No power past 4000 RPMs

[cdc_uky]

1990 Mercruiser 4.3 with alpha gen 1 lower unit

• It gives me nothing beyond 25 MPH/3800-4000 RPMs under load
• It does not bog down when I push the throttle further at that speed…just no response
• Starts fine, idles fine, revs well beyond 4000 RPMs on muffs/in neutral
• The engine sounds normal and isn’t smoking or anything like that
• It has good gas in it
• The last tune up with new cap, plugs and wires was about 4 seasons ago
• The carb was rebuilt 3 seasons ago
• It was winterized properly last season

My immediate thought is the carb but it is hard to believe with it running so well in every other aspect. The boat has always "cut out" just below 40 MPH since the day I bought it 7 years ago. Should I be thinking ignition coil? Is it actually the carb?

 

[strum]

Just a shot in the dark here but make sure all plugs are firing before anything else then from there I be looking at the carb. main jet(s)

 

[dubs283]

• The last tune up with new cap, plugs and wires was about 4 seasons ago
• The carb was rebuilt 3 seasons ago
• Is it actually the carb?

Start with a full tune up, replace fuel filter(s) and inspect contents. I'm guessing this is thunderbolt ignition? Have you replaced the ignition sensor with the latest OEM three wire and properly grounded?

Could possibly be the carb, check fuel pressure/vacuum

 

[Scott06]

Start with a full tune up, replace fuel filter(s) and inspect contents. I'm guessing this is thunderbolt ignition? Have you replaced the ignition sensor with the latest OEM three wire and properly grounded?

Could possibly be the carb, check fuel pressure/vacuum

Start with compression test then In addition to this check the ignition timing not only initial timing at idle but that you are getting the correct advance as rpm raises

 

[achris]

Those had a Rochester Carb, yes? If so, that sounds like a clogged power valve (or a fuel supply problem).

Chris........