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Author Topic: Running a fuel pump on a carbureted motorcycle?  (Read 1709 times)
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wobblywalrus
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« Reply #15 on: April 20, 2018, 08:33:35 AM »

A 1000cc cylinder has over 2.5 times the displacement of a single cylinder of a 750 Triumph.  The harley guys are putting carbs on cylinders of nearly that size.  They might have the info you need.  RB Racing might be a good source.
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Vinsky
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« Reply #16 on: April 20, 2018, 09:40:42 AM »

This is a modification you may want to check out. Claims to increase flow.
https://daytonaparts.com/daytona-carburetor-float-valve.html
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John
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« Reply #17 on: April 21, 2018, 12:46:47 PM »

The Daytona Float valve sounds a little suspicious to me. If it were that great, why wouldn't they show some real flow numbers to back up their claims?  In my opinion, for a given fuel pressure, the only way to get more flow would be to increase the size of the float valve seat.  But that could lead to flooding as the float would need to be larger in volume to counteract the force which is determined by the fuel pressure multiplied by the area of the float valve. Furthermore, their claim of being able to control the air/fuel ratio with the design of the float valve is certainly not something I use to control the A/F ratio.  Yes, a bouncing float can cause flooding and insufficient flow can cause a lean condition, but if the float valve is large enough, that shouldn't happen.

Attaching a large external float bowl with a larger float volume  (as generatorshovel has done) would seem to be the better solution as the larger float and probably larger float valve should be able to cope with the pressure of a fuel pump as well as allow more flow especially as required when using Methanol.  The Amal Matchbox float (although I haven't got one here to measure) may be large enough to cope with higher flows and the increased pressure of a fuel pump, and it mounts on a diaphragm to isolate it somewhat from vibration.  You may also want to increase the size of your fuel lines and petcock as you want to make sure the float valve is the most restrictive part of the fuel system if you don't use a pump.

Tom
« Last Edit: April 21, 2018, 12:51:25 PM by Koncretekid » Logged

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wobblywalrus
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« Reply #18 on: April 21, 2018, 02:02:01 PM »

That is the problem I had.  The fuel pump produced too much pressure for the big needle valve that I needed to pass the flow.  The needle valve would not close.  That is why the two tank system with a bypass and gravity flow to the carb was used.

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panic
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« Reply #19 on: April 24, 2018, 02:52:24 PM »

Mikuni makes different size needle & seat assemblies at least for the VM (I think the # is the size in mm: 1.5... 3.5) with the small ones for snow machines with pumps and the largest for gravity only.
The Mikuni diaphragm fuel pump cannot use venturi vacuum, it relies on pressure cycling, so better crankcase evac reduces efficiency.
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mtiberio
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« Reply #20 on: April 24, 2018, 03:49:07 PM »

Mikuni makes different size needle & seat assemblies at least for the VM (I think the # is the size in mm: 1.5... 3.5) with the small ones for snow machines with pumps and the largest for gravity only.
The Mikuni diaphragm fuel pump cannot use venturi vacuum, it relies on pressure cycling, so better crankcase evac reduces efficiency.

I stand corrected on vacuum source. Thanks.
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Koncretekid
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« Reply #21 on: April 24, 2018, 07:29:03 PM »

Mikuni makes different size needle & seat assemblies at least for the VM (I think the # is the size in mm: 1.5... 3.5) with the small ones for snow machines with pumps and the largest for gravity only.
The Mikuni diaphragm fuel pump cannot use venturi vacuum, it relies on pressure cycling, so better crankcase evac reduces efficiency.

I use an HSR 42 flatslide Mikuni, and they were only able to offer my supplier one other size needle valve and float than stock, which is almost immeasurably larger.  I think I would have to use a pump to change it to Methanol.

As to the Mikuni diaphragm pumps, I was wondering if they would work with a reed valve type breather valve which  decreases the crankcase vacuum to about 11" H2O by my measurements (BSA B50, which breathes thru the primary case).  In my opinion, there should still be changes in vacuum due to piston movement similar to two-stroke motors.  But at what flow rate?

Tom
« Last Edit: April 24, 2018, 07:30:37 PM by Koncretekid » Logged

We get too soon oldt, and too late schmart!
Life's uncertain - eat dessert first!
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