Mercruiser 350 compression numbers

skytow · Nov 30, 2008

All,

I'm looking at a boat that has full inboard Mercruiser 350 / 260. (not I/O) I had the compression checked and all cylinders were between 145 and 150. I know the consistency is good just don't know how "good" the numbers are. What does a marine 350 start our life with?

The motor is a 1987 with about 890 hours on the clock. Oil pressure and temperature looked fine.

I plan on a rebuild when the budget allows but I'd like to get a season or two out of this, say 50 to 100 hours.

What do you think??

Thanks,

JD

Don S · Nov 30, 2008

Re: Mercruiser 350 compression numbers

Those numbers are fine, and you don't need a rebuild.

captmello · Nov 30, 2008

Re: Mercruiser 350 compression numbers

That sounds about right for those hours. My 350's between 150 and 160 with fresh heads. You could run with that motor for years!!!

Welcome to Iboats!

skytow · Nov 30, 2008

Re: Mercruiser 350 compression numbers

Thanks!

You guys are quick! I may not need rebuilds today but.....just planning for the future.

The motor is freshwater cooled but the risers are original from 1987. The manifolds are also freshwater cooled according to the mechanic I had look at it. I think, having been in salt water, I should definitely have the risers replaced as insurance. Thoughts?

Thanks again!

JD

Don S · Nov 30, 2008

Re: Mercruiser 350 compression numbers

You guys are quick! I may not need rebuilds today but.....just planning for the future.

Have it propped right, don't run it at wide open throttle all the time, keep it tuned up, keep up with the preventive maintenance, winterize properly and that future will be 10 or 20 years away.

skytow · Nov 30, 2008

Re: Mercruiser 350 compression numbers

Don,

Thanks again. I certainly don't NEED to spend anymore money!!

JD

chiefalen · Nov 30, 2008

Re: Mercruiser 350 compression numbers

Good numbers.

If it ever starts to run at near 180-190 degrees than you may be looking at new risers.

Fresh water makes all the difference in the world.

Take ya money that you saved and buy x-mas gifts.

Welcome aboard !

captmello · Nov 30, 2008

Re: Mercruiser 350 compression numbers

The motor is freshwater cooled but the risers are original from 1987. The manifolds are also freshwater cooled according to the mechanic I had look at it. I think, having been in salt water, I should definitely have the risers replaced as insurance. Thoughts?

You lost me on this one. I'm an I/O guy but I'm pretty sure your manifold and risers are cooled with the same water. If the boats been run in salt in saltwater on a regular basis, the manifolds would have been shot long ago. Maybe I misunderstood. wouldn't be the first time.

If the boat has been in fresh water only, they may all be good. Don't start replacing expensive parts "for insurance". Watch the temp close and get a feel for the way the boat runs.

chiefalen · Nov 30, 2008

Re: Mercruiser 350 compression numbers

Woa been in salt water?

Watch temp.

Same risers hard to believe. Had my salt water riser off after just 4 years, know i got another 2 years maybe on it.

This one i have is a osco.

Been keeping a eye on e-bay for another one.

The manifold looks good to me and thats even more surprising.

Thats original from 86'.

mthieme · Nov 30, 2008

Re: Mercruiser 350 compression numbers

captmello said:
You lost me on this one. I'm an I/O guy but I'm pretty sure your manifold and risers are cooled with the same water. If the boats been run in salt in saltwater on a regular basis, the manifolds would have been shot long ago. Maybe I misunderstood. wouldn't be the first time.

If the boat has been in fresh water only, they may all be good. Don't start replacing expensive parts "for insurance". Watch the temp close and get a feel for the way the boat runs.

By fresh water cooled he means it's a closed cooling system with antifreeze.
No seawater touches the important pieces.

captmello · Nov 30, 2008

Re: Mercruiser 350 compression numbers

By fresh water cooled he means it's a closed cooling system with antifreeze.
No seawater touches the important pieces.

You're probably right, closed cooling. However, not the mani's and risers? Someone straighten me out.

skytow · Nov 30, 2008

Re: Mercruiser 350 compression numbers

Guys, you got it right..

It uses a closed water system with antifreeze and a heat exchanger for cooling. My understanding is that some closed systems, such as this one, also used the closed side of the system to keep the manifolds cool but raw water enters the heat exchanger and exits through the risers and out the exhaust system.

My concern is that after 21 years the risers are pretty far gone and could allow raw water to enter the engine if they corrode through. As far as I can tell, or the records show, they are original. Perhaps they were replaced at some time but I can't be sure. Risers alone aren't all that expensive. Not sure how difficult job it would be to change them??

Thanks,

JD

captmello · Nov 30, 2008

Re: Mercruiser 350 compression numbers

thanks for the explanation, I've learned something new!!

I'm tempted to speak more about the risers but I'll let the Pro's take over.

Go Luck with the Boat!!

Don S · Nov 30, 2008

Re: Mercruiser 350 compression numbers

You started out wanting to overhaul/rebuild the engine. Now down to maybe a set of risers.
Best thing to do is pull them and check them oui. Look at the gasket surfaces and check the passages. Here is a picture I did for checking the manifolds, but the same thing goes for the risers.

After you pull the risers and inspect, PLEASE use the proper OEM gaskets to put it back together.

skytow · Nov 30, 2008

Re: Mercruiser 350 compression numbers

Don,

Thanks a million for the advice!

Believe me I don't want to spend any more than I have to, however, I need to (want to ) be prepared financially in the event I have to do major repairs. I've been planning the budget trying to cover the contingencies and plan to do what it takes (and what makes sense) to get as much life as I can from the motor.

Thanks again,

JD

Mercruiser 350 compression numbers

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