Mercruiser 4.3 Wont Reach WOT RPM

needarp

Cadet
Joined
Aug 21, 2013
Messages
9
Hi all,

I've got a 1997 19.5' Spectrum with a 4.3 LX Gen + Alpha 1 setup. I bought the boat 2 years ago. It runs great, but I can't get it to rev past 4100 rpm. Originally, I figured the boat was over-propped with a 21 pitch prop, so I bought a new Black Max 19 pitch and installed it. Still bang on 4100 rpm.

I performed a lot of maintenance on the boat this winter:

-Pulled the heads, all new gaskets from the block up
-New coil, plugs, wires
-New intake manifold, thermostat
-New fuel filter/separator & carb inlet filter
-Checked base timing, bang on 8 degrees
-Compression check: 168 - 172 across all cylinders
-Rewired all power cables (a lot of corrosion on old cables)


Engine starts, idles, seems to run perfectly. Feels under-powered on the bottom end and still will not rev past 4100 rpm.

So far I know:
-Timing appears to be good
-All new ignition components except ICM and distributor sensor
-New fuel
-Carb seems to be opening completely on full throttle
-No fuel filter restrictions

I've seen a lot of threads on this particular issue over the years, but I haven't seen any where the issue has been resolved. I'd appreciate any insights/advice.
 

CFM56-7

Seaman Apprentice
Joined
Oct 22, 2013
Messages
49
Re: Mercruiser 4.3 Wont Reach WOT RPM

Hello needarp,

Just throwing this out there, but have you confirmed your timing is advancing correctly? I can't give you any specifics for your motor, but if you have the correct manual, there should be a chart depicting the timing curve.
 

MikDee

Banned
Joined
Jun 6, 2007
Messages
4,745
Re: Mercruiser 4.3 Wont Reach WOT RPM

Recheck your timing, was it set in base mode procedure to 8*? Plus, is your flame arrestor (air filter clean, or clogged?). Also, is your fuel tank vent clear? Finally is your fuel tank tube strainer, & anti-siphon valve clean, & clear? One more item, is your carb opening all the way?
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,468
Re: Mercruiser 4.3 Wont Reach WOT RPM

Hi all,

I've got a 1997 19.5' Spectrum with a 4.3 LX Gen + Alpha 1 setup. I bought the boat 2 years ago. It runs great, but I can't get it to rev past 4100 rpm. Originally, I figured the boat was over-propped with a 21 pitch prop, so I bought a new Black Max 19 pitch and installed it. Still bang on 4100 rpm.

I performed a lot of maintenance on the boat this winter:

-Pulled the heads, all new gaskets from the block up
-New coil, plugs, wires
-New intake manifold, thermostat
-New fuel filter/separator & carb inlet filter
-Checked base timing, bang on 8 degrees
-Compression check: 168 - 172 across all cylinders
-Rewired all power cables (a lot of corrosion on old cables)


Engine starts, idles, seems to run perfectly. Feels under-powered on the bottom end and still will not rev past 4100 rpm.

So far I know:
-Timing appears to be good
-All new ignition components except ICM and distributor sensor
-New fuel
-Carb seems to be opening completely on full throttle
-No fuel filter restrictions

I've seen a lot of threads on this particular issue over the years, but I haven't seen any where the issue has been resolved. I'd appreciate any insights/advice.

Ayuh,..... Welcome Aboard,..... I'm sure yer answer is in this link from Don's Adults Only section,...

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

jrampey

Petty Officer 2nd Class
Joined
Jan 22, 2005
Messages
157
Re: Mercruiser 4.3 Wont Reach WOT RPM

Is your tach accurate?
 
Top