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