Yamaha 4.3 with Rochester Carb Flooding

[BigChris]

My father's boat.

Yamaha outdrive 4.3L V6 with Rochester Quadrajet on it. Electric fuel pump.
Fuel pump looks exactly like this : http://marineenginedepot.com/MP/Mari...-Low-Pressure/

Floods the carb out at idle. With the spark arrestor off, looking down the primaries, fuel can be seen pouring in the primaries and out the throttle linkage.

So I replaced the needle and seat. With engine off, and fuel pump running (either from cranking or supplying power to the pump) it's fine. But as soon as you fire the engine, the pump overwhelms the needle and seat.

I put a fuel pressure gauge on it, pump is making 8psi, which in my experience with carburated engines, seems a little much. Shouldn't it be closer to 5psi?

This problem is relatively new, and occured like a light switch, it ran fine earlier this year and then Saturday starting doing this.

Is there something I'm overlooking?

 

[Bondo]

This problem is relatively new, and occured like a light switch, it ran fine earlier this year and then Saturday starting doing this.

Ayuh,.... If it was runnin' just fine with this fuel pump before,.....

I'd sooner think a float is sinkin' in the carb,....

Pop the top off, 'n weigh the floats,....

 

[Capt Ken]

I didn't read anything about cleaning the carb? Clogged vents can cause the same problem.

 

[BigChris]

I didn't read anything about cleaning the carb? Clogged vents can cause the same problem.

The vents for the float bowl on the Quadrajet are centrally located in the carb, one is near the fuel inlet, the other near the air filter mounting stud. These vents are approximately .300" in dia and will easily allow an object like a pencil to pass through them. They are not clogged.

 

[BigChris]

Ayuh,.... If it was runnin' just fine with this fuel pump before,.....

I'd sooner think a float is sinkin' in the carb,....

Pop the top off, 'n weigh the floats,....

I guess in a round about way I was asking if a possible failure mode of this type of fuel pumps is to provide too much pressure. Counter-intuitive, I know, but stranger things have happened.

A new float can be had for $17. I guess I'll just go that route.

 

[Bondo]

Ayuh,..... I agree 8 psi is atad high, but I believe within reasonable,....

I've run that very same pump in several applications, Without a problem,....

I'd think the carb Should be able to handle that pressure,....

Floats, 'n needle valves are the only thing I can think of that'll cause that condition, but I don't do Quadra-jets either,....

 

[Capt Ken]

From your reply I see you are not familiar with the workings of the Rodchester and the Venturi vents so good luck.

 

[BigChris]

From your reply I see you are not familiar with the workings of the Rodchester and the Venturi vents so good luck.

Oh please do explain, I make mistakes all the time.

 

[jimmbo]

8 psi is on the high side, but usable. I suspect the float is set too high. Or the float has absorbed fuel and is too heavy. One other thing, you said you changed the needle and seat? Did the new seat have the same size inlet as the old one. I assume you did check the float height after the replacement?

 

[BigChris]

Carb visibly flooded, you could look down the carb and see it as engine struggles to stay running and eventually stalls.

Changed needle and seat. Differences were the old needle had a blunt tip that was factory. New needle came to a point. Old seat had two slots machined into the side of it, new did not. Upon visual inspection, both orifaces in the seat appeared to be the same size.

Replaced needle and seat as pair, along with steel shim under seat. Kit wanted 3/16" float height from top of carb, I gave it 1/4".

Installed air horn, started engine, no change. Engine runs fine until fuel can be observed pouring into primary venturis.

Removed air horn, verified alignment of float, watched brief youtube video on needle and seat of the rochester. I made the mistake of hanging the little needle "retriever wire" from one of the holes in the rear of the float arm, I corrected that. (that was the way it was in there when I opened it up)

Removed power source to coil, held float pivot clip down with finger, had my father crank engine, float appeared to "hold". Reassembled, fired engine, no change.

Removed air horn, rigged remote power supply to fuel pump. Held float pivot clip down with finger (allowing float to pivot normally with out air horn holding it down).
Powered fuel pump. Only when the pump is making its maximum pressure does it force open the needle and seat ever so slightly. I suspect when you add the vibration of the engine running it only helps the pump force open the needle and seat. You can hear the pump really work here when its dead-headed.

Just to take a big swing at it, I changed float height to 1/2" below top of carb. performed above test with air horn still off, held pressure. Reassembled, still floods, although it seams like it takes a little longer for it to happen.

So I use the old seat to fabricate an adapter for my fuel pressure gauge (which is also my vacuum gauge, it does both, it's one for roundy round race cars), and I see 8psi. So I think to myself, that seems like alot. In dirt oval, we'd never run more than 5, I should ask those guys on the forum bout that.

Block does not have provisions for mechanical pump.

New float, and another rebuild kit arrives from Summit today. I'll fiddle with it some more down at the Cheasapeak this weekend when I go down. Once I get it sorted, I'll be re-replacing the air horn gasket as I've had it on and off quite a few times.

 

[jimmbo]

8 psi is at the high end of the preferred range.
You could install a fuel pressure regulator and drop the pressure down to 5 psi.
You did use a gasket under the seat?

 

[Daves70]

Gasket under the seat like the other poster said and anytime I have a rochester carb apart I replace the float. I've in the same situation as you before and it's been the float. I don't like electric fuel pumps pumps on boats period but one reason is most carbs can only handle 6 psi. Sometimes you get away with it sometimes not.

 

[BigChris]

Replaced needle and seat as pair, along with steel shim under seat.

New float = problem solved.