2004 25 HP 2 stroke compression should be...

Sailor John

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Hello all,
I have a new-to-me 2004 25 hp 2 stroke Mercury engine that seems to start and run well. During a cold engine compression test, the cylinders both hit exactly the same at 100lbs. My question is: What should the compression number be? This engine is very clean with very minimal use.
My older 1982 18 hp hits 150 lbs in both, but it is a real pig to manually start.

Any thoughts?
Thank you,
Sailor John
 

Texasmark

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The manual on my 2002 90 says "For compression readings below 120, problems could be experienced". Spec on that engine is 150 also as I recal; don't have my manual any more.
 

hemi rt

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Compression tests are done with the engine at operating temps, all plugs out and the carb/s wide open. A cold engine will have lower compression readings.
 
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flyingscott

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You want to be at 120 for that motor they have compression release holes in the cylinders. Seeing 150 will be hard, I plugged the holes on a motor I had I could barely pull it over. It was fast for about 15 minutes.
 

Texasmark

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You want to be at 120 for that motor they have compression release holes in the cylinders. Seeing 150 will be hard, I plugged the holes on a motor I had I could barely pull it over. It was fast for about 15 minutes.

"It was fast for about 15 minutes."..........and then????????? Naw I don't wanna know. LMAO, sorry couldn't help it.
 

flyingscott

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"It was fast for about 15 minutes."..........and then????????? Naw I don't wanna know. LMAO, sorry couldn't help it.
It was a motor that I had it was getting tired and not worth rebuilding so i plugged the holes. Fast for 15 minutes then melted the pistons to the block was not pretty..
 

Texasmark

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Sorta what I figured you'd say. I had a 1975 70 hp rude that I bought well used having been passed around numerous POs. I was young and dumb. Before OT alarms. T stat froze shut at WOT. The LU on the engine must not have had an overrun clutch like Mercs because the minute it locked the prop slammed on the brakes and almost threw me out over the bow. .030 and $1k for the machine work and parts, I did the labor, oh and a NEW Tstat. Grin
 

Sailor John

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Thanks for the tips hemi rt,
I did a warm compression test and it resulted in little compression difference. Maybe 105 lbs now. I also noted that when I gave it some throttle, the engine died. After running for about 20 minutes at idle, it seemed to throttle up better. I changed the plugs and put in fresh ethanol free fuel. I will try some Seafoam in the carb too, to see if that makes a difference in the throttle up.
When the engine warmed up, it seemed to run quite well and pushed the dinghy along beautifully. Quick too. Although it did have some lag as it throttled up.
Any other tips are appreciated.
Sailor John
 

Texasmark

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Open the low speed jet a little at a time....like 1/16 turn at least till you are open half a turn more than you are now till it recovers to suit you. If it doesn't you have something else restricting your fuel because it takes fuel to produce power.
 

flyingscott

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Thanks for the tips hemi rt,
I did a warm compression test and it resulted in little compression difference. Maybe 105 lbs now. I also noted that when I gave it some throttle, the engine died. After running for about 20 minutes at idle, it seemed to throttle up better. I changed the plugs and put in fresh ethanol free fuel. I will try some Seafoam in the carb too, to see if that makes a difference in the throttle up.
When the engine warmed up, it seemed to run quite well and pushed the dinghy along beautifully. Quick too. Although it did have some lag as it throttled up.
Any other tips are appreciated.
Sailor John


Don't worry about warm compression compression always goes up when it's warm. Cold compression is the most important one,that is what gets you running and keeps you running until it warms up,. I would try a different gauge
 

hemi rt

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Don't worry about warm compression compression always goes up when it's warm. Cold compression is the most important one,that is what gets you running and keeps you running until it warms up,. I would try a different gauge

And where did you learn this from - coming from a licensed mechanic with over 45 years in the trade
 

hemi rt

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Thanks for the tips hemi rt,
I did a warm compression test and it resulted in little compression difference. Maybe 105 lbs now. I also noted that when I gave it some throttle, the engine died. After running for about 20 minutes at idle, it seemed to throttle up better. I changed the plugs and put in fresh ethanol free fuel. I will try some Seafoam in the carb too, to see if that makes a difference in the throttle up.
When the engine warmed up, it seemed to run quite well and pushed the dinghy along beautifully. Quick too. Although it did have some lag as it throttled up.
Any other tips are appreciated.
Sailor John


If you are going to put seafoam through it do it with the seafoam in the gas, spraying it through the cab will clean the throat and the combustion chamber but will not help clean the internal parts of the carbs and this is what you want.
 

flyingscott

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And where did you learn this from - coming from a licensed mechanic with over 45 years in the trade
Sorry you are offended by what I know. I always take cold compression never warm things expand get tighter. Ever try to get a cold motor to idle with bad compression or start. If i want to know compression loss I can just throw some oil in the cylinder that will tell me the difference. That motor has low compression I would have thought you would tell him to get a different gauge to double check. I don't have 45 years of experience but I know that
 
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hemi rt

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Sorry you are offended by what I know. I always take cold compression never warm things expand get tighter. Ever try to get a cold motor to idle with bad compression or start. If i want to know compression loss I can just throw some oil in the cylinder that will tell me the difference. That motor has low compression I would have thought you would tell him to get a different gauge to double check. I don't have 45 years of experience but I know that


Not offended but please give the proper information out.

