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4.3L Mercruiser running slow

L

ledwell

New member
I have a 2007 Tahoe Q6 with a carburated Mercruiser 4.3L V6. I just put the boat in the water a couple of days ago for the first time this year. The problem I am having is it is running slow. The boat will normally top out around 53MPH(Checked with a GPS). Now it will only run aprox 40MPH. It runs very smooth, just doesn't seem to open up. I ran it for a while no change. Today I took it out and it would only run maybe 10-15mph for a few minutes then it got to where it would top out around 40MPH. I replaced the water separating fuel filter and connected the motor directly to a fresh can of gas(to rule out bad fuel in the tank). The problem didn't change. :(

I would appreciate any advice someone might have. Thanks
 
sandkicker said:
What about RPMs? (@ 53 mph and 40 mph)
Click to expand...

Hi Sandkicker,

Thanks for the reply. I haven't paid attention to the RPMs in the past to compare it to. But it doesn't seem to be reving high.(in other words, I think the RPMs are probably normal for 40mph, it's just not normal for 40mph to be the top end) It just acts as if the throttle isn't opening all the way.

Thanks,
Jamie
 
So, pay attention to the tach, WOT should be 4600-4800rpm.

Things that will slow your engine down.

A dead cylinder
no compression in a cylinder
a bad dist cap
failing ignition coil
too much oil in the crank case
incorrect ignition timing
Fouled Carb
not able to achieve full throttle at the carb.
 
Docksidemarineservices said:
So, pay attention to the tach, WOT should be 4600-4800rpm.

Things that will slow your engine down.

A dead cylinder
no compression in a cylinder
a bad dist cap
failing ignition coil
too much oil in the crank case
incorrect ignition timing
Fouled Carb
not able to achieve full throttle at the carb.
Click to expand...

i.e.., failure of an engine to run within the factory specified WOT RPM range is one of the FIRST things you look at. Speedo is irrelevant. For any kind of engine life, you should not normally run your engine continuously any faster than 80% of the maximum RPMS that your engine achieves ( assuming said max is within the factory specified, 4600-4800, range).
 
paraphrasing an old list that has been kicking around forums for a while

Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
Marine growth on hull and outdrive
Wrong gear ratio in outdrive
Restricted carburetor air intake (clogged flame arrestor)
Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
Poor cylinder compression
Carburetor issue - secondaries not opening or choke not opening
Fuel pump pressure and vacuum
Boat overloaded, improperly loaded, or improperly trimmed.
Engine Overheating
Engine timing and ignition system operation or cam jumped a tooth
Remote control cables and linkage for proper travel to open throttle plates fully
Shift Interrupt system switches sticking or pulling current to ground through partial short in switch or wiring
 
Thanks for all the replies. I will run it and check the tach.

Considering it is running smooth and the fact it was running fine when I put it away for the season, I think I can rule out several things.

Here are the possibilities I'm thinking:
The little fuel screen where the fuel line connects to the carb(that I didn't know existed)
The choke not opening
Fuel pump not putting out enough pressure
Other carburetor issues

I'm going to mess with it some more and see what I can figure out.
Thanks
 
I got it. I did the old test of pulling off plug wires one at a time looking for RPM drop and noticed it didn't drop on one of the cylinders. Changed the spark plugs and it seems to be running fine. I'm very surprised how smooth the motor seemed to run on 5 cylinders.

Anyway, thanks for all the suggestions.
 
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