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1972 Mercury 650 Rebuild Questions

johnm

New member
"I have a 1972 650 Mercury 3cy

"I have a 1972 650 Mercury 3cyl. I got it from a guy who had the powerhead rebuilt due to a weak #1 cylinder after a boat swamping. (I later found the water output tube plugged with dried mud, which is how I think he got into problems) I got the head from him and remounted to lower unit as-is. I ran a few hours with it and it was running poorly on full throttle, so I took it to a marine repair to have it tuned. They supposedly tuned it up (last season) and I ran it for about two weeks and it seemed to run always great. I always saw what looked like a solid stream of cooling water ejecting from it, but he told me that I still needed a new impeller. I pulled a skier with my 16ft boat no problem. This year I took it out once and it ran OK for a while (half hour) then would not develop any power when pushed to full throttle. I was on vacation and could not get any marine mechanic to look at it, so I tried checking everything external to it (spark, fuel, distributor, coil, wires, etc). I was pretty sure that it was a sticking float/needle on one of the carbs because when I pumped the ball hard, gas would leak from the carb throat.

But, every once in a while over several days, it would all of a sudden start running fine for a while at a time. (hence I still thought carb, but wasn't about to remove them in the water, and I didn't have enough tools with me.) It did sound slightly rough, and was still rather low on top speed power.

So I took it to him for a checkup and he informed me that #1 cylinder was again low in compression. The $1200-$1500 for the rebuild he quoted me was more than I was interested in, so I took it back and decided to work on it myself. I've rebuilt many car engines, but this is the first time I've done any major engine work on a boat. I ran a compression check myself and found 1-2-3: 35-135-145, so I guess I believe him. I squirted some 80weight oil into #1 and got 150psi out of it. I am wondering if this means that the cylinder is probably not too far gone for a rebuild if some oil (heavy oil I realize) got me that much increased pressure. I hate to tear it down if it is probably hopeless.

I'm afraid if it was rebuilt or rebored once ( I have no way of finding out what the first mechanic did to it) I will not be able to get another rebore out of it, and I assume that the blind bore design cannot be sleeved by any machine shop. I looked through the exhaust port and the rings had no spring and were pretty well stuck to the piston. The engine never seized while I ran it. I don't know why this problem happened again, unless it was the few hours I ran it with the old impeller, although it looked like it was squirting OK to me. Is there any marine machine shop I can send the bare block to that will not gouge me?

Also, how can I remove the flywheel without the "special Mercury flywheel removal tool"? Or how can I get one? Do I need any other really strange tools, or are my basic engine overhaul tools all I need? I just ordered and bought the Merc. factory service manual.

Also, should I replace all pistons, or just the one that's bad? Do I need to rebalance it, and what's the best way to remove any weight from the piston/rod assembly?

If anyone is still awake after reading all this, I appreciate it. You can either post, or send me email at: [email protected]

Thanks,
John
Rochester, NY"
 
"John,

You won't be abl


"John,

You won't be able to tell exactly what you need to do until you take the engine apart. One thing is for sure; that top cylinder needs work.

My guess is that the engine needed to be bored for an oversized piston and was not. They just put new rings on and put it back together.

You can have it bored for about $40 per cylinder, and you should probably do all three, but before you order any parts take it apart and inspect everything very carefully. You may have some trouble in the exhaust system on the port side of the block, which is very common in these engines. You can run into some serious trouble here so pay close attention to this area, especially the condition of the mating surface of the block and the condition of the exhaust ports.

"
 
" Thanks, Tony.
I had assumed


" Thanks, Tony.
I had assumed what you did, that they didn't repair it correctly the first time, but supposedly the guy that did it was supposed to have known what he was doing. Oh well. Do you recommend overboring all three cylinders even if the other two appear to be within spec, or just hone them?

What exactly are you looking for on the exhaust?
Leaks between the water jacket and exhaust or what? Is there any easy way to test, or am I just looking for normal mating surface flatness? I already unbolted the port to look at the rings on the piston, and I didn't see any evidence of gasket failure when I took it off.

Thanks for your reply and any help.

John "
 
"John,

Based on what you ha


"John,

Based on what you have said, I would be inclined to bore all three cylinders. This is always the safest route when rebuilding a powerhead. But first you have to determine if the block has already been bored, and to what specs. You can usually tell by examining the pistons and finding the part number. Then you will know what size the pistons are.

Examine the exhaust ports for pitting, and look for signs of water in the exhaust. Water does mix with the exhaust, but this happens in the mid-section just beneath the block. There should not be any water in the exhaust manifold except for the water jackets there.

"
 
" John, To add to what Tony sa

" John, To add to what Tony said. If you don't know for sure how old your water pump is, replace it. I replace the pumps on all my personal engines every two years reguardless of the number of hours used. (they go bad just from sitting still also) If your pump quits, your motor overheats, if your motor overheats, your motor is toast. Also, replace your thermostat while you rebuild that motor. Its a pain in the but to get to, so do it while you have it apart. I've seen scores of 3 cyl mercs overheat and burst a chunk of metal out of the top combustion chamber. "
 
" The water pump impeller was

" The water pump impeller was replaced last year. I don't know how to judge what is good or not. It looked like the telltale tube was peeing out a pretty strong stream even before I changed the impellor. There is a plug hole tapped into the cylinder head cover that I could screw a water pressure gauge into to see what the pressure is in the water jacket around the block. The hole is up at the top near the #1 cylinder. What would/should I look for in normal PSI reading?

Also, is there a place I haven't seen to attach a water temp unit? I would be glad to monitor it also, if I knew what normal readings to expect.

As far as the thermostat, I cannot find one listed in the manual anywhere on this engine. I have the Mercury exploded parts list and I have gone over it line by line and do not see anything called a thermostat. Is is possible that this type of engine does not have one?

My mechanic showed me a block in his shop with the blown out hole in the top of the combustion chamber, so I know what you mean.

Thanks,
John "
 
" John, If your motor had a th

" John, If your motor had a thermotsat, it would be located under the exhuast port cover. I'm pretty sure thats where it would be. I don't know if merc made one without a thermostat or not. Maybe someone else can jump in here with better resource books than me. Anyway, if you are rebuilding the powerhead, you will probably stumble into the thermostat. If it's got one, it's gotta be located in the water jacket somewhere.
Jeffery. "
 
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