Mercruiser Intermittent Boggin

[Amati]

My friend has a Mercruiser 228 Gen 1 (Chevy 260 V8 double barrel carb) on a 1982 Cris Craft. The engine block is from a 2000 Chevy truck which was outfitted with new Mercruiser parts in 2005. The outdrive is entirely new within the past 15 months. It’s got a Thunderbolt IV ignition.

He’s having the most unusual problem. I’ve read through the posts and see symptoms which are similar but one element or another is different from what he’s experiencing. I’m hoping someone can help. We can’t make sense of the various symptoms which change somewhat with each new attempt to identify the problem and to make matters worse --- it’s an intermittent issue. And each time we change something at first it seems we have it fixed then we find out the threshold point where the problem kicks has moved up in the RPM scale for a while and slowly the problem moves back down closer to idle.

There are times when the boat ‘bogs’. The boat feels like it’s surging (or choking). There are no unusual sounds from the engine, although sometimes it sounds like it’s running rougher. The problem can start as soon as we push off from the dock, other times he can run all day with no problem. Next time we take it out, it may happen cold or after some period of time running.

At first he thought it might be the wiring harness because when we played with the connector the problem would go away. He changed the engine side wiring harness – ran fine for a day. Parked the boat in the driveway…..two weeks later (no use between times) put the boat in the water, problem returns.

He’s changed the fuel pump, the water separator, checked the spark, cleaned water out of the carb. He pulled the choke all the way open. We even changed the ignition module.

Here’s the part that’s weird. Some days it bogs not much over idle, other days it waits until it planes out (not during the load of reaching plane but afterwards). We’ve tried to correlate any elements to see if something is common which we just aren’t finding – sometimes it happens under load; sometimes at idle. Sometimes @ high RPMs, sometimes very low. Sometimes it’s running fine, we stop for dinner; and then it starts bogging when we leave. Usually after he changes a part, it will work well for an hour or so and then start again – other times it will work for one or two days.

Maybe it’s a coincidence but several times we’ve held blocks of ice on the ignition (“brain”) and it seems to help at least for a little while.

One symptom implies it’s fuel related but why would ice on the brain make it better? Sometimes he thinks the secondaries aren’t working correctly but then it’s not always when it’s under load and it seems random based on weight or use. Distributor? Carb? Ignition module (although it seems highly unlikely that two different modules would cause the same symptom).

We’re totally at a loss as to what to think or try next. Any suggestions?

[kenJ]

Intermittant ususally means electrical. There are just so many variables, hot/cold, humid/dry, smooth/rough water etc that can play havoc with the system. I would start by making sure all the electrical connections are clean and tight, including the grounds. Are the plug wires new? Is the coil new? Is the Thunderbolt system new or used?

[tuantom]

Water in the fuel tank? A few bottles of heet are a cheap test - though the separator should catch this. Simple first.

[Amati]

After much experimentation - turns out the distributor was bad.