Low or no compression

[redart]

Hi,
I've kinda asked this question in another post (thumb compression test) but didn't really get a clear answer, so I'll try again. Question is......will very low compression in both cylinders of a 6hp 2 stroke engine stop it from firing?. I mean not even a cough or splutter!.
I have spark and fuel. I can hold a thumb over either plug port and feel very little pressure when I cycle the engine by manually turning the flywheel. I've taken the head off and the gasket is intact but looks pretty crudey. I also noticed the bolts weren't that tight. Pistons and cylinders looked ok to me. After cleaning the gasket and putting the head back on, still nothing, not a pop. Am I on the right track thinking this is a compression problem ?.

 

[Andy in NY]

Re: Low or no compression

sounds like you should use a real compression tester to get an accurate reading.

start with the troubleshooting spark and fuel.

 

[Tim Frank]

Re: Low or no compression

Hi,
I've kinda asked this question in another post (thumb compression test) but didn't really get a clear answer, so I'll try again. Question is......will very low compression in both cylinders of a 6hp 2 stroke engine stop it from firing?. I mean not even a cough or splutter!.
?.

Yes! It will stop it from firing. (is that clear enough....)

I can hold a thumb over either plug port and feel very little pressure when I cycle the engine by manually turning the flywheel. I've taken the head off and the gasket is intact but looks pretty crudey. I also noticed the bolts weren't that tight. Pistons and cylinders looked ok to me. After cleaning the gasket and putting the head back on, still nothing, not a pop. Am I on the right track thinking this is a compression problem ?.

The advice that you got in that other thread was that the readings that you will get off your thumb may not fall within the accuracy that is desirable in a compression test. You are looking for both cylinders to be within about 10% of each other....even if you use the same thumb on both cylinders you may not be able to rely on the results.
You need to really roll the motor over to get the compression reading....even if you could judge "adequate" compression with your thumb, you won't be able to pull the cord and use your thumb with good results.


Also, most folks in here would not recommend reusing a head gasket. It may have been perfectly good to start, but now probably isn't. I would replace it after I checked compression WITH A COMPRESSION TESTER. Otherwise you are shooting blanks....in the dark.

You can check spark using your thumb, but I wouldn't advise that, either.
You did not mention HOW you verified that you have spark, but if you did the "plug grounded to the block" test, that is by no means foolproof and you would be better to use a proper spark tester and ensure your spark will jump 3/8"+.