TOPIC: '72 Merc 800 Starting problems, WTF?

My buddy has a '72 Merc 800 in beautiful condition, has been babied for years. He has had an intermittent problem with starting and now it is quitting in mid flight.
I told him I would ask the pros on the Fiberglassics web site and I knew you would ask two questions: is it getting gas, is it getting spark?
We can smell the gas, the plug was wet.
When we went to test the spark, I turned it over while my buddy placed the plug on some metal and boom...not only did it start right away but it gave him a killer zap.
Why, my brothers in Merc's, did this action start the engine? Vapor lock?
Come back and thanks for your opinions.

Headley,
After he got zapped and the engine started - did you guys put that sparkplug back in and then did the beast start and run okay?
The most common reason he got zapped was that perhaps whatever surface of metal he held the sparkplug against was rubber mounted with no ground strap going to the main powerhead.
There should be a large ground strap running from the starter to the starboard mounting bolt of the front cowl support, another large ground strap running from the front cowl support's port mounting bolt over to the ignition adjustment plate on the front of the distributor, another either large or small ground wire coming out of the internal harness and going to the lower pan - besides the small black wire that goes to one of the small terminals on the starte solenoid, a small ground strap between the distributor body and the crankcase cover, and several small ground straps that connect the aft end of the powerhead to the lower pan, and a couple small ones hidden by the lower 'U' shaped cowl that cover the powerhead mounting nuts - but these last (2) aren't necessary for this issue - just for protection against corrosion.
It sounds possible that either the battery terminals need to be cleaned, and the 12vdc to the starboard white terminal be tested.
It could be as simple as hidden corrosion at the battery terminals - especially if the connectors being used have the primary leads clamped to them with two small capscrews - OR - perhaps the wiring in the control box is beingging to faulter due to age & corrosion.
I'm just guessing on these issues.
Doc Frankenmerc

Thanks for the note Doc...oh by the way I found the box of parts for that Johnson I gave your buddy last year. I will send it to your house.
The motor turns over well, always has, it just won't catch. He has been having this problem for a while. I told him we had to be sure that the motor was getting spark so we did that test. It started and ran on 3 cylinders just fine. Turned it off, installed plug, and it started and ran just fine.
The shock was just one of the little surprises that happens, no biggie.
What happened in between is my question. Why would it not run until I pulled the #1 spark plug? I kept thinking it was some kind of vapor lock, or something that kept if from getting fuel. Then when I took out the plug everything started moving.
We did discuss the control box. It is older and he probably could use a new one (where should I get?), but why would taking a plug out make it catch and start?
Thanks for your input Thom. So happy to see you are still the man...

Thom

Most common fault with that one is the harness plug on the side of the engine. That ignition needs 12V to run and it sounds like the connection is giving some grief. Other possibilities are the mercury tilt switch and the ignition switch. Hotwire the switchbox 12V lead and try it again.

thanks for the input, really appreciate it GF.