Closed carb mixture screws and 5.0L still runs

[rebuilder45]

Bought the project boat with a rebuilt engine to install so not a lot of history on it.

It's a 5.0L +/- 1999 vintage in 1976 crestline, has rochester 2BBl carb and an aftermarket electric fuel pump.
Engine runs good but consumes a lot of gas - pulling kids on tubes, turning off when changing kids burns about 5 gallons an hour.
I pulled plugs and looks like I'm running rich so I started it up, warmed up and started adjusting the two misture screws per manual. I can screw both of them in fully and the engine still runs and there is only a small improvement if I back them out.
I have sprayed carb cleaner around the outside to check for vacuum leaks and it dosn't appear to be any.

What is the next thing I should check. Electric fuel pump is new and manufacturer stated no regulator needed as it is set at 5-7psi.

Thanks

 

[achris]

Re: Closed carb mixture screws and 5.0L still runs

5 gallons an hour is not a lot for a boat engine, especially using it for tow-sports.

I think your chasing a non-existent problem.

Chris.......

At cruising speed my boat (see below) uses about 25 litres an hour (6.6 US gallons)

 

[alldodge]

Re: Closed carb mixture screws and 5.0L still runs

Bought the project boat with a rebuilt engine to install so not a lot of history on it.

It's a 5.0L +/- 1999 vintage in 1976 crestline, has rochester 2BBl carb and an aftermarket electric fuel pump.
Engine runs good but consumes a lot of gas - pulling kids on tubes, turning off when changing kids burns about 5 gallons an hour.
I pulled plugs and looks like I'm running rich so I started it up, warmed up and started adjusting the two misture screws per manual. I can screw both of them in fully and the engine still runs and there is only a small improvement if I back them out.
I have sprayed carb cleaner around the outside to check for vacuum leaks and it dosn't appear to be any.

What is the next thing I should check. Electric fuel pump is new and manufacturer stated no regulator needed as it is set at 5-7psi.

Thanks

Agree with Chris on a boat burning a lot of fuel. Being able to screw the idel jets in and it barely changing the idle leads me to the carb may need replacing. If the idle jets are turned in and closed and the engine still runs the jets are not working. Keep in mind these are idle jets only, they have no effect on running above idle. You could spend some money on fixing this issue but it may not save you anything at the pump. This is your engine and determining wheather the plugs are carboning up and may cause other issues later but we have not seen them

 

[tank1949]

Re: Closed carb mixture screws and 5.0L still runs

ALLDODGE is 100% correct. Get a buddy and a fire extinguisher. With flame arrester off, cup hand over butterflies and manually advance throttle while moving hands back and forth so motor will not stall. The sucking will scare the crap out of you but I have never heard of someone loosing hand via this method. This ususlly sucks small amouts of trash through carb and out but may not always work.

 

[NHGuy]

Re: Closed carb mixture screws and 5.0L still runs

If it runs with the idle screws closed there is a carb problem. Have you backed down the idle to the specified rate? It should be down around 750 rpm and at operating temperature while doing the adjustment. Once the carb is open enough to run on the main supply the idle mix screws don't do anything.
Also take a look down the carb when the throttle lever is all the way back. That's straight up on most controls. There should be no gap where air can pass. When the throttle is closed the throttle blade blocks the throat. Otherwise, if the throttle is at all open, it's above idle.

 

[NHGuy]

Re: Closed carb mixture screws and 5.0L still runs

Tank's trick will cause air velocity around the inside of the carb throat. It's a quick and easy way of trying to free up stuck particles in the carb and get them burned in the fuel charge. If that doesn't work it's time to fix up the carb.

 

[rebuilder45]

Re: Closed carb mixture screws and 5.0L still runs

Thanks for the help, I did not realize the mixture screws only effected at idle.