scoutabout
Lieutenant Commander
- Joined
- Oct 14, 2006
- Messages
- 1,568
Maybe some of you will have some ideas. In trying to resurrect my Merc 7.5 (Thunderbolt Ignition series) I had the switch box and stator replaced. The switchbox wires had disintegrated, which seems to be the usual fate for those parts and I was also told that one stator coil was bad.
So, used parts were sourced off a similar motor and installed. I've had it out a couple of times and each time it displays the same behaviour. Although it will start and run, after about 45-60 minutes cruising smoothly at 3/4 throttle it suddenly begins to misfire, dropping down to about half that speed and running quite raggedly. It will also jump between the two speeds quite suddenly - almost as if one cylinder is losing spark intermittently.
After this development the engine also won't idle at all and stalls if the throttle is pulled back to less than 10% or so.
I'm fairly sure this isn't a fuel starvation issue unless something is intermittently getting blocked in the carb. The fuel bulb remains firm too.
Any suggstions what to investigate and in what order much appreciated.
So, used parts were sourced off a similar motor and installed. I've had it out a couple of times and each time it displays the same behaviour. Although it will start and run, after about 45-60 minutes cruising smoothly at 3/4 throttle it suddenly begins to misfire, dropping down to about half that speed and running quite raggedly. It will also jump between the two speeds quite suddenly - almost as if one cylinder is losing spark intermittently.
After this development the engine also won't idle at all and stalls if the throttle is pulled back to less than 10% or so.
I'm fairly sure this isn't a fuel starvation issue unless something is intermittently getting blocked in the carb. The fuel bulb remains firm too.
Any suggstions what to investigate and in what order much appreciated.