Worrying compression results HELP PLEASE

[MystyBlue2]

Hi folks i have a mariner 25hp 2 stroke 1980,

After a few symptoms i decided to buy a compression tester and do some digging.

I did the test with engine up to temp with pull cord out both plugs removed and throttle wide open.

RESULTS

Did 3 tests on both cylinders and got 85PSI on both on all 3 attempts. There is no differance between cylinders which is a good tbing but is 85psi too low??

Help would be much appreciated folks

 

[Bilgediver]

Sounds a bit low. Try putting a small amount of oil down the plug holes and see what the difference is. I would expect 100 plus psi

 

[vas]

have you got an engine in good nick to test the gauge against it? 80 is low, if a good engine does also 90-100 expect the gauge to be the culprit (or not using it properly, although seems that you've covered this as well, plugs out, crank WOT)

 

[[178529]]

Good news that the compression on both is the same. 85 is lower than I've had on 2 stroke outboards. I've normally seen 100-110. It still should run at that compression but might be hard to start. Piston rings, read vlavles and crankcase seals could all cause probs on an older engine

 

[vas]

my yamaha 2cyl 2stroke 700cc engine on my jetrib had 150-155psi (20yo, no idea on hours run) OK, starts on first press of the starter if there's fuel in the carbs

 

[kashurst]

85 sounds low to me too. However as both cylinders are the same it could be something easy like stuck/broken piston rings. I think you will have to take it to bits and have a proper look.

 

[gordmac]

I would check the guage or try a different one before pulling the engine apart. Strange coincidence that both would be faulty and reading the same. Not familiar with two strokes, would a fault with the crankcase seals say cause the same low reading on both?

 

[vas]

I would check the guage or try a different one before pulling the engine apart. Strange coincidence that both would be faulty and reading the same. Not familiar with two strokes, would a fault with the crankcase seals say cause the same low reading on both?

indeed they could (apparently), crankshaft seal as well.

 

[MystyBlue2]

Thanks so much for all your help and advice guys,

Just ran another test. This time engine was cold and throttle open but i sprayed some 2 stroke oil in both cylinders and the readings i got was 95PSI on both cylinders. So compression is slightly rising when oil is in. Can anyone tell me is this a good or bad thing and would my readings be sort of normal for a 40year old engine?

 

[[178529]]

Thanks so much for all your help and advice guys,

Just ran another test. This time engine was cold and throttle open but i sprayed some 2 stroke oil in both cylinders and the readings i got was 95PSI on both cylinders. So compression is slightly rising when oil is in. Can anyone tell me is this a good or bad thing and would my readings be sort of normal for a 40year old engine?

It is likely showing some bore wear. Normally compression is better when the engine is warmed up as the piston and rings have expanded to correct dimensions. With a 2 stroke, I would think new reed valves, new piston rings and change the crankcase seal would be cheap and make a big difference.

 

[MystyBlue2]

It is likely showing some bore wear. Normally compression is better when the engine is warmed up as the piston and rings have expanded to correct dimensions. With a 2 stroke, I would think new reed valves, new piston rings and change the crankcase seal would be cheap and make a big difference.

Im very basic when it comes to mechanical hands on stuff so that would be a job for the pros, how much would you estimate these jobs would cost in total or how many hours roughly for a pro to do these jobs as the local boat mechanic charges £60 per hour and if its going to work out pricey i might have to look into a new motor if its goong to work out to a similar cost

 

[MystyBlue2]

Does anybody know how many "crank seals" there are and if more than 1, which one would probably be the culprit?

 

[MystyBlue2]

I know im sounding like a complete noob here and probably annoying but im a little confused...

The hot test gave readings of 85psi in both cylinders and the cold but "wet" test gave 95psi in both cylinders.

My question is if it was blown crank seals would the "wet" test still give me a higher reading or would it just stay at 85

In my head if seals are blown there should be no differance in compression wet or dry as no matter what the compressed air/fuel is leaking passed seals and going to give a low reading regardless, so am i right in thinking this could be a stuck ring issue as figures slightly rised when i add oil into cylinders and oil sat around the piston increasing the results???

Thanks for all your help guys very much appreciated

 

[jim.howes]

Hi a crank shaft has 2 ends so reckon on 2 seals. Why would 1 be more knackered than the other? What ever gave you 85psi to both cylinders is common to both cylinders.

 

[MystyBlue2]

Hi a crank shaft has 2 ends so reckon on 2 seals. Why would 1 be more knackered than the other? What ever gave you 85psi to both cylinders is common to both cylinders.

Thanks for clearing that up

 

[MystyBlue2]

Ok well last night i gave a healthy mix of 2 stroke oil and deisel into cylinders to soak piston and rings (see if a ring is seized into piston) and to my delight this morning i checked and all of my mixture is all over the floor and none at all left in the cylinders.
I squirted mixture in, left plugs out and tilted engine up on a chock of wood to allow it to cover pistons. Is this normal or is this a total giveaway of knackered crank seals?

Cheers folks

 

[BruceK]

Not sure how knackered crank seals would effect compression values albeit from your test above something is obviously leaking but it could be coming out the exhaust port. The fact that it is leaking if both pistons were in a transition above the valves / ports would indicate some ring clearance issues.
I'd be looking at the head gasket (yes most watercooled two strokes will have a waterjacket with head) but also take it off and inspect bores for state of cross hatching or mild seizes If the cross hatchings have worn off or the bores scored it's probably not worth the engineering costs to rectify. If they are in good order I'd check the ring tolerances. As they age the gap widens and loses pressure. It could just be age related wear on the rings and a cheap fix. PSI values for your engine I'd have pegged at 110 -130 all being good. 85 is very low and I'd expect some idling issues.

 

[gordmac]

If you are filling the chamber with oil the piston needs to be above the exhaust port or, as you have discovered, it just leaks out.

 

[MystyBlue2]

Not sure how knackered crank seals would effect compression values albeit from your test above something is obviously leaking but it could be coming out the exhaust port. The fact that it is leaking if both pistons were in a transition above the valves / ports would indicate some ring clearance issues.
I'd be looking at the head gasket (yes most watercooled two strokes will have a waterjacket with head) but also take it off and inspect bores for state of cross hatching or mild seizes If the cross hatchings have worn off or the bores scored it's probably not worth the engineering costs to rectify. If they are in good order I'd check the ring tolerances. As they age the gap widens and loses pressure. It could just be age related wear on the rings and a cheap fix. PSI values for your engine I'd have pegged at 110 -130 all being good. 85 is very low and I'd expect some idling issues.

Yes this sounds right and it starts and runs 1st time every time but extremely low on power and rough bumbly/stumbly idle, thanks

 

[MystyBlue2]

If you are filling the chamber with oil the piston needs to be above the exhaust port or, as you have discovered, it just leaks out.

Ok so would you recomend having both pistons half way up bore then repeat test?