Now you keep doing it your way and I'll keep doing it the correct way; remember that all compressions given for any engine are the operating temperature compressions.

Just for the heck of it here's a question, when does the combustion chamber reach its' operating temperature considering that everything is working properly?
 

Sailor John

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Good morning all!
Thanks hemy rt.
Just an update....I did as you suggested and put sea foam in the fuel, AND I sprayed some into the carb. When looking into the carb, I could see that it was yellow, like nicotine. Lots of stuff to clean out. I will borrow another compression tester to see if it gives a different reading. The engine starts with a half pull, and it seems to be throttling up much better than before. I have not worked on the low speed jet yet.
And I will bite...When does the combustion chamber reach it's full operating temperature?

Sailor John
 

flyingscott

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Not offended but please give the proper information out.

Now you keep doing it your way and I'll keep doing it the correct way; remember that all compressions given for any engine are the operating temperature compressions.

Just for the heck of it here's a question, when does the combustion chamber reach its' operating temperature considering that everything is working properly?

So how do you do a compression test on a motor that doesn't run. Do you stick it in the oven to warm it up I know I don't have 45 yrs in the trade. I know that I can go do a compression test on my 25 hp mercury hot or cold and it will be 120+ each time. Good for you if you can charge for 2 tests. One question How can the rings get tighter as they get warmer if they did that wouldn't they score the wall. That is usually what happens when it overheats. When did boat mechanic become a trade, This question is being asked by my electrician friend.
 
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hemi rt

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So how do you do a compression test on a motor that doesn't run. Do you stick it in the oven to warm it up I know I don't have 45 yrs in the trade. I know that I can go do a compression test on my 25 hp mercury hot or cold and it will be 120+ each time. Good for you if you can charge for 2 tests. One question How can the rings get tighter as they get warmer if they did that wouldn't they score the wall. That is usually what happens when it overheats. When did boat mechanic become a trade, This question is being asked by my electrician friend.


What do you think ring gap is for? I realize you can't do a proper compression test on an engine that doesn't run and your "stick it in the oven" is just plain stupid. A licensed Marine Tech has been around here for years (Canada). Can't answer my question about combustion temps?
 

flyingscott

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Ring gap is for expansion somewhat of an effect on compression. If the gap wasn't there when it heated up, well hopefully you get the idea. Do you honestly think a 20-30 year old motor with good compression still has a factory ring gap. Combustion temps reach their maximum on a wide open run that's when your temps are the highest. I am not going to teach you anymore as you already know everything. He had a gauge that read bad compression on a good running motor all I suggested was a different gauge before anything.You are just mad because I disagreed with you about warm compression get over it.
 
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hemi rt

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Ring gap is for expansion somewhat of an effect on compression. If the gap wasn't there when it heated up, well hopefully you get the idea. Do you honestly think a 20-30 year old motor with good compression still has a factory ring gap. Combustion temps reach their maximum on a wide open run that's when your temps are the highest. I am not going to teach you anymore as you already know everything. He had a gauge that read bad compression on a good running motor all I suggested was a different gauge before anything.You are just mad because I disagreed with you about warm compression get over it.


And what is there in an engine to wear the end gap? nothing. The combustion chamber doesn't change temps from the first time an engine is started until it is shut off. The piston and cylinder walls(block) do increase in temp but NOT the combustion chamber. But seeing as you know everything this conversation is going no where. Have a good day Mr. Know-it-all
 

flyingscott

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And what is there in an engine to wear the end gap? nothing. The combustion chamber doesn't change temps from the first time an engine is started until it is shut off. The piston and cylinder walls(block) do increase in temp but NOT the combustion chamber. But seeing as you know everything this conversation is going no where. Have a good day Mr. Know-it-all

Now I know I am right because of the name calling, that's all you got. Rings get thinner and the block wear over time ring gaps change. Do you really think a stock 40 yr old motor will have a factory ring gap. By the way at wide open the motor is firing more and more friction from more RPMs that's all affects the combustion chamber. You are also adding more fuel and more air along with advanced timing it's firing under more pressure.. But hey don't let the facts get in your way. The name calling nice touch. I have taught you enough I am done.
 
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