Mikuni Carb/fuel delivery issue

Mini Bike & Go-Kart Parts

mustangfrank

Well-Known Member
#23
For testing I'd take the fuel pump out of the loop and just gravity feed the fuel, maybe put a 90 degree fitting right at the carb to eliminate the high arc. Unless I'm seeing it wrong in the pics, it looks like the tank is high enough above the bowl/inlet.

Not causing your issue but I would just eliminate the filter after the pump, the filter right out of the tank is all you need. I'm not a fan of the sintered metal filters but the clear bodies are nice.
 
#24
I'm not a minibike fuel pump expert.
but i have had a lot of success using minibike fuel pumps.
every minibike fuel pump i have seen, needs a vacuum pulse from the intake manifold.
Taking it from anywhere else is pressure, not vacuum.
Not sure why people just hook the fuel pump up to whatever they happen to see?

My fuel pump needs a vacuum pulse, which requires an intake manifold with a vacuum fitting.
intake can be drilled/tapped to accept a barbed fitting



Lower line is vacuum, upper line is fuel to carb. Motor turned over 9k RPM with zero fuel issues.

"the front of the block"
is not the proper place to power the fuel pump imho.
you need a vacuum source.
Also filter should be before pump in my experience
you are trying to apply a pressure signal (crankcase pressure) when a vacuum signal (the complete opposite and is only generated by the piston at the intake manifold) is required.
 
Last edited:
#25
For testing I'd take the fuel pump out of the loop and just gravity feed the fuel, maybe put a 90 degree fitting right at the carb to eliminate the high arc. Unless I'm seeing it wrong in the pics, it looks like the tank is high enough above the bowl/inlet.

Not causing your issue but I would just eliminate the filter after the pump, the filter right out of the tank is all you need. I'm not a fan of the sintered metal filters but the clear bodies are nice.
So I tried gravity feeding it and same thing still happened. Even with a full tank the fuel doesn’t go the line into the carb. But when I drain it into a gas can, it flows perfectly fine. I also tried getting rid of the filters and still no fuel down the lines.
 
#26
I'm not a minibike fuel pump expert.
but i have had a lot of success using minibike fuel pumps.
every minibike fuel pump i have seen, needs a vacuum pulse from the intake manifold.
Taking it from anywhere else is pressure, not vacuum.
Not sure why people just hook the fuel pump up to whatever they happen to see?

My fuel pump needs a vacuum pulse, which requires an intake manifold with a vacuum fitting.
intake can be drilled/tapped to accept a barbed fitting



Lower line is vacuum, upper line is fuel to carb. Motor turned over 9k RPM with zero fuel issues.

"the front of the block"
is not the proper place to power the fuel pump imho.
you need a vacuum source.
Also filter should be before pump in my experience
you are trying to apply a pressure signal (crankcase pressure) when a vacuum signal (the complete opposite and is only generated by the piston at the intake manifold) is required.
I’ve tried using every port available on my engine even the valve cover. Tried the sucking and blowing thing to simulate it pumping fuel, but still nothing is working.
 

mustangfrank

Well-Known Member
#27
So I tried gravity feeding it and same thing still happened. Even with a full tank the fuel doesn’t go the line into the carb. But when I drain it into a gas can, it flows perfectly fine. I also tried getting rid of the filters and still no fuel down the lines.
So I tried gravity feeding it and same thing still happened. Even with a full tank the fuel doesn’t go the line into the carb. But when I drain it into a gas can, it flows perfectly fine. I also tried getting rid of the filters and still no fuel down the lines.
Try this...keep the gravity feed for now, install the carb with the bowl removed. Leave the float and needle installed and manually work the float to check for fuel flow and cut-off and diagnose where the blockage or issue is. Obviously use a catch pan to keep fuel from getting all over the frame and floor.

This engine still runs though, correct? I know you mentioned the stalling off choke but I'm starting to wonder if you're seeing air or bubbles in the fuel line which may be normal, especially with that high loop going into the vertical carb inlet and utilizing a fuel pump. My lawn tractor fuel system always looks wrong to the eye, gas only sits at less than halfway filling the clear filter and just trickles through the line to the carb, never filling the line. Been that way since new, ran great but I thought I was getting air in the lines from somewhere, took me a few weeks to see some other tractors just like it at a dealer were exactly the same.
 
