Compression question

[thatone123]

I have did a lot of compression tests and usually do 3 or 4 pulls. Does it matter how many pulls. Have a 40 and it takes more pulls to get up to 90-100 psi.

 

[jimmbo]

What engine? Some old 40 hp OMC engines had a compression relief system that lowered compression when hand starting. Eventually the compression would rise.
Scored cylinders/stuck rings can also result in slow build up too, but they usually reveal themselves with just constant low readings.

Try a different gauge, that's one more variable

 

[thatone123]

Actually it is another brand, I have just found that this forum has more knowledgeable experts than the other forums although some of the OMC experts can be found time to time on the other forums. It's a general question and I think the comoressiion readings are not dependent on how many spins of the flywheel happen but they are cumulative to get the right, real or true compression reading. This is what I am asking, but maybe it is self evident.

 

[thatone123]

BTW, I have always wondered why more outboards do not have the relief system and I did not know that OMC 40's had that. None that I have owned that is for sure as they are one of the hardest to pull mid range outboards ever as far as I know. I have heard of a commercial fisherman who tore his bicep muscles when one of the old commercial 40's backfired or something to that effect.

 

[Scott Danforth]

Actually it is another brand, I have just found that this forum has more knowledgeable experts than the other forums although some of the OMC experts can be found time to time on the other forums. It's a general question and I think the comoressiion readings are not dependent on how many spins of the flywheel happen but they are cumulative to get the right, real or true compression reading. This is what I am asking, but maybe it is self evident.

not all brands have same compression. ask your question in the correct forum to get the correct answer

 

[F_R]

Pull it till the gauge doesn't go any higher. The PSI in the cylinder is virtually the same with each pull, however the pressure in the gauge innards has to build up till it matches what is in the cylinder. Compare to inflating a flat tire. When you start, the tire pressure is zero PSI, but the air compressor tank might be 100 PSI. Does the tire instantly go to 100 PSI? Of course not. It takes some time for air to enter the tire to the point that it equals the tank pressure (or hopefully you remove the hose)

 

[thatone123]

FR Thanks for a simple and complete answer! Just what I was looking for!