[18 Shabah]

Hello. I'm new to forums and apologize, in advance, for any cultural mistakes that I might make with my request for help.

I have a 1997, 18' Shabah with an MCM 4.3LX GEN + with a 2BBL and an ALPHA 1 drive. I bought it 2 years ago. It started with some acceleration problems that I remedied over time with some fuel additives and only running high octane, non-ethanol fuel through it. I have continued using the same fuel. It ran great and according to spec until late last September.

It would start and idle well. The problem was with acceleration. In both heavy-loaded (even towing) and light-loaded conditions, in WOT from a stop, it began to accelerate very slowly, almost like it was missing on some cylinders. The engine didn't sound rough, it just stayed at low RPM.

After several minutes and very slow acceleration of RPM and speed, all of a sudden, it would kick in to a full horsepower output and spec performance. It almost felt like a vehicle does when you stomp on the accelerator and the automatic transmission drops into a "passing gear". I was standing during one of these transitions and the rapid acceleration knocked me back into my seat. The transitions seemed to hit around the same RPM/speed, though I don't recall where it was...maybe 10 to 15 MPH or so.

Even after running at spec performance for several minutes, after a stop with or without turning the engine off, the acceleration problem persisted. The accelerator pump is pushing two streams of fuel into the carburetor, so I don't think that's the problem.

I would be so grateful for any help that you can provide.

[Hudson River Steve]

Sounds like it could be a weak spark. I would start by checking/cleaning the electrical connections from the igniton module in the distributor to the coil. Don't forget to clean the connection from the coil bracket to the engine. Inspect the coil itself for cracks.

[18 Shabah]

Thank you so much for your quick response and for your guidance. I am going to take it out of winterization early next month and I'll let you know the results. Again, thank you.

[18 Shabah]

You nailed it. It was definitely spark. As I was looking for cracks in the coil, distributor cap, etc., I removed the distributor cap to find that the rotor had about 10 degrees of play in it. It must have been an aftermarket rotor from a previous owner. Anyway, I secured the rotor to the shaft with some gasket sealer, adjusted the timing, and it runs better than it has since I purchased the boat.

Thank you so much for your guidance. You saved me a lot of time and frustration.

[ColoradoDiablo]

You'd best get a new rotor!
Gasket sealer ain't gonna last long. Not to mention , it'll quit at the worst of times

[18 Shabah]

Agreed.

I came to that conclusion about 4 hours after I secured the rotor. Oh well. I'm going to get one and put in the boat. I'll be able to fix it on the fly within about 30 minutes since I keep all my tools on the boat.

Thank you for your input.

[ColoradoDiablo]

Now your thinkin!!

[18 Shabah]

Hello again. I have what may be a related problem.

With your guidance last year, I replaced the aftermarket distributor with an OEM distributor and got my engine back to spec, running at full horsepower and 4500 RPM at WOT. It was very reliable and I am so appreciative of your help.

For my first trip out this year, I went by myself to run around the lake and do a little fishing. I spent very little time in idle and the boat ran perfectly, “buzzing" at spec and full horsepower.

For my second trip, I took the kids. We spent a lot of time idling at the dock and while the kids were getting on and off the tube we were towing. After times of idling, when we’d accelerate, the engine would “chug”, only reaching 3500 RPM (or so) and obviously not hitting full horsepower. After chugging for a while (up to 10 minutes) it would suddenly jump to buzzing at full horsepower and 4500 RPM, without any change in throttle position. I noticed that the longer we spent in idle, the longer it took to jump from chugging to buzzing under throttle. So I started to minimize the time spent in idle. I’d just cut the engine off rather than stay in idle. With this practice, the chugging problem went completely away.

So the obvious answer is “minimize the time in idle and get on with your life.” But there are times when I travel through no-wake zones and have to stay in idle for a while. So I’d really like to fix this issue.

Regarding ignition - this engine does not have a computer, but it does have an ICM module. Is this chugging a possible symptom of the module failing? Plugs and distributor are new and the whole system operates according to spec while buzzing. A new ICM is over $400, so I’d really like to be sure this is the problem before making that purchase.

Regarding fuel - is there a chance that the periods of idling are “flooding” the carburetor and it takes a little while under throttle to return to correct operation? How can this be remedied?

Regarding air supply - could it be that while idling the engine compartment does not get the amount of air that it needs for correct operation and it takes a little boat movement to flush new air into the compartment (I doubt this one because we’d be moving at 30 MPH for up to 10 minutes before it would suddenly jump from chugging to buzzing)? I do have the intake vents partially blocked (25% to 50%) with bouys and tools. Would this make a difference?

Your input has been so helpful in the past and I’d truly appreciate whatever guidance you can provide.

[ColoradoDiablo]

The carb is loading up , causing a very rich burn , fouling the plugs.
Just a thought , have you changed the plugs after all the probs you had?
Try turning out the air screws about a half of a turn and see what that does.

[18 Shabah]

Thank you so much for the quick response.

They are new plugs this season, with less than 5 hours on them. The plugs from last year looked good when I changed them out at the beginning of this season. Maybe they had enough good time on them at the end of last season to burn off any fouling.

I will adjust the mixture screws the next time I go out and I'll get back to you with the results.

Thanks again.