1990 4.3 LX 6 cyl Mercruiser that stutters and bog in the high rpms

Looking for the best way to diagnose why this engine stutters and drops RPMS when it hits 3400 RPm. When I hit this RPM range the tach starts to go crazy and drops RPMs. Boat starts and runs fine up until that point on the tach. I did not seem to have this problem before I put it away in the fall.

So far I have changed fuel water separator, changed plugs, put some heet (water absorber treatment in a can) in the ethanol free gas that was in the tanks. I also added quite a few gallons of ethanol free gas and the problem still exists.

The pulled plugs looked alright with the exception being my Number 1 cylinder had a sticky substance like oil tar on the plug. I have never seen this before ad it was only found on the one plug. I did change out the oil the same day I pulled the plugs.

I have asked a mobile mechanic to diagnose the problem but he has not gotten back to me. Normally he is fairly quick about these things. I think he views this as a hornets nest and doesnt want to touch it. Difficult to haul the boat out of the water. so I am looking to the experts for a plan of attack for diagnosis outside of replacing parts that are not the root cause. Carb was rebuilt last season. Any one with a 4.3 LX who has had this issue or anyone else with an opinion care to offer how you would diagnose and fix the issue?

Lastly I think it is wise to not run the boat in the lower RPM range without having this problem fixed. Do you agree or is it not a problem if I keep it below 3000 RPM?

Re: 1990 4.3 LX 6 cyl Mercruiser that stutters and bog in the high rpms

In reading some other threads on this problem, I think the idea of running the engine from a portable tank and seeing if the problem still exisits will help me to isolate alot. I am thinking that should be step one....

Re: 1990 4.3 LX 6 cyl Mercruiser that stutters and bog in the high rpms

When was the las time it had a full tuneup? New plugs, cap and rotor? I would also suggest a compression test before you do the on the water test with the remote tank.


List of possible causes of low WOT (Wide Open Throttle) rpm.

In no particular order

Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel
2. Propeller pitch or diameter, damaged blades
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
5. Marine growth on hull and outdrive
6. Wrong gear ratio in outdrive
7. Restricted carburetor air intake (clogged flame arrestor)
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
9. Poor cylinder compression Compression Test
10. Carburetor defective, or wrong type.
11. Fuel pump pressure and vacuum
12. Boat overloaded, improperly loaded, or improperly trimmed.
13. Engine Overheating
14. Engine timing and ignition system operation
15. Remote control cables and linkage for proper travel to open throttle plates fully.

Re: 1990 4.3 LX 6 cyl Mercruiser that stutters and bog in the high rpms

When I hit this RPM range the tach starts to go crazy and drops RPMs.

Click to expand...

Ayuh,... Disconnect the Tach, 'n Try it....

just a Guess, but maybe the tach has an internal short....

Re: 1990 4.3 LX 6 cyl Mercruiser that stutters and bog in the high rpms

Hello,
All Good Ideas. I checked the tach for tightness of connections but never thought about removing it and running the boat. This can be done in short order since I still have all the screws out of the dash.
Don S. It has not had a full tune up since 2007 or 2008. As far as the numbered list the follwing can be checked off for complete/done.
1,2,4,5,6,7,10,11,12,13,&15. This leaves open to check #3, #8(not sure how to check that one), #9, & #14. This is a newer rebuild with aprox 100 hours so I didnt think to check compression. This is a twin set up and I have 2 gas tanks. This stumbling is not happening on starboard engine. I guess I could close off the tank that feeds the stumbling engine so it would be forced to get gas from the other tank.. This would be like the portable tank test but not as good.

Since Don s recommended compression test and I cant explain the sticky tar like substance I found on cylinder 1, I decided this morning to have the engine rebuilder come in and take a look at this and to diagnose the stutter. I like doing things myself but the boat is an hour away and it just seems more efficient to have a local expert who rebuilt the engine 100 hours ago diagnose this. I did get the 2nd engine compression tested last month and it was still good. The stumbling engine usually is stronger than the one that compression tested good so I didnt think about compression testing it. I will report back on what the issue was, once found.

Re: 1990 4.3 LX 6 cyl Mercruiser that stutters and bog in the high rpms

Jlee815 said:

Hello,
All Good Ideas. I checked the tach for tightness of connections but never thought about removing it and running the boat. This can be done in short order since I still have all the screws out of the dash.
Don S. It has not had a full tune up since 2007 or 2008. As far as the numbered list the follwing can be checked off for complete/done.
1,2,4,5,6,7,10,11,12,13,&15. This leaves open to check #3, #8(not sure how to check that one), #9, & #14. This is a newer rebuild with aprox 100 hours so I didnt think to check compression. This is a twin set up and I have 2 gas tanks. This stumbling is not happening on starboard engine. I guess I could close off the tank that feeds the stumbling engine so it would be forced to get gas from the other tank.. This would be like the portable tank test but not as good.

Since Don s recommended compression test and I cant explain the sticky tar like substance I found on cylinder 1, I decided this morning to have the engine rebuilder come in and take a look at this and to diagnose the stutter. I like doing things myself but the boat is an hour away and it just seems more efficient to have a local expert who rebuilt the engine 100 hours ago diagnose this. I did get the 2nd engine compression tested last month and it was still good. The stumbling engine usually is stronger than the one that compression tested good so I didnt think about compression testing it. I will report back on what the issue was, once found.

Click to expand...

Did you ever resolve this? and if so what was the cause?

Re: 1990 4.3 LX 6 cyl Mercruiser that stutters and bog in the high rpms

Who knows? I assume he paid someone to fix it.
At least he could have the courtesy to post the solution to his problem.

Re: 1990 4.3 LX 6 cyl Mercruiser that stutters and bog in the high rpms

Hello Sorry Guys, I missed this post. have been away from the forum. This issue has not been totally cured yet. I just don't run in the high end anymore. We took the shift interrupter,wiring out of the harness and wired it separately thinking that would fix the issue since we thought it was there it did not. We totally eliminated it being fuel related or carb issue by swapping those out temporarily and running of portable tanks. It seems like it may be the Ignition sensor. This happens above my normal cruise speed only. The other thought is it could be the neutral cut out switch in the actual throttle near the driver seat. We have spent 5 hours on diagnose and still no joy. it is getting expense to fix.

Re: 1990 4.3 LX 6 cyl Mercruiser that stutters and bog in the high rpms

I had a similar experience a couple times in my 1989 Sea Ray 280 w/twin 4.3LXs. The battery to that engine (port) was weak, if the engine didn't start in the first couple tries I had to jump them to get the engine to start.

I only experienced the stuttering after I had jumped the batteries to start. Putting in new batteries seems to have cured my problem (knock on wood).

Try swapping your batteries and see what happens. It may not seem logical but it is something quick and easy to try.

Re: 1990 4.3 LX 6 cyl Mercruiser that stutters and bog in the high rpms

you didn't mention if you did a FULL tune-up or disconnected the tach

its possible the sensor is the issue, unless its the OEM latest style (solid black plastic, three wire) it should be replaced, along with your filters, cap, rotor, plugs, wires, etc...

Re: 1990 4.3 LX 6 cyl Mercruiser that stutters and bog in the high rpms

The other thought is it could be the neutral cut out switch in the actual throttle near the driver seat

Click to expand...

Ayuh,.... That is Not the problem, it only comes into play during Starting...