'75 merc 650 questions

[jbjennings]

A few questions pretty please.....

1. My merc 650 3 cylinder runs awesome WOT.
It idles fair. I've cleaned the carbs, BTW. However, it isn't as smooth as I'd like and at slightly less than WOT, it has what seems to me to be bad timing, possibly like I have a cylinder that's missing every once in a while.
Is there a way that the timing could be off a little and still run perfect WOT? It seems to have a wee bit of bogging midrange, but has plenty of power--mainly just SOUNDS like it's bogging.
Does throttle cable adjustment affect timing?

2. It has brand new plugs. I have not checked the rotor and distributor. Is it worth taking the distributor cap off and cleaning the rotor and contacts? Could this be the problem?

3. Last of all, I've read a little here and there the last couple of years about what bad motors these are. What are the problems with these motors? My dad said he noticed they didn't have near the speed or power compared to the 4 cylinder 650's. SHould I get rid of this thing while it's still running good? I actually kind of like the motor, but am a little afraid of its bad reputation. I have one on my barge that has a huge crack all the way down the water jackets under the exhaust cover and runs good but has water jackets made partially of jb weld.

Thanks,
JBJ

 

[jbjennings]

Re: '75 merc 650 questions

Still hoping..........

 

[Fuzzytbay]

Re: '75 merc 650 questions

I would guess its maybe just age. At WOT you are notgoing to notice a drop in performence. Yet at lower speeds youwill. My bet is on plan old age. The reeds are ageing, the compressions dropping a bit. All perfectly normal issues.
If she runs good, doesn't give problems, then for confidence sake, do a compression test. It an easy test, and if it comes back good, (no more than 10 lbs between cylinders) then its worth investigating, if its distributor cap,n rotor replacement time, etc.

 

[jbjennings]

Re: '75 merc 650 questions

Thanks,
Compression is good, with 145 on top, 150 on middle and bottom cylinder.
JBJ

 

[Faztbullet]

Re: '75 merc 650 questions

These engines never really idled that well and always had a shimmy at idle. This is do to 2 carbs feeding 3 cylinders as center cyl was fed from top and bottom carbs and is a balancing act to keep it from running rich. I myself think their a great little engine for light boats and racing. Back in the late 70's mid 80's Merc made this engine in a race version that was a 650x and 650xs. The 650x was around 90hp at 7000 rpm and the 650xs was around 80hp at 6200 rpm, These engines where very competive in the sst70 division .

 

[jbjennings]

Re: '75 merc 650 questions

Thanks Fastbullet,
I just rechecked compression with the engine warm and found it to be 147 on top and middle and 148 on bottom. I'm thinking it doesn't get much better than that. It does have a darn shimmy at idle just as you describe. I readjusted the carbs on the muffs but will probably have to redo it on the lake AGAIN with a little backpressure on the exhaust. I don't think I'm going to worry about the distributor and rotor unless you guys think I should. It's running nice and cool and has plenty of power, so I guess I'm going to enjoy it like it is. I'm really pleased that the lower unit holds water out. I think I'll just run 'er til she flies apart. If I can get just 100 hrs. out of this motor I'll be well pleased. I payed $400 for the boat, motor, and trailer and the old fiberglass boat is actually in great shape with a good floor. It also has a manual hydraulic tilt which I think is really handy! Here's a pic. It wasn't running when I bought it. I had to clean the carbs. and rewire it which didn't cost much at all.
If anyone else has any thoughts on the motor I'd love to hear it.
Thanks,
JBJ

 

[hibbert6]

Re: '75 merc 650 questions

JBJ,

I have the same engine. I bought it (on the boat) 13 years ago and it still runs great. I've had a few strange issues with it, but 99% of the time I'm extremely happy with it. I've never tuned anything. I replace the plugs and impeller every couple of years but that's about it.

That hydraulic tilt - mine has it, I think, but basically it seems to be a shock absorber for something. I have to hoist the motor up from behind and stick the pin through to set the trim. A 2 person job. Fortunately, my 10 year old son "gets it" now and can do the pin. My wife likes to pretend she doesn't understand so I won't ask her to do it.

Dave