engine repair
 

I have an 84 Johnson V 6 outboard where the engine starts to miss under load. Runs fine at idle. Any ideas on how to trouble shoot which coil(s) might be causing the problem?


Thanks

Bill

A weak ignition may work ok at idle but it's harder to fire a spark when cylinder pressure is high (large throttle opening), so what you describe may be an ignition problem, although it could also be a lean misfire condition where a main jet is slightly clogged and only shows up after the carbs switch from the idle circuit to the main jet system. If I were you, before I started screwing around with the carbs, I'd build a spark checker similar to what was described in service manual and that I built years ago for my old 1975 V-4. Here's a thread where I described how to make it.
http://www.classicseacraft.com/commu...=spark+checker

It checks the entire ignition system including sensor coils, power pack, ignition coils and wiring, so a weak or no spark during cranking could be any of those components. If it checks ok cold, you could still have a bad sensor or ignition coil with a crack or break in the windings that would only show up when hot after the coil expands to open up the break. In that case, check it again after you've run motor for a bit. I had an early 50 hp V-4 with a magneto system with a crack in magneto that ran fine when cold and still ran ok when hot because the magneto generated so much voltage at high speed, but it would die as soon as you retarded throttle and wouldn't start till it cooled off!

My 96 130 Johnson's did the same thing turned out to be bad gas
Run a 6 gal portable first cheap way to check fuel

Ditto, and put a new water separator filter on. Dump the old one and see what comes out.

I wanted to give everyone an update on this… after many months of trying to trouble shoot the problem. I let the mechanics take a shot at it. They could not reproduce the problem at the shop and needed to run it on the water. After 10 minutes of use on the water, they found the problem. The whole right side of the motor was blown. Water stopped circulated on that side of the motor due to a rusty/clogged cooling port at the base of the block. The left side of the motor had the thermostat attached to it and was reading normal temperatures. Now it’s time for a new block.