Re: 1982 Mercury 70 runs then stalls and wont start back up HELP!
Larson70,
I am also in the mid-70's to late-80's 3 cyl Merc club. I have a 1978 70hp. What you describe is exactly what went on with my motor over the last season or two (including it becoming increasingly crotchety and my "crutch" fixes), until it did finally corrode all the way through the exhaust baffle plate like Oldman and Max above are talking about. In my case saltwater then got into the cylinders and crankcase, destroying nearly everything in there. I have a used, supposedly fresh-water only, 1988 powerhead on the way right now.
Just a few ideas/warnings for you:
If the motor has seen a lot of saltwater use, there is very little chance that you will get the exhaust cover (port side cover) bolts out (some 12 or 13 of them IIRC) without breaking off most of them. Even in a freshwater-only motor, you'll have a tough time with that, especially with the two smaller (7/16" head, IIRC) bolts right in the center of the plate. Corrosion sucks. And if it's seen salt, there is almost no chance that the exhaust baffle (under the plate) will come off without breaking apart, even if it is currently not yet fully corroded through.
If you want a clue as to the condition of the piston skirts and rings, and to check for corrosion on your connecting rods (which would be a sign of water intrusion), you can pull the transfer cover instead (starboard side; plate that the fuel pump is attached to); then you can see these things through the intake ports. Since the transfer cover sees air/fuel/oil instead of water, it will be far easier to remove (no corrosion). You will have to remove most or all of the stuff (switchbox/rectifier and plate, ignition coils, etc.) on that side first.
I came up with what I believe will be a way to check whether the baffle plate is corroded through (or if there are other water passage leakages) without removing the exhaust cover, and--assuming I can't get it's exhaust plate off without an epic struggle--I plan to do this to my used powerhead when it shows up; you would have to remove the entire powerhead to do this though:
-Plug/block tell-tell brass hose nipple outlet at top rear starboard side of cylinder block.
-Invert cylinder head so it rests on it's top; carefully!
-Fill cylinder head with water through the passage that the water pump feeds (red arrow in pic).
-It should fill between the exhaust cover and baffle, and into the space around the cylinders under the rear cylinder cover, until it just starts to come out where they used to put the tstat/poppet in the early 70's (blue arrow), and out the cylinder block drain hole (green arrow); my guess is about 2-3 quarts total.
-Now, let that baby sit awhile, and see if the water level drops. If it does, you got a leak somewhere; and if it's not external, it's internal. If it drops fast, you probably got a real bad baffle plate hole.
I am thinking about whether the same test might be do-able without removing the powerhead; I'm thinking you'd have to:
-Drop the LU.
-Plug/block the plastic nipple on the starboard side of the exhaust plate that feeds cooling (?) water to the fuel pump.
-Create some sort of manometer-like device to attach to the tell-tale brass nipple, like some clear tubing held straight up somehow.
-Figure out a way to force water up the water pump copper tube until it fills the block and goes partway up your tell-tale manometer device, then quickly plug/stopper the water pump pipe so you can then chekc whether the water level drops over time.
Or alternately, plug the water pump pipe, and slowly fill it through the tell-tale/manometer thingy?
Way I see it, if the baffle's already gone, it's already ingesting water, so a little more won't hurt it.
