1984 75hp OB no spark

I had it running last year after alot of screwing around. It ran fine and could start it all day. Next day nothing again. So I get back into it, and there's no spark again. I have a book and have tested the stator, replaced trigger assembly due to brittle wire, and coils. All checked out good. The book says if all that is good replace the switchbox. Is there any tests that can be ran on it to determine if it's bad? Or is that usually the way it goes with a 25 year old motor? Is the switchbox going bad that common of a thing? I can't think of anything else to test. I cleaned the carbs, rebuilt the fuel pump, had to buy new gas tank since there wasn't one. Don't know what else to try. Just hoping for a "did you try this", before I start spending more money.

Re: 1984 75hp OB no spark

Solved my spark problem. I got it out on the water and it idles good. Not great but good. As soon as I put it in foward or reverse it dies. Any ideas? When I have the earmuffs on it won't do it. I checked the timing and it's at 4.5 degrees, so I don't think it's off. I need some advice please.

Re: 1984 75hp OB no spark

When you cleaned your carbs you may have adjusted the idle mixture to best idle on the hose which is far too lean once on the water. Back them out an additional 1/4 turn to get it to run on the water. Then readjust again on the water at an idle in Forward gear to best idle speed, then back them out an additional screwdriver blade width.

Test Acceleration from idle in gear to immediate WOT, any bog, hesitation, cough or sneeze, then back the idle mixture out an addition blade width and retest, repeat as necessary until ALL traces of hesitation are gone.

Re: 1984 75hp OB no spark

Thanks. I'll try it again next weekend when I get back in town. Can the same results be acclomplished in a test tank. I was thinking of getting a new 55 gallon drum from work and using that. Even if it will get me closer to being tuned right and still have to do it on the lake, it would make it alot easier for me rather than spending half the day screwing around on the lake by myself hoping I don't kill a battery.

Re: 1984 75hp OB no spark

So I got a drum from work and cut it down to adjust it a little more before going out on the water. Got everything running pretty good I thought. Hit the key a couple times and it would fire right up. This is after I replaced the trigger plate assembly. The wire was so brittle it broke all the way up. I thought maybe that had been the problem for it running eratically. So get out on the lake takes for ever to get it started. There can't be that much of a difference between an hour ago when it would start right up and now I thought. Well get it going. Check timing in gear at 750 rpm. It's about 9 degrees. Try to adjust it dies numerous times. Get it at about 7. On the lake full throttle can't muster more than 2500 rpm and it's at full timing. 27 degrees. Go back to the launch to fiddle with it a little more. Can't get it started again. Timing light blinks acouple times then nothing. On all wires. Hook it to the truck it works. Back on the boat nothing.

Is it time for the switchbox since there's no way to really test it?

Re: 1984 75hp OB no spark

Any ideas at all?

Re: 1984 75hp OB no spark

Sounds like an insulation breakdown. Your spark seems to be ok when not in the water but put it in the water and bad or no spark. Make sure that all wire connections and their surrounding surfaces are clean. You may see something arcing if you run it in the dark.