Running a fuel pump on a carbureted motorcycle?

[tylerm]

Hello all, I wanted to see if anyone could speak from experience as to running a fuel pump (either electric or diaphragm) on a carbureted motorcycle. I race vintage Triumphs (two cylinder, two carburetors) and want to try using a fuel pump to make sure I maintain fuel supply through my runs. The idea would be very small fuel pump to supply positive pressure to the carburetor bowls, where the needle/seat will stop excessive flow.

What I am wondering is roughly how many PSI are needed to maintain this positive flow without causing any overflow or other issues?

I have seen small electric pumps (Facet brand) that are available in a series of gph and a range of PSI (3-4.5psi for instance, another one being 9-11.5psi). Not sure if this would be the way to go, versus a fuel pump and separate regulator to fine tune adjustment.

Looking forward to any input anyone may have. Thank you!

Tyler

[mtiberio]

I never did this on a race bike, but I did this on a street bike. I have seen guys that used to use ram air pressure to pressurize their tanks at Daytona.

With the standard gravity feed floats and float valve, I wouldn't go any higher than the 1 or 1.5 psi put out by the Mikuni vacuum operated fuel pump. This I have used on 2 different street bike where I had float bowl starvation issues.

I used the single: https://www.amazon.com/Mikuni-Genuine-Single-Fuel-DF44-211-D/dp/B000GTV9AK

but they make a double output: https://www.amazon.com/Mikuni-Fuel-Pump-Outlet-DF52-73/dp/B000GZQ65C

they are ugly little f*ckers and require a rats nest of short hoses, but they work. If you really want to go with a higher pressure, depending on your brand of carb, you can get lighter floats and smaller float valves to prevent overfilling.

[tylerm]

I actually have a Mikuni diaphragm pump, one inlet, two outlet. I tested it a year or two ago on another race bike I have. I ran the pulse line to a fitting off of my generator block off plate.

I had one fuel line from petcock to the inlet on the pump, and one fuel line from each output on the pump to the carburetors. While the engine was running it would pump out of one of the two outlets only. I have my crankcase vented in a couple of spots, so I believe there isn't enough pressure to run the diaphragm pump, at least with two outlets.

Ideally I want to run a diaphragm pump for simplicity, though I don't want to have to 'de-vent' my crankcase to do so.

[mtiberio]

I actually have a Mikuni diaphragm pump, one inlet, two outlet. I tested it a year or two ago on another race bike I have. I ran the pulse line to a fitting off of my generator block off plate.

I had one fuel line from petcock to the inlet on the pump, and one fuel line from each output on the pump to the carburetors. While the engine was running it would pump out of one of the two outlets only. I have my crankcase vented in a couple of spots, so I believe there isn't enough pressure to run the diaphragm pump, at least with two outlets.

Ideally I want to run a diaphragm pump for simplicity, though I don't want to have to 'de-vent' my crankcase to do so.

I think you are supposed to use intake venturi vacuum, not crankcase vacuum. Plus the line from the vacuum source to the pump need to be stiff and minimal.

[BVCBR]

Honda, and other manufacturers, ran electric pumps on carbureted bikes in the late 80's/early 90's. Take a look at a 87-96 or so CBR1000F and CBR600F2/F3. I know the 1000 used an electric pump that can support up to at least 150hp and produce 5-6 psig of pressure. You may have to adjust your float level or maybe even change the float valve to compensate for the increased pressure.

[Stainless1]

We pumped fuel to bike carbs... turned up regulator till they leaked, then backed it off... The tough part was getting to not leak at idle.
Do a search, this has been discussed before. 

[sofadriver]

This is easy. First get a Pingel hi-flow petcock. Expensive but worth it.
Use one of those low pressure 3-5 psi pumps but get one that pulses rather than one with an impeller. They use less power and are more efficient. (a pulse pump will buzz - the impeller pumps whirr.) If in doubt any speed shop will have them in stock.
 
(Since it's vintage British I'm assuming you are using all brass barbed fittings)

Run a single large fuel line and tee off to each carb from that single line at the carbs.

