4.3 Merc won't rev past 4000 rpm

[johnnyb-21]

My 2002 Regal 1800LSR with 4.3 mrec 2bbl carb won't rev past 4000 rpm. It's been that way since I bought it earlier this year. Engine runs flawless in all throttle ranges including WOT, but hits 4000 rpm at approx 75% throttle and won't rev any farther. I had 21p prop, would top out at 41mph, tried 19p prop, speed dropped to 36mph but still 4000 rpm. This boat should do 52mph at 4500rpm with 21p prop.
Neutral switch tested ok. Replaced distributor cap, it has electronic advance, fuel and water filters replaced, oil change (made sure not to overfill). Throttle plates open fully at WOT. Lower unit has correct ratio.
The boat only has 100hrs on it. I'm out of anwsers, does anyone have any suggestions?

 

[Bondo]

Re: 4.3 Merc won't rev past 4000 rpm

My 2002 Regal 1800LSR with 4.3 mrec 2bbl carb won't rev past 4000 rpm. It's been that way since I bought it earlier this year. Engine runs flawless in all throttle ranges including WOT, but hits 4000 rpm at approx 75% throttle and won't rev any farther. I had 21p prop, would top out at 41mph, tried 19p prop, speed dropped to 36mph but still 4000 rpm. This boat should do 52mph at 4500rpm with 21p prop.
Neutral switch tested ok. Replaced distributor cap, it has electronic advance, fuel and water filters replaced, oil change (made sure not to overfill). Throttle plates open fully at WOT. Lower unit has correct ratio.
The boat only has 100hrs on it. I'm out of anwsers, does anyone have any suggestions?

Ayuh,... Welcome Aboard,.... Lotsa suggestions, click here...

 

[cr2k]

Re: 4.3 Merc won't rev past 4000 rpm

Maybe your tach is set to 8 cyls. My Regal was.

 

[90stingray]

Re: 4.3 Merc won't rev past 4000 rpm

This boat should do 52mph at 4500rpm with 21p prop.

Is that what the manufacturer states it will do? Are you trimmed out? Also, the prop change numbers don't make any sense. Is the motor backfiring or surging at WOT?

 

[achris]

Re: 4.3 Merc won't rev past 4000 rpm

Maybe your tach is set to 8 cyls. My Regal was.

I've run the number through my prop 'angle of attack' calculator and they all seem right, so I suspect the tacho is reading ok. The other numbers indicate that what you are seeing is accurate, so the problem really is that the engine is only making 4000. Just check that you have the right ignition module....

I'd start hunting this one down with a timing light and the manual for testing all the different systems within the ignition system. If the knock sensors or module is giving a false signal that could easily account for the loss of revs at the top.....

Chris........
EDIT: Just done some reading up on the Thunderbolt V system, and 4000rpm is the magic number. It's where the acceleration advance and the MBT advance both cut off, but the knock control retard continues beyond that. I would definitely be studying the advance curves in the manual and checking with a timing light. The other possibility is that you have a really bad tank of fuel and the ignition system is doing all it can to protect your engine....