Low Compression in both cylinders but identical psi

[jsweber82]

Hey Guys, I recently purchased a Jon Boat with a 1972 Merc 9.8. I took, it out Saturday and it does run, though it took a couple minutes to crank. Once It was warmed up it seemed to run ok. I just bought a compression tester from Harbor Freight with the quick connect hose coupler. I tested both cylinders twice at about 5 pull starts each. Both cylinders topped at 45 psi exactly. What would both cylinders at this compression indicate? Is there anything I can do short of a rebuild? I have seen suggestions to run seafoam through a tank of gas to try and clean out carbon deposists and free stuck valves etc. Does this actually help though? Any suggestions welcome!

Thanks,

Jason

 

[JB]

Re: Low Compression in both cylinders but identical psi

Different gauges measure at different psi, particularly rope start outboards. 45psi is at the low end of what you should expect. I would get another gauge and try it again. The cylinders measuring the same is a good sign as long as there is no head gasket.

 

[Silvertip]

Re: Low Compression in both cylinders but identical psi

45 PSI is pretty low and even though both cylinders are the same, it indicates an engine that is simply tired. SeaFoam can be used to free up stuck rings IF that is the issue. But there is a better way to do it rather than just adding a small amount to the fuel. It is called a "De-Carb" (de-carbonization) and is well documented in this forum. You make a very strong mix of SeaFoam and one gallon of gasoline preferably in a separate tank.. There are many mixtures recommended but you cannot use too much SeaFoam so don't worry about it. Be aware that the engine will smoke like crazy while it is running. You run this mixture to get the engine warmed up and then just let it sit for an hour or so and repeat the process several times. Then head to the lake with a load of fresh fuel and run the be-jeepers out of it. Then check compression. If it is still the same you have a tired engine that requires a re-ring as a minimum to restore compression. Another method is to mix SeaFoam and fuel in a spray bottle. Remove the airbox. Then start the engine and run it at a fast idle (2000 rpm or so). Then spray the mixture into the carb. Spray until the engine begins to bog. Let the engine recover and hit it again. Repeat this process until you can no longer see your neighbors house (or until they call the police/fire dept). Then give it one more good dose to kill the engine. Let it sit overnight. Next day head to the lake and again, run the be-jeepers out of it.

 

[Texasmark]

Re: Low Compression in both cylinders but identical psi

My Merc manual says anything below 120 one could expect problems. Different folks use different numbers but with the cylinders being within 5-10 % of each other says things are ok.

On an engine of that size, how do you wear out the rings on a 2 stroke to that degree? If you ran it without oil it would merely seize up and wouldn't turn over, much less run. In my opinion if your compression was really that low, on a 10 hp engine, the rest of the engine would be a visual piece of crap.

On using a pull rope, you need to spin the crank as fast as you can. Removing both sparkplugs help you to do that. Having a fresh (Charles Atlas) friend do the pulling while you do the monitoring helps too. Ensuring that you have the rubber pushed into the hole with "vigor" helps 3.

Try again. If low again, squirt some 30w oil in the cylinders ensuring that you get an even coverage all around the piston and try again. If you don't get a "marked" increase in compression, check your methodology. I find gauges pretty accurate and have been using HF tools for 25 years with stellar results as long as you use them within the realm for which they were designed.......non commercial production/repair.

Mark

 

[Faztbullet]

Re: Low Compression in both cylinders but identical psi

Its a combination of the gauge and pull starting, it would be a b@@@@ to start and be way low on power if compression was 45psi.

 

[jsweber82]

Re: Low Compression in both cylinders but identical psi

Thanks guys. I think to rule out a faulty guage I will rent one from autozone tomorrow and try again, and also try the 30w oil trick. I just read some reviews on this guage from HF and there were quite a few bad ones that said the results were very low, and using another guage showed dramatic differences. Fingers crossed it is just the guage. I think I will run a can of seafoam through it regardless since I have heard a lot of success stories. Thanks for your help!

 

[eggs712]

Re: Low Compression in both cylinders but identical psi

I just read on the HF website, and the reviews for their compression gauges were bad. Apparently they have a check valve when they should instead have a Schrader valve, which causes low compression readings. One guy got 90 psi with the HF gauge but 145 psi with a good Sears gauge. HF is hit and miss sometimes.

 

[Way Far North]

Re: Low Compression in both cylinders but identical psi

If you truly had 45 psi compression the motor wouldn't run at all. Compression check should be carried out with the engine warm, both plugs out and WOT. With any 2 cycle anything below 100 psi it will not preform very well. Just my $0.02 though.

 

[Scott Danforth]

Re: Low Compression in both cylinders but identical psi

i would borrow a gauge from Autozone or advanced auto

 

[Texasmark]

Re: Low Compression in both cylinders but identical psi

If you truly had 45 psi compression the motor wouldn't run at all.

I agree. It may idle, or run in gear on muffs, but without compression to develop torque, as soon as you need some it won't be there. If your engine pulls a good load, it can't be very low on compression.

Mark