Early 70s merc 650 3-cyl, 2 carb.. 1 bad cylinder. WORTH REBUILD??

[mnRIVERrat]

I have a previously strong running 73 or 74 650 3cyl with the 2 carbs and after a stall on the lake this year determined I have a slow leak down on top cylinder. All the wiring, hoses and many parts were replaced on this motor shortly before I got it, and I am wondering, assuming just pistons, rings, and crank and rod bearing will be needed along with a hone or bore job. My question for those of you with more experience, is this motor worth putting the time and money into? I am very mechanical and will do all labor myself unless bore cylinder bore is needed. Or is this a more problem prone motor that is not worth the effort. Wanting to start rebuild soon so any info or experience on these motors would be greatly appreciated. Thanks.

 

[jerryjerry05]

Re: Early 70s merc 650 3-cyl, 2 carb.. 1 bad cylinder. WORTH REBUILD??

Most cases the only thing needed is a new piston/rings and possibly a rebore on the affected cylinder.
Pull the head and check for damage.
It could be just a bad head gasket.
Take it apart,post some pics.
Profile/location?

 

[Faztbullet]

Re: Early 70s merc 650 3-cyl, 2 carb.. 1 bad cylinder. WORTH REBUILD??

Pull the head and check for damage. It could be just a bad head gasket.

This engine has no head or gasket to remove, you have to split the crankcase to inspect or a intake/exhaust cover. What is compression of engine as you said it has a slow leak down??

 

[mnRIVERrat]

Re: Early 70s merc 650 3-cyl, 2 carb.. 1 bad cylinder. WORTH REBUILD??

I didnt have the equip to do a leak down test so my mechanic did it. Said after an hour #1 cylinder lost 80% and #2 and
#3 stayed over 98%. I just put the motor in my basement to start tearing down and inspecting. Will post pics as I go but will probably start a new thread on rebuild/repair so other guys will have it as reference. I didn't find much for my particular motor when I searched.

 

[jbjennings]

Re: Early 70s merc 650 3-cyl, 2 carb.. 1 bad cylinder. WORTH REBUILD??

I own 2 of those 3-cylinder 650's. One has a horrible crack down the side of all 3 cylinder's water jackets, which is a problem common to those motors. It has been running for 3 years now with JB weld holding the water jackets together. It does crank easy and run good. If the problem on yours required more than new rings and a hone, I'd scrap it. They aren't the best motors mercury ever made. There's lots of used ones around that I see on craiglist that you could get a used powerhead off of, but there's no telling without pulling the exhaust cover if they have a cracked water jacket or not.
That's just my opinion----I'm no outboard motor mechanic so take it for what it is.
BTW, with 2 carbs and 3 cylinders, I find it nearly impossible to get mine to idle JUST RIGHT...... They're not my favorite motors. I'd like to have a 4-cylinder merc 650, myself. I think they're far better engines.
JBJ

 

[jbcurt00]

Re: Early 70s merc 650 3-cyl, 2 carb.. 1 bad cylinder. WORTH REBUILD??

I own 2 of those 3-cylinder 650's. One has a horrible crack down the side of all 3 cylinder's water jackets, which is a problem common to those motors. It has been running for 3 years now with JB weld holding the water jackets together.

Sorry to hijack your thread RiverRat @ the 3 cyl merc 650

But JBJ is this possible:

Has anyone done or had done the lock-n-stitch repair done on a 3.0 block that had a external crack?

http://www.youtube.com/watch?v=Pq0wfU4ZaKk

Thanks Wes

Back to RiverRats stuff: I have also heard bad stuff about the 3cyl 650's, and I have a '71 Merc 650 4cyl, long leg. I've been looking for a blown head short shaft or an inexpensive one, but @ here they are few & far between, and all but 1 have been long.

Hope the 650 you've got turns out to be in good shape if parts are as hard to source as I've heard.