Put soldering flux into one leg of a barbed brass Tee along with a big enough chunk of solder to fill that leg when you heat it with a propane torch. Stand that leg on a flat, steel surface when you melt the solder. It won't stick to the steel and won't run out. JB Weld works fine too.
Drill a 1/16th hole in the solder. Connect that Tee inline as close to as possible to the junction of the carb lines. Run a hose from the soldered leg back to the fuel tank. Connect a low pressure gauge to one of the carb lines and turn on the pump. You will probably see about 2.5psi and that's too much. Pull the plugged Tee and drill a larger hole (1/8 will probably do it). Keep enlarging that hole until you read just below 1.5psi

[wobblywalrus]

This is what I do. It is to have the lower main tank and a higher and smaller auxiliary tank.  The fuel pump fills the little tank and its overflow goes back into the main tank.  Sort of a recirculation system.  There is no air pressure in the little tank.  The little tank has a petcock with gravity feed to the carbs like a normal motorcycle.

[sofadriver]

this is all based off of running an S&S super E carb, but it should cross over:  i have a facet fuel pump (5 psi max?) and a mr. gasket brand puck style fuel regulator.  i use the Harley "race size" needle and seat.  just sitting, not running, the race needle and seat will hold any and all pressure the pump can throw at it.  running (remember this is a sportster), the motor will shake/bounce the needle off the seat at apprx 2.5 psi per the regulator.  so, on race day, start the bike and warm it up with the regulator set at 1.5 to 2 psi. or what ever number the seat will hold, roll up to the line, turn the pump up to 4 or 5 hoping no one sees the carb start to over flow and make a run.  i am dead heading the pump (tank to pump to reg to carb, no return line to tank).  also, i am running a pingle guzzler petcock w/ -6 or -8 an fittings/lines to the pump to get plenty of fuel flow to the pump.  but, check everything else before you throw money at a too small vent, a too small needle and seat, a too small stock petcock, crappy kinked fuel lines etc., etc., etc..  lots of carbureted bikes around the world go really really fast without fuel pumps.

I agree with mc2032 on this (except the overflowing float bowl part). If your tank is higher than your carbs you can't go wrong by majorly  oversizing your fuel supply lines and petcock. It will give you volumes of "reserve" fuel at the bowl and increase the head pressure. The higher your gas tank is, the more head pressure you'll have at the carbs. Also, mount clear float bowls if they are available. You can watch them at dyno time and really see whats going on there. I've had motors that would cause the fuel in the bowls to foam at high RPMs. Would have never known it without clear float bowls.

[mtiberio]

would be good to know what brand of carbs, and what fuel type/class

[tylerm]

Thanks for the info, everyone!

The bike I am currently building is a 1950 Triumph A-VG with a stroker crank, iron head, running in the 750cc class. I have set records in this class before with a bored out 650, but it was only about 670cc. This new bike I am building right now, and the engine I am building for it, is just a test bed for a new dual engine Triumph I am going to build this winter for 2019. I built a dual Triumph that ran in the A-VG 1350 class and set several records back in 2012 / 2013.

The last few seasons I raced I ran Mikuni TM32 flat slide carbs. For this new bike/motor I am going back to Amal 932 concentric carburetors just for ease and testing. I typically run 110 leaded gas, and as for fuel, that is new to me. I have set fuel records on both of my race bikes in the past but just running gas. This year I aim to play and learn with methanol.

[saltwheels262]

 roll up to the line, turn the pump up to 4 or 5 hoping no one sees the carb start to over flow and make a run.  

So, you don't mind purposely piszing gasoline onto a hot engine ?

[mc2032]

sorry about the overflow comment, was trying (and obviously failing) to be funny.  I do not advocate spoiling the salt flats in anyway shape or form, up to and including purposefully overflowing fuel onto the surface.  thank you for your concern.

[generatorshovel]

Thanks for the info, everyone!
 I have set fuel records on both of my race bikes in the past but just running gas. This year I aim to play and learn with methanol.

If you are having fuel supply problems with gas, you have a LOT of headaches ahead with methanol, although, once set up correctly.
  I use a single TM-32 Mikuni on a 250 blown Honda & also had problems with adequate fuel flow, the Mikuni needle & seat cant handle much over 1.5 psi.
The facet type "rattle" pumps bounce the needle off the seat, so I went with a low pressure (5 psi) high flow Mallory electric pump (5 amp draw) & to get around the pressure problem with the Mikuni, adapted a Holley float bowl that feeds into the empty Mikuni bowl
Mikuni float bowls are, in my opinion, too small for methanol use.
You will need to increase your jetting to flow 250% more than gas when using methanol (All flow, not just mains)

[wobblywalrus]

The Mikuni carbs seem to be the best choice for oddball fuels and race applications, based on my experience.  They make special parts for almost any application.  The trick is to figure out what is needed and where to get it.  Try Sudco.  Do some internet research, too, on any specific problems you have.  Chances are someone else had the same problem and they developed a fix.