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compression 90 psi 1967 65 hp 650

R

rwirtzfeld

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I purchased an old 1967 Mercury 650, compression test came out at 90psi on all cylinders. Is that enough for it to run well, thing seems to smoke alot, just wanted to know if I should proceed with a tune-up? Dont want to waste money.
 
Re: compression 90 psi

pull the plugs and shoot some mystery oil into the cylinder, rotate by hand, let sit 10 min, rotate by hand again, and let sit again, and check your compression once more. Has the engine been sitting a while? Once running again, hit the carbs with seafoam or powertune, that might help your smoking issue.
 
Re: compression 90 psi

The engine had been sitting for several years. I wanted to check the compression to see if it was worth sticking money into. I did try squirting some regular motor oil in one cylinder and then checking compression, didn't improve compression at all, so I take that as a sign its not the rings/cylinder walls causing the loss of compression?

I will try the mystery oil trick, does that free up the rings a little?
 
mystery oil does many mysterious things....HAHA, no really, my understanding is it helps lubricate the rings and seals, especially if they have dried a little. It needs time to penetrate, thats why i you wait, then turn the pistons, wait, and do it again, give a chance to get in all the nooks and grannies so to speak.
 
Compression is a funny thing.The important thing is to be even or all cylinders close to the same.
You guage might read different than mine.If the readings are 90 on all then I'd go for it.
Get a different guage.You pull all the plugs when you did it the first time?
If it ain't ran in a while the smoke will go away.J
 
I did pull all the plugs when I did the compressions checks. I will replace all my fuel hoses, they are in need. I took the carbs apart and they actually looked pretty good, only thing I notice is the float needles show some wear, not sure if they were holding tight. So I will rebuild them, fuel pumps and all new hose.

I got to ask a stupid question, why do the carbs. not have any filter on the air intake?

thanks for the reply
 
Last edited:
Hallo Guys,

Ok ,maybe a stupid question,but i,m from another country,so,not so clued up on what products is available and under what name.I take it,if you guys talk about mystery oil,is that like in penetrating oil,like Q20,or something else.I always thought,that by putting something like that,on pistons,to say,loosen the rings,it can actually damage also the sleevs,as it can loosed carbon between the rings,that can scrape the sleeves.
 
Merc sells a product called Power Tune that frees up the rings. You add it to the gas and go boating. Seems to work.

Jeff
 
Mystery oil is more of a lubricating oil than penetrating oil, but has cleaning propeties. Not sure what q20.
 
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