Re: 1972 800 thunderbolt ignition problems
I bought a 1974 merc 850 on craigslist for $350, the only thing I tested was compression, and it was 120 on all four cylinders. When I got it home I went through the same thing as you where all the tests were good but motor wouldn't run. I finally caved in and took it to a "pro." I told him what I paid, and that I would not spend a bunch to fix it but I would happily spend 100 dollars to know the problem. He said, Ill tell you the problem for free and fix it for $150. The problem, WIRING, WIRING, WIRING. He said I bet you all your electronics are great and you prolly just need new wiring. Then he said something like those MERC guys in the 1970s had rocket scientists building their blocks and drunken blind monkeys doing the wiring, after they went and found the ****tiest insulated wire on planet earth. The insulation cracks and creates shorts that arc sometimes, and sometimes not. He said it is impossible to trace the single short because there are prolly 20 of them, but in his 300 years of experience he has re-wired 4-5 million 1970s mercs and can do it in a couple of hours now he has done so many. He was exaggerating about the experience thing Im sure, but when he said "when I am done, Ill never see you again," he was being serious. That was several years ago and he was right. Take the time and REWIRE EVERYTHING. Ended up costing me $225 for the pro to do it, but it was worth it.
