1999 Merc 25 2 stroke compression?

[rg2]

Can anyone give me the compression specs for a 1999 Mercury 25hp 2 stroke motor?

 

[will941s]

Re: 1999 Merc 25 2 stroke compression?

As long as both cylinders are within 15% of each other your ok. I have one that has 110 pounds even and one that has 120 pounds even. Both run great and you cannot tell any difference in the performance between the two. I think the average I've seen are around 120-130psi. Pull the plugs, open the throttle and crank that thing til you have the highest reading...with the engine at operating temp.

 

[will941s]

Re: 1999 Merc 25 2 stroke compression?

BTW...post your readings when you do it....it's always interesting to see what other people have and people on here can get some good info out of it.

 

[rg2]

Re: 1999 Merc 25 2 stroke compression?

Just so I know I'm testing it right. Once the engine has been warmed up I remove both spark plugs wires and remove one actual plug - I then thread the compression guage in to that cyclinder - turn over the engine by depressing the electric start or pull starting. (Does it mattter which method I use?) and read the guage. Is there a certain time I should allow it to crank or number of times I should try to pull start it?

I tried this method earlier when the engine wasn't warm and I got 90 and 100. I'll try it again with engine warm.

 

[rg2]

Re: 1999 Merc 25 2 stroke compression?

Well I retested the compression once the engine was warm and I got 75psi on one cylinder and 90 on the other. I then let the motor cool back down and again read 90 and 100. When the engine was running it was running smooth - it just did not seem to have a lot or as much power as it had last year. What do you guys think.
Should I try a "wet" test or is that not necessary with a 2 stroke since oil is already present in the gas that is entering the cylinders during the test. I just bought this motor last summer and I always add stabil (marine type) and sea foam to the gas. Any thoughts on what I should do next?

 

[will941s]

Re: 1999 Merc 25 2 stroke compression?

Those readings seem a little low, but every gauge is different. The only way to see where you really stand it to test another 25 merc with it and that will get you in the ball park of where the gauge is. You can get some Seafoam deep creep, tilt the motor up, take the plugs out, and pour some on the cyliders. Rotate the motor a few time and pour a little more in there. Replace the plugs and let it sit for a few days. Start the motor to burn it off then re-do the compression test. That might free up the rings and get you some compression.

Option 2: Get a 1 gallon jug, mix a gallon of gas/oil to ratio in the jug. Pour a enitre can of Seafoam in there. Drop your gas line in there and let her rip. Run it for 15 minutes, shut it off for 15 minutes. Keep doing this until it runs out of gas. Re-do the compression check. This strong version of decarbing may free up everything stuck in the motor and WILL clean out all the carbon, and it's going to smoke like crazy.

If this don't change anything, well....you may have a internal engine problem. Let us know what happens.

 

[rg2]

Re: 1999 Merc 25 2 stroke compression?

Will, thanks for the suggestion - I'll try it out.