engine repair

[Bushwacker]

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!