#29
Try this...keep the gravity feed for now, install the carb with the bowl removed. Leave the float and needle installed and manually work the float to check for fuel flow and cut-off and diagnose where the blockage or issue is. Obviously use a catch pan to keep fuel from getting all over the frame and floor.

This engine still runs though, correct? I know you mentioned the stalling off choke but I'm starting to wonder if you're seeing air or bubbles in the fuel line which may be normal, especially with that high loop going into the vertical carb inlet and utilizing a fuel pump. My lawn tractor fuel system always looks wrong to the eye, gas only sits at less than halfway filling the clear filter and just trickles through the line to the carb, never filling the line. Been that way since new, ran great but I thought I was getting air in the lines from somewhere, took me a few weeks to see some other tractors just like it at a dealer were exactly the same.
Ill give that a try in the morning where I am. Also the thing you mentioned with the tractor, that is exactly what mine is doing. Did you ever find a solution to it? Maybe that could help?
 
#30
Does your intake manifold have a pulse fitting installed?
Valve cover is also pressure, not vacuum.

Maybe the diaphragm in the fuel pump is faulty?
There is no pulse fitting on my manifold. Also, I see people using the exact same pump as me and they have no problem using the valve cover as a source of vacuum and the pump works fine, only when its directly hooked up to the carb does it not work
 

mustangfrank

Well-Known Member
#31
Ill give that a try in the morning where I am. Also the thing you mentioned with the tractor, that is exactly what mine is doing. Did you ever find a solution to it? Maybe that could help?
It was just the way they are, the fuel line was a long, lazy routing from the rear of the tractor mounted tank to the fuel filter and pump. The fuel filter made the line sag a little so it was an uphill run to the carb, I think the bowl vent or tank vent would let air in the line, it was just normal for that system to always have a 4-5 inch "bubble" in the line that made it look empty. Replaced several fuel filters over the years and even the pump (same round type) once on a mis-diagnosis for no-hot restart but it always operated the same way and ran perfect.
 

desert rat

Well-Known Member
#32
I've been trying to stay out of this one cause I don't want to do all the typing but lets go. Not open for discussion is the best location to get the pulse for the fuel pump. That's right these pumps need both vacuum and pressure to work at their best. The best location to get that is the crank case. Before you all go there yes the valve cover is a location that is like that but it ain't the same, it is too far away from the piston to get a clean +/- pulse. OK on to point #2 these pump push fuel much better than they pull so pump at or below the tank pushing fuel up to the carb. Point #3 tank above carb there is no need for a pump. Case in point Honda 750cc four cylinders have four carbs bigger than what we are dealing with here with no pump on a single fuel line. So now is time for the fix, I know you are on the edge of you tool box waiting for it. You have a plugged float bowl vent, kinda like trying to blow air into a mustard bottle. Not happening right? So to check me on this fix, mount the DRY fuel tank above the carb with a clear fuel line direct to the DRY carb with nothing in between. Put fuel in the tank and see if fuel flows to the carb, if not move to step two. Step two loosen the four screws holding the float bowl to the carb, don't remove them just a single turn on each should be fine. If fuel begins to flow the problem is confirmed to be a clogged bowl vent. Once that problem is out of the way you can reintroduce the pump and do whatever you can get away with and we can fix that.
 
#33
All i know is that i have a 33mm Mikuni on alcohol (twice as much fuel volume requirement) and i get the signal from the intake manifold. Motor makes ~25hp on chassis dyno and turns 9k RPM and never had a single issue.
Look at literally any junior dragster engine and they all have the same setup.
Crankcase pressure is very high and will push oil out into the catch can, from the valve cover and from the side cover.
Not sure if it's different because it's a flathead VS OHV?
 
#34
I've been trying to stay out of this one cause I don't want to do all the typing but lets go. Not open for discussion is the best location to get the pulse for the fuel pump. That's right these pumps need both vacuum and pressure to work at their best. The best location to get that is the crank case. Before you all go there yes the valve cover is a location that is like that but it ain't the same, it is too far away from the piston to get a clean +/- pulse. OK on to point #2 these pump push fuel much better than they pull so pump at or below the tank pushing fuel up to the carb. Point #3 tank above carb there is no need for a pump. Case in point Honda 750cc four cylinders have four carbs bigger than what we are dealing with here with no pump on a single fuel line. So now is time for the fix, I know you are on the edge of you tool box waiting for it. You have a plugged float bowl vent, kinda like trying to blow air into a mustard bottle. Not happening right? So to check me on this fix, mount the DRY fuel tank above the carb with a clear fuel line direct to the DRY carb with nothing in between. Put fuel in the tank and see if fuel flows to the carb, if not move to step two. Step two loosen the four screws holding the float bowl to the carb, don't remove them just a single turn on each should be fine. If fuel begins to flow the problem is confirmed to be a clogged bowl vent. Once that problem is out of the way you can reintroduce the pump and do whatever you can get away with and we can fix that.
Thanks for the advice. In fact, Step 1 of your solution is exactly what is happening to me when I try and do that. I have to go to work soon so Ill give step 2 a shot later on today.
 
