COMPRESSION

[Opsguy1979]

Quick question, how do you go about checking the compression of a engine? Whats a good level/bad level and what tools do you need??

Cheers

Ops /forums/images/graemlins/cool.gifguy

 

[Rick]

More info required - what type of engine for starters?

 

[Opsguy1979]

Hi Rick

Its a Mercury 3.3 unfortunatly age unknown

 

[Rick]

To accurately check, you need a compression guage, and a spark plug spanner, however with such a small engine, it's probably not that important, other than it has enough compression. Why are you testing it, or what makes you think it's low?

 

[Opsguy1979]

Cheers

The only reason was I just brought a second hand one and was just curious. But ill take your advice and leave alone I think.

 

[charles_reed]

Mercury 3.3

is actually a re-badged Tohatsu 3.5 as are most of the small 2-stroke OBs.

The 1 weak point in those small Tohatsu was the alloy cooling pump - in the 3.5 that's in stainless.

Leave well alone, just drain and flush the tank, change the spark-plug, check the HT lead, the dog-clutch actuator (the nylon top handle sometimes siezes) and you've got an engine which will last about 15 years.

Oh!, protect the cowl from UV, not so bad in UK but that's the part that deteriorates fastest

 

[oldharry]

[ QUOTE ]
Cheers

The only reason was I just brought a second hand one and was just curious.

[/ QUOTE ] You wont get a realistic reading off a small hand start 2 stroke like that. In any case the top end compression on a 2t is not as important as the crank case pressure, which is virtually impossible t measure anyway. If it starts easily the compressions are OK. If it does not then the crankcase seals are the most likely culprit, not the piston rings, unless they are broken or sticking. As the piston has to come out if you are changing seals anyway, that is the only time its worth checking them.

 

[Lakesailor]

Concur with that. 2 strokes only achieve their proper compression under running conditions as the speed of the piston travelling up and down creates the airflow that's needed for the valve timing, which is a product of the piston travel anyway.
It's like extreme camshafts on race engines only achieve the proper airflow at higher revs. You're just pumping volume out of the exhaust, which means lower compression readings.