2002 70hp 2 stroke compression test results

[luckydutch]

Looking into buying a boat with 2002 70hp 2 stroke... compression test came back 100, 100, 105 .. good, bad, ugly?

 

[vinnie1234]

Hard to say without knowing what the engine is supposed to have, but you have only a 5% variance, which is typically an acceptable amount. I don't see any red flags here.

 

[luckydutch]

My Yamaha Master certified technician says it SHOULD have around 115-125. I like everything else about the boat but feel that this is a potential deal breaker

 

[vinnie1234]

did you perform the test with the engine warm and at w.o.t? Did you ensure that the carb is opening all the way? Did you do a wet test? If the compression is low, the question is why. It could be a simple as the carbs not opening fully. I don't know a lot about 2 strokes, but reed/rotary valve replacement or crank seals may be the other reasons for poor compression.

The other thing that popped into my mind is the adapter on your compression tester. Are you using the right one? Is it sealing properly? If you disconnect the throttle linkage and manually open the carbs all the way you may get different results.

If you are certain that the test is done right and the tester setup is proper, then I think you know the answer!

 

[99yam40]

2 strokes do not need carb opened as it will get all the air it needs from the exhaust port to compress.
4 strokes are different and need to suck air from intake side.

Warm up motor and use different compression gauges to make sure it is not just the gauge reading a little low

 

[boscoe99]

Well, the air that gets into a two strokes cylinder has to come through the carburetor, even though it goes through the block to get to the cylinder.

I vote for a redo of the compression check with a known good gauge, a hot engine and the throttles wide open.

The compression you reported does not indicate anything is wrong with the motor. Does it start, idle and otherwise run well?

 

[99yam40]

Well, the air that gets into a two strokes cylinder has to come through the carburetor, even though it goes through the block to get to the cylinder.

I also thought that was true on the 2 stroke, but a good man on this site told me to do it both ways next time to see if I see any difference.
I checked with throttle open and with it closed and ended up with the same results.

Try it next time to see for yourself.

Might be different if exhause is under water, but never tried it that way so cannot say.

 

[boscoe99]

It has never crossed my mind to do a compression test with the throttle closed. So, I can not say from first hand experience if it makes a difference. I would have thought so but if you say it does not, that is good enough for me.

Having said this, it would seem to be a good maintenance practice to do the check with the throttles wide open. Be consistent and use a procedure known to be accurate with two or four strokes. Better to do a two stroke check with the throttles open than a four stroke check with the throttles closed. IMO.

 

[99yam40]

I agree with that, I have always done it with throttles open.
just thought I would mention it as I was supprized also by what I found out.

You doing any good this time down there catching some fish?

 

[boscoe99]

Have not been fishing yet. Trying to get a trip set up for this coming Sunday. Am dealing with the locals and they are always on manana time.

 

[WIMUSKY]

Rule of thumb is to test at WOT. The main thing you want to see is relatively even numbers across the board, which you have. However, they do appear to be low. Doesn't mean you couldn't get a few years before a rebuild.

 

[racerone]

Throttle position does NOT MATTER on a 2 stroke !-------------Exhaust ports are wide open before the piston closes them off.---Only then does compression start.