#35
All i know is that i have a 33mm Mikuni on alcohol (twice as much fuel volume requirement) and i get the signal from the intake manifold. Motor makes ~25hp on chassis dyno and turns 9k RPM and never had a single issue.
Look at literally any junior dragster engine and they all have the same setup.
Crankcase pressure is very high and will push oil out into the catch can, from the valve cover and from the side cover.
Not sure if it's different because it's a flathead VS OHV?
From what I remember (correct me if I'm wrong) flatheads are a little different in terms of vacuum/pressure, so what you might have to do for a fuel pump, might be different for people running OHV. Also that looks/sounds like a mean ass flathead. I looked at the picture before and noticed that thing is tricked out, cool build!
 

SAS289

Well-Known Member
#36
Do these pumps have a return spring for the diaphragm? If so there's no need for any positive air pressure. Once negative pressure releases the spring will return the diaphragm. If it has a spring it should work anywhere on the engine that get's a "pulse" to negative.
 
#37
Do these pumps have a return spring for the diaphragm? If so there's no need for any positive air pressure. Once negative pressure releases the spring will return the diaphragm. If it has a spring it should work anywhere on the engine that get's a "pulse" to negative.
I know mine does not use a spring. Others may use a spring?
 

desert rat

Well-Known Member
#38
All i know is that i have a 33mm Mikuni on alcohol (twice as much fuel volume requirement) and i get the signal from the intake manifold. Motor makes ~25hp on chassis dyno and turns 9k RPM and never had a single issue.
Look at literally any junior dragster engine and they all have the same setup.
Crankcase pressure is very high and will push oil out into the catch can, from the valve cover and from the side cover.
Not sure if it's different because it's a flathead VS OHV?
You are in a whole different class of motor.
 

desert rat

Well-Known Member
#39
Do these pumps have a return spring for the diaphragm? If so there's no need for any positive air pressure. Once negative pressure releases the spring will return the diaphragm. If it has a spring it should work anywhere on the engine that get's a "pulse" to negative.
MOST do not use a return spring which is why they are needing the +/- pulse
 
#40
I've been trying to stay out of this one cause I don't want to do all the typing but lets go. Not open for discussion is the best location to get the pulse for the fuel pump. That's right these pumps need both vacuum and pressure to work at their best. The best location to get that is the crank case. Before you all go there yes the valve cover is a location that is like that but it ain't the same, it is too far away from the piston to get a clean +/- pulse. OK on to point #2 these pump push fuel much better than they pull so pump at or below the tank pushing fuel up to the carb. Point #3 tank above carb there is no need for a pump. Case in point Honda 750cc four cylinders have four carbs bigger than what we are dealing with here with no pump on a single fuel line. So now is time for the fix, I know you are on the edge of you tool box waiting for it. You have a plugged float bowl vent, kinda like trying to blow air into a mustard bottle. Not happening right? So to check me on this fix, mount the DRY fuel tank above the carb with a clear fuel line direct to the DRY carb with nothing in between. Put fuel in the tank and see if fuel flows to the carb, if not move to step two. Step two loosen the four screws holding the float bowl to the carb, don't remove them just a single turn on each should be fine. If fuel begins to flow the problem is confirmed to be a clogged bowl vent. Once that problem is out of the way you can reintroduce the pump and do whatever you can get away with and we can fix that.
ok so I did what you told me to do and this is what happened. When I unscrewed each bolt 1 turn, reattached it to the manifold, I tried to feed it fuel but it was flowing a little bit, but when I took it off the manifold, gas just rushed in and started to overflow the carb and gas got all over me. Again I reattached it, and all fuel flow stopped dead in its tracks. Also, where would the float bowl vent be on a mikuni 22? Ill try and clean it and see if it fixes anything.
 

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