Compression ?

Mon Dec 29, 2003 1:25 am

How much compression should the engine have. I am having a hard time trying to start my 49 cub. Every thing checks out ok as far as getting fuel, spark, air and timing. I did a compression test and I get 72/75/70/75 lbs. in each, dry. I didn't have a chance to do a wet test. :? It seem a little low. :?

Mon Dec 29, 2003 7:36 am

That is low, but enough that it should run. Since they are uniformly low, I imagine the rings/cylinders are where the compression is being lost.

Mon Dec 29, 2003 10:29 am

It ran about 6 months ago. But now I just get it to pop once in awhile with a little starting fluid. Would a wet test tell me if its the rings?

Mon Dec 29, 2003 10:54 am


It should run with that compression, even though it is low. Adding a little oil would tell you if it has ring problems, and also help it start... providing it doesn't foul the plugs. There's a very good chance they are fouled now from multiple starting attempts. I'd take a wet test and then try to start it with fresh plugs.

If the wet test only gives slightly better readings, the valves are leaking, but if the readings are 20# or so higher it's a ring problem.

Marilyn's Cub had only slightly better readings on the first dry test (82, 96, 82, 94) but it runs like a well oiled watch. It sometimes starts on the very first 1/2 turn of the crank. It does smoke a little, so it will get new rings.

Mon Dec 29, 2003 11:23 am

My '49 was just slightly higher in readings as yours. My #2 was the worst as the cylinder wall was scored. I had a combo rings/valves problems so I went in and did my best lapping the valves and swapping out the rings. If you do decide to swap the rings, you will most likely require a ridge reamer to remove the pistons. Get a good service manual and you should have no problems. However, like the other guys have said, you should be able to run it, perhaps you have a clogged fuel line, or poor spark/timing problems. Here is a site I found handy for generic gas engine info. It is not specifically related to tractors, but the gas engine principles still apply, I found it quite informative for the regular Joe, as I am.


Mon Dec 29, 2003 3:06 pm

If your Cub ran before, why would you think it would not run now? What has changed from 6 months ago?

If you have worked with the iginition - I would double check it first - just because you can get a spark does not mean your timing is corect. I would bet you have a timing problem - when you get the "Pop", does it come out the exhaust? Normally that pop is comming from firing on the beginning of the exhaust stroke. (Very Late timing)

It's true you have low compression; but, if its all in time, it should run. I don't know how much power you'll have.

Good Luck,


Tue Dec 30, 2003 2:48 pm

I finally got it running at 1AM this morning. I did a wet test which brought the comression up 15lbs. on average. I then try starting it with starting fluid and it ran for a few seconds. I then took the oil fouled plugs out burnt the oil off with a torch and cleaned then with a spark plug cleaner. Put back together an it started right up. I let it run for 15min. to burn any oil off and now it starts fine. Go figure. It dosen't give off any smoke when running that would be from oil. At least it running.
:D :D Thank you all for the help. :D :D