Mercury 650 problems – is it worth an overhaul?

[shoestringmariner]

Hey all, I ran my new to me 1976 **CORRECTION 1975** Merc 650 tonight for about four hours. I took it out fishing. Did a fair bit of trolling. Did not seem to like the idle very low for long periods of time. Had to use the idle lever to keep the revs up just a wee bit. Anyway, concerning part was when I was wrapping up;
I just stowed everything away and went to give it a hole shot. It responded and started to climb on plane and then died suddenly. I thought it was out of gas. I checked the fuel and it was still good. Primer bulb still had gas in it. It was very hard to start again. Wide-open throttle, extended cranking, chug chug starting. Took what seems like forever to clear and run properly again. Then it lit up and I made a good full-tilt run back to the launch.

During the difficult starting, I thought I saw a bit of vapour escape from the cover plate. I was not using the choke

There are other known issues with this motor. The wiring is split and rotten in some places. I haven’t gone through and replaced anything yet. It seemed to run fine until tonight.

Another big issue is no reverse gear as posted earlier.
I thought about looking for another 60hp plus to re-power, but they seem scarce, look beat or are well out of range.

With these known problems, would this motor still be worth deeper digging in your opinions?
130-132psi even across all 3 cyls. I don’t mind spending money on this if it will be a good reliable motor for years to come. But I hear negative things about these 3 cyl models 🙁

Lastly, if I was to re-power this boat, what would your suggestions be?
I believe the boat is rated for up to 90 hp

[kerry]

The wiring could very much be an issue, but check the spark plugs. Prolonged trolling may require a hotter plug to stay clean. Also, try a tank of fuel with synthetic oil to see if fouling is less. Compression numbers seem good to me, but I don’t know what’s good for your motor. Since it runs good on plane, would adding a small 3-5hp trolling motor work on your boat? A couple things to think about…

If you have too many, AND not enough, you're a collector.

[shoestringmariner]

Thanks Kerry. Small troller is on my shopping list, though likely next year or after overhauling this one or its replacement. I’m checking the plugs this afternoon. Already running a soil synthetic.

What about regular vs premium. I can’t get regular without ethanol in it though I do use stabilizer. Manual says don’t use premium

[amuller]

In general you can’t expect any such motor to idle for long periods of time without acting up. Plug fouling and loading up of mixture in the crankcase is more or less the nature of the beast. Use a trolling motor for trolling.

If compression is good, as it seems to be, and there are no bad noises, I would incline to put some effort into it. Cleaning up and replacing wiring is tedious but can be done bit by bit. It just takes the right materials and workmanship.
Carbs? One cylinder running lean can be fatal. Are the plugs all the same appearance? Fuel filter(s) clean?
Fuel? I don’t think E10 gas causes as many problems as get blamed on it, but it’s wise to check fuel lines for internal deterioration that could shed bits.
Just some idle thoughts……

[kerry]

I forgot, not a bad idea to add a bit of Seafoam to the fuel. Helps keep things clean inside.

If you have too many, AND not enough, you're a collector.

[tarcigam]

They have many detractors. Few fans. Try Johns site, I know there is at least one person there that loves em. I have some parts motors.

[mercuryspecialist]

Hello,
Personally, I very long ago tired of reading of how these 50 cubic models are supposedly a poor choice. We sold a pretty fair quantity of them, and they were not problematic to us. The 1976 year model brought a distributor-less ignition (also true for 4-cylinder models), and stainless lower unit shafts. Capacitor discharge ignition should not be having spark plug fowling issues, but may have carburetor(s) or fuel pump issue, or be mis-adjusted. Mercury wiring issues are a fact, but 25 years to somewhat more, is often the limit. You should open the remote control, and address any rotten wiring inside as well. The use of premium grade fuel would not be an issue. Like most all outboards, do not run it out of the water, replace the impeller every few years, check/change the lower unit lubricant, prop correctly for the boat, and mount the outboard correctly (you might be surprised at how many outboards are incorrectly mounted). A lower unit ratio change would be needed for a heavy or large boat (again, it is a 65 flywheel rated horsepower with 50 cubic inch displacement outboard motor, not a noticeably larger outboard). A few questions, or if some advice is wanted: info@fergusonpoolemarine.com

Regards,
Joe

[51seahorsetn27]

I’d go thru the wiring since it is known bad, another guy had a similar problem and it was a grounding issue with bad wiring and a broken switch, do all the wiring/ switch stuff dont skimp here. If problem persists check fuel issues. Vapor from the carb is usually unburt or over fueled cylinder from no spark so go to wiring freshin it up, then if still problem check ignition modules and or box(s). Compression sounds good, if all is well there then go to fuel pump then carb. 10 years as a mechanic says look for a ground short in bad wiring.

[shoestringmariner]

Thanks guys. This motor is going to the doctor next week for full assessment. I can plug through a lot of different things but I want this thing to be reliable. We’re going to have a look at it and see if the bones are solid enough to fix the issues.

I also think it might be a 1975 not a 76

Serial number starts with 426. The Seloc repair manual I picked up says 1976 650 started with serial number 438.

I’m somewhat disappointed with this manual. There’s lots of info for every other model except for this one.

[shoestringmariner]

So I had a mechanic have a go at this motor. He said the wiring is looking rough but should be ok for a season or two, which is ok by me.

He put a new water pump in it and the pee stream looks much better.
He said it had the wrong prop (too big & not enough pitch) and wrong plugs in it and he adjusted the carbs. I’m not entirely pleased with it though. It was sneezing at first start/warmup. I ran it at 90% open and crusing speeds for almost 2 hours. Seemed to run well but I was concerned it was over revving. (No tach) seemed to run higher rpms this time out..

My issue now is at the end of the day, it was a handful at the launch. Would not run right at idle. Hard to start. Tank had 10-15% fuel in it. Didn’t want to start without choke. Is the primer bulb supposed to be firm? It was soft. Vent was open

Thanks in advance

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