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Poor fuel economy hard to start runon issues

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echinton

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"What should a Chevy 4.3L Vort

"What should a Chevy 4.3L Vortex get for fuel economy? This motor is attached to a Cobra Stern drive and pushes my 21' Bayliner (a Full Cabin) which I run entirely in the ocean. I've had lots of issues with this motor and replaced the distributor and carburetor last spring.

I've attempted to check the fuel consumption twice by filling the tank up, going out for a few hours, running the boat on step (25mph @ 2600 RPM ) and then filling it again after the trip and doing the math. I'm coming up with 1.2 to 2.2 miles per gallon.

I think this is lousy mileage but have nothing to compare it too. Also, I believe I have related issues

-First: The motor is difficult to start when cold and won't remain at idle until the motor is warmed (usually I warm the motor at 1200 RPM for approximately 10 minutes). This “troubled idle issue” goes away after the motor is warm and will then idle at approx 600 RPM for the rest of the day without further problem.

-Second: If I run the boat at power for a while and then pull it back to idle and shut the motor off, I get lots of “Run-on” and the motor will not turn off. HOWEVER, if I idle the motor for a few minutes (usually 3 to 5 minutes) to cool it down, the motor shuts off just fine.

My exhaust smoke seems normal (no blue or dark smoke) however, when I run the motor with a garden hose and water muffs on the stern drive, I get soot on the ground from the exhausty water. I’m under the impression that this is normal as I’ve had 4 stroke outboards which did the same thing.

What could be the problem?"
 
"Well there is a start and run

"Well there is a start and run procedure for carbed motors and you described it, almost to a tee.

When one wants to start a carbed motor that is cold one would pump the throttle fully down 3-4 times in neutral leaving the throttle about 1/3 one the way down, in neutral and crank the motor.

After about 3 times cranking no more then 5-7 seconds then pump again 2-3 times.

If after 2-3 times going thru the cycle and the boat does not start then stop and don't kill the battery something is wrong.

When started one needs to allow the motor to warm us and allow the choke to fully open.

Your boats idles good at 600 then your right on.

When running hard the motor and coming to a stop pull back easy and when you do stop allow the motor to run at idle, for 3-4 minutes before shutting down the motor.

Gas question who knows, heavy boat, alot of fat people on board, too mush stuff who knows.

I got a 140 hp 4 banger 86' and use between 4-6 gallons a hour running. 19' cuddy. It's a heavy old style boat.

You got hidden water in the hull maybe? Water logged stringers maybe?"
 
"[i]25mph @ 2600 RPM[/i]
you

"25mph @ 2600 RPM
you might want to try a bit less prop pitch, so you are cruising above 3000 rpm.

seems to be a sweet spot around 3050 rpm on my V6 (pre-vortec). I bought a fuel flow meter for about $140"
 
"I mis-wrote, that was 25 mph

"I mis-wrote, that was 25 mph at 3600 rpm. With the water condition, I wanted to remain consisteant and figured that would be a good RPM to do the fuel check at. I normally cruse at 30 mph / 4000 rpm. I can go faster, but am worried I'd screw somemthing up by running it so hard (waves on the hull or over temping the cylinders). According to SELOC, the max RPM should be 4200 to 4600. I've never gone to 4600 rpm, the conditions where I live are rarely favorable for me to run it that fast.}"
 
"Speed is of no matter. You ha

"Speed is of no matter. You have to dial in the wot.

You will have to see if your motor runs between 4200-4600 rpm.

All you have to do is run at wot for 2 minutes.

If with the prop you have the right rpm then you know it's ok if not might have to go down a pitch or 2.

Verify with a gps.

My sweet spot is 3000 also, seems to be the sweet spot for my friends boats also.

PS i run in Raritan bay rough also, seems 20-25 is fast enough."
 
"Eric:

the soot in the exha

"Eric:

the soot in the exhaust is carbon that gets flushed out by the cooling water. most likely cause is a rich mixture.

I wouldn't call it 'normal' but it is quite common. Could be due to a number of things. Unless you spend a lot of time idling or trolling, it's probably not cost effect to have someone try to fix it.

The advice above is sound. That sweet spot is the thing to shoot for, once you make sure the prop is right with the engine in good shape.

Remember, the faster the engine runs, the more gas it will consume."
 
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