'74 merc 650 3 cylinder---need some help

[jbjennings]

Merc 650.......150 psi on all 3 cylinders. I've cleaned the carbs. Replaced wiring and rectifier. Water pumping good and it doesn't seem to be hot when I get the problem.
Here's the problem:

When I run the motor wide open, it will do fine for about a mile then miss a little and then totally come off plane because it's losing a cylinder. When I take the plug out of the cylinder I've lost (top cylinder), it has what appears to be powdered aluminum buildup on the surface gap plug, bridging the plug and preventing spark. I cleaned it off, and it took off again fine, then not too far it has done it again on the middle cylinder (the middle cylinder's plug was clean when I took it out after the first problem).
I keep having this problem, and the problem starts at the identical location as the previous time (same distance from launch).
The other plugs aren't steam cleaned, and the problem happens on different cylinders. There also isn't any milky stuff on the plugs so I don't feel like it's water intrusion. It also doesn't appear to be getting hot, although I'm suspecting this because the problem shows up about the same distance from the launch each time.
Do these symptoms give you guys any clue what the problem could be?
I've changed the plugs with good ones that haven't ever given any problem on my other '73 merc 650 which runs good with bad water jackets.
Thanks,
JBJennings

 

[Faztbullet]

Re: '74 merc 650 3 cylinder---need some help

Well that is preignition but finding the source..
If it was mine this is what I would do:
1) Replace t-stat and replace the poppit valve grommets
2) Replace impeller and upper & lower housing
3) Check the max timing to see if out of spec's
4) Check WOT rpm to be sure it not overpropped(lugging)
5) Rejet carbs high speed .002 bigger due to todays fuel and compression
6) Correct heat range spark plugs (NGK BUHW)

 

[Texasmark]

Re: '74 merc 650 3 cylinder---need some help

Are you mixing TCW-3, 2 cycle marine oil at the rate of 50:1 with your gasoline? 50:1 is a pint to 6 gallons of gas if I recall.....16 oz to 132 oz x 6 gal = 16/732 = 49.5:1....yeah that's right. If you haven't been then that alum on your plugs is your cylinder walls grinding away due to lack of lubrication. Otherwise where is the alum coming from; it surely isn't normal.

Get back with me on this.

Mark

 

[jbjennings]

Re: '74 merc 650 3 cylinder---need some help

Are you mixing TCW-3, 2 cycle marine oil at the rate of 50:1 with your gasoline? 50:1 is a pint to 6 gallons of gas if I recall.....16 oz to 132 oz x 6 gal = 16/732 = 49.5:1....yeah that's right. If you haven't been then that alum on your plugs is your cylinder walls grinding away due to lack of lubrication. Otherwise where is the alum coming from; it surely isn't normal.

Get back with me on this.

Mark

Thanks Mark....
I've never forgot to put oil in my tank but there's a first time for everything. THat sounds logical for sure. I'm going to follow fastbullet's advice and check my water pump (switch the foot to my known good one) so I know the pump is working good, and it'll have the other prop on it for my barge so I know it's not lugging (shouldn't be a problem because this motor has been running this prop since '76 for sure. I'm also, of course, going to mix up a new tank of gas so it has oil for sure. I'm getting a little oil on some of my plugs so I'm thinking it has oil in the gas, but will try that anyway. I'm also going to run it and use my infrared thermometer on all parts of the engine and see what it says.
I'll report my results ASAP.
Thanks for the help guys,
JBJennings​

 

[Faztbullet]

Re: '74 merc 650 3 cylinder---need some help

Hey Texasmark... Not trying to be a wisea** but cylinder liners in outboards are cast iron except some 25's & 200"s which are chrome plated. Inlines suffer from piston erosion on the crown(deflector) of the piston when it has preignition. This is actually melted aluminum thrown onto plug and have seen the spark plug hole completely closed due to this.

 

[jbjennings]

Re: '74 merc 650 3 cylinder---need some help

Fastbullet,
I honestly had no idea that my motor had cast iron sleeves in the cylinders. I had to replace some of the wiring and battery cables, and had to take loose the cables and such. To be honest, I don't really know how to adjust the timing on this motor. Is it by adjusting the stop screws on the armature plate? I do have a timing light. I know you don't have time to write how to do something that simple, but would just like to know if it's by the stop screws.
Also, could overheating cause preignition? Obviously so or you wouldn't mention the poppit valves and water pump, right?
Last of all, if I can fix this problem is the motor probably still in o.k. shape as far as longevity? I hope so because the lower unit is nice, it has good compression still, and I kind of like the rig. (Got if for $400) Here's a pic... If I can get it where I trust it, I think I may try to find some stick steering because that danged console and chair takes up all the room in the boat!
thanks,
JBJennings

 

[Faztbullet]

Re: '74 merc 650 3 cylinder---need some help

Jbjennings.. look in the cowling in the little packet where pull rope is stored, the plastic timing pointer should be in it,(OEM#62397)if not look in your other engine and I will try to dig one up. The pointer fits in a slot on the block/bracket. Then you can check & adjust timing. You can also loosen the max advance screw and turn it in 1 full turn (3 degrees) and retest.