'97 MCM 4.3LX GEN + acceleration problem
 

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.

Re: '97 MCM 4.3LX GEN + acceleration problem
 

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.

Re: '97 MCM 4.3LX GEN + acceleration problem
 

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.

Re: '97 MCM 4.3LX GEN + acceleration problem
 

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.

Re: '97 MCM 4.3LX GEN + acceleration problem
 

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 :(

Re: '97 MCM 4.3LX GEN + acceleration problem
 

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.

Re: '97 MCM 4.3LX GEN + acceleration problem
 

Now your thinkin!!:wink_thumbup:

Re: '97 MCM 4.3LX GEN + acceleration problem
 

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.

Re: '97 MCM 4.3LX GEN + acceleration problem
 

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.

Re: '97 MCM 4.3LX GEN + acceleration problem
 

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.

Re: '97 MCM 4.3LX GEN + acceleration problem
 

Hello again,

So all the consistency that I had on my last trip did not transfer over to this trip. The amount of time in idle made no difference. Idle mixture adjustments made no difference. It seemed totally random. Both my son and I noticed how random the performance was.

It starts and idles perfectly.

Sometimes it would buzz at full HP right from the start. Most of the time, it would chug for a while and then jump to buzzing. I did notice that upon acceleration, when I bumped the RPM up a bit by bringing up the trim little, it tended to reduce the time it took to jump from chugging to buzzing.

Sometimes, when pulling a towable loaded with kids, it would chug. I could quickly pump the throttle from idle to WOT a couple of times to reduce the time it took to buzz. This worked sometimes. Other times, I’d have to drop back to idle and try again. Sometimes it would chug, and other times it would buzz right from the start. I could always get it to buzz eventually and once it was buzzing, it did not return to chugging until the boat had returned to idle.

It seems that it is a problem when at WOT and low RPM, primarily at start.

The fuel filter/separator has less than 10 hours on it now so I don’t think that’s the problem.

I discovered that I have a Mercarb 2 BBL that only has 1 idle mixture screw.

My next step is to check the carburetor fuel inlet filter and then ensure that the accelerator pump is working consistently. I’ll also put some cleaner through the fuel system before my next trip out.

Any other ideas?

Re: '97 MCM 4.3LX GEN + acceleration problem
 

Hello Larry,

So after much frustration, I got it fixed. The engine manual detailed that this motor came with 2 different Thunderbolt Ignition systems. One without a timing control module and one with both a timing control module and a knock control module. Mine fooled me because it has the engine control module, but not the knock control module…so it looked like the other system. So when I set the timing after changing the distributor, I set it to the wrong spec. I finally figured it out, set the timing correctly, and it is running perfectly now. Thanks for your help with this. I just thought I’d close the loop with you.

Take care ~ David

Re: '97 MCM 4.3LX GEN + acceleration problem
 

Glad to hear everything came out good for you David.:wink_thumbup:
Mid year changes in the system can sure be a pain in the butt for sure. :(