Is there a way to reduce vacuum to the fuel pump

[Superchief500]

Still at it... this Chaparral G50B is kicking my butt. After months of fiddling and conversations, I am at my last effort to fixing this thing. I am using a VM38 Mikuni carb. The vacuum fuel pump is overpowering the floats causing fuel to dribble out of the overflow. I either need to turn down the pressure to the pump or reduce the vacuum to the pump. I have had many conversations with Sudco and the tech thought I needed to drop the gap needle and seat to a 1.5 so I ordered one installed it and there is no change. The needle and seat hold 10psi with no leaks. Then he suggested I reduce the line size by putting a smaller line inside the bigger vacuum line so I did that and the problem is worse by far. I am thinking I need to go with bigger line. I am using 5/16 now. Does anyone have any other suggestions? Thanks

[james gerber]

I have been through issues in the past like this. Make sure your VM38 is the one that is set up for a fuel pump and not gravity feed. if i remember right the carb that fit mine and worked correct was a Mikuni VM-38/21 (i still had to jet it to fine tune)
I hope this helps and good luck.

[Bennett20200]

if all else fails, would you consider a low pressure electronic fuel pump? set it for 2 pounds of pressure & see if you still have an issue. If you do still have the same issue, you know its not a fuel pressure issue, something must be wrong in the carb. If you don't have the issue anymore, then you could consider just bypassing the other fuel pump and leaving the electronic one on there. Those little clickity-clack pumps are pretty cheap too, right around $20-25 dollars.

[jerseybigfoot]

Without going for a Mikuni snowmobile carb you can add a Tee to the carb fuel inlet and connect a return hose back to the tank . bring the return hose a few inches higher than the carb. Unfortunately you have to make a return port in the tank.

[dave phelps]

What vacuum pump are you using??

[Superchief500]

if all else fails, would you consider a low pressure electronic fuel pump? set it for 2 pounds of pressure & see if you still have an issue. If you do still have the same issue, you know its not a fuel pressure issue, something must be wrong in the carb. If you don't have the issue anymore, then you could consider just bypassing the other fuel pump and leaving the electronic one on there. Those little clickity-clack pumps are pretty cheap too, right around $20-25 dollars.

I haven't thought about that. It's worth looking into. Do they require a return line? Thanks!

[Superchief500]

I have been through issues in the past like this. Make sure your VM38 is the one that is set up for a fuel pump and not gravity feed. if i remember right the carb that fit mine and worked correct was a Mikuni VM-38/21 (i still had to jet it to fine tune)
I hope this helps and good luck.

I wondered if there was a different carb for a fuel pump application. I have been searching high and low for information on such a carb with no luck. I will have to look into that! That is great information thank you very much!

[Superchief500]

Without going for a Mikuni snowmobile carb you can add a Tee to the carb fuel inlet and connect a return hose back to the tank . bring the return hose a few inches higher than the carb. Unfortunately you have to make a return port in the tank.

That makes sense. I was toying with the idea of an adjustable fuel regulator but needed a return line for that too. I abandoned that idea because I couldn't find a leak proof fitting for the tank. With my luck, if there is such a thing, it would leak then I am screwed

[Superchief500]

What vacuum pump are you using??

I am using a Mikuni vacuum pump. From what I understand, they don't produce more than 4psi.

[Superchief500]

I wonder if a fuel shut off valve in the vacuum line would work. Close the valve a little at a time to reduce vacuum just enough to stop the fuel from dribbling out. I know... I am grabbing straws here

[jerseybigfoot]

it will create a pressure drop at a specific flow rate. reduced flow but will still probably flood at idle. you could loop the fuel line back to the fuel pump inlet through a tee at the carb but you should add a a valve to atmosphere in the return loop to vent the air out of the line to make sure the the line is full. air in the line will probably give the pump fits..

[racerone3]

you could put a "T" in the pulse line and use a needle valve to vent a controlled amount to atmosphere

[Bennett20200]

I haven't thought about that. It's worth looking into. Do they require a return line? Thanks!

Nope, no return line. Just make sure you get a low pressure pump and you should be good to go. If you're still overpowering your needle with only 2-3 PSI coming from the fuel line then you know the issue is your needle and seat and not a pressure issue. Good luck, let us know how it works out!

[budzinsk]

I am dealing with this same issue. I got a new Mikuni Pulse pump and I have a 73 Attex with a mikuni carb on it. New floats and new needle. Please let me know what you find out...

[dave phelps]

I am dealing with this same issue. I got a new Mikuni Pulse pump and I have a 73 Attex with a mikuni carb on it. New floats and new needle. Please let me know what you find out...

I just went through this myself. Mikuni vm30 original superchief carb and the fuel pump was overpowering the needle and seat. put new needle and seat in and still the same. Ended up replacing pulse pump with a new one and problem solved. Would've never guessed that!!