merc compression specs

[tamarackstyle]

does anyone know about what the compression should be on a 1975 model 500,50 hp?

 

[12Footer]

Re: merc compression specs

Nope.. Well, they aren't in the manual. This is probably because the numbers aren't that vital.<br />What is vital is that both cyclinders are equal or very close to the same number. A Merc115 i werked on was 90-100 across the board.A 20HP was 90 on both. My baby 7.5HP merc is 90 on both.<br />They all run great.

 

[ODDD1]

Re: merc compression specs

Tamarack, you should see 120 psi on a healthy 50 hp 4 cyl....under 100 and she is hurt....

 

[tamarackstyle]

Re: merc compression specs

o.k here it is. I replaced the valve in my compression tester and it must have been the wrong one because when I tested the motor it ran 30 across the board. I went out and tested the truck which I checked and noted before I took it on a 4000 mile trip. they read at 150 before the trip and with my wacky tester read now at 40. this means either my mercury is reading on all four at 140 or I lost some in the truck from the trip. Is 140 too high for the mercury?

 

[Kuni]

Re: merc compression specs

compression is a guide only it may range from 110 to 150psi on different engines the key is make sure its even within 10-15% of each other and if your concerned about a cylinder take the spark plug out and use a light to see inside with the piston on bdc you should be able to see any harsh marking or forgein object damage, and you can compare what you see with all the other cylinders to give you a good idea of your engines health.<br />(when you look in look at the clinder walls)<br />an old instrument light from an old speedo or tach taped to a piece of welding wire makes a good light that won't obstruct your vision

 

[tamarackstyle]

Re: merc compression specs

they were even all right. the difference between the highest reading and the lowest was only 4 psi. Which would make it about 1.5% difference. looks like I got a good one!!<br /> Thanks everyone for all your help!!!!!