Will a fuel pump help with my fuel flow issues?


CB750 Enthusiast
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St George
CB750P 1981, Keihin CR carbies and pods with a 5/16" inlet and I have had consistent issues with fuel flow where fuel won't flow out of the tank.
It came with the carbies on it, and it was running "ok".
I have adapted it from 1/4" to 5/16" at the fuel petcock.
I have done all the usual checks, fuel is clean, tank is super clean, and have emptied it a number of times, to put fresh fuel in, petcock doesn't have any restrictions that I can find.
The problem is intermittent, but it does seem to be starving, even when running.
Starts first go every time.
I have added an inline filter, but it was doing it without it.
I removed the in tank filter "straw" because I have the inline one, and it did seem to fix it for a bit.
Thinking it might be an airlock in the filter and have tied it in a different position, which is working, but still think it is starving under power.
When it stops, there is no fuel in the filter, and you can fill it by blowing in the tank, or wait and watch it trickle in.
Is it something to do with the non gen carbs, as I have the originals which need rebuilding, and some airbox parts?
Would fitting a fuel pump help, and what pressure range would it need to be?
No don't use a fuel pump. I have heard of some people having issues with certain inline filters but I haven't experienced that. Are the carbs vented properly?
Not sure what you mean by venting, see carb photo.
Keihin CR.jpg
I've had inline fuel filter problems before. Some filters do a good job of allowing air back to the tank, some don't. And more importantly, filters are rated by microns. If you buy a 10 micron filter, you might have a similar issue.
the one i use today has a bronze filtering medium in a transparent housing, looks like a cone. Got it at autozone in the small engine area but don't remember the brand. Works great and I don't have any vapor lock or fuel flow issues.
Put more gas in the tank. 2 gallons of gas weighs over 10 pounds. So you have 10 pounds of fuel pressure with no pump.
By "carbs need to be vented", Loose Chain is referring to vent tubes that go to the carb bowls. Air has to be able to flow in and out of the carburetor bowls as fuel displaces it, and most carburetors do this simply by having a hole drilled from the carb bowl going up and out to atmosphere. Lots of bikes will have a rubber hose attached to this and route it somewhere relatively safe like up under the tank. Some manufacturers will even put a little filter on the end.

If you look at the carbs you should see some kind of small tube feature cast or (if it's brass) pressed into the body of all four and pointing upwards. That will likely be the bowl vent. These need to all be clear of obstructions.

Are you getting plenty of fuel flow from the petcock itself, with and without a fuel filter attached via a fuel hose? Check that, and if it's flowing plenty of fuel then you know the issue is inside the carbs.
Lots of bikes will have a rubber hose attached to this and route it somewhere relatively safe like up under the tank. Some manufacturers will even put a little filter on the end.
That's where I was going with the question. I'm wondering if the vent hose had been blocked off for some reason and the carbs not venting. That's why I asked 'Are the carbs vented properly?".
Thanks everyone,
I will check the filter that I showed earlier and make sure it is allowing plenty of air to the carbs, as this is one thing I haven't checked.
I don't believe it is a fuel cap problem because there is no pressure relief when I undo it, and fuel doesn't just start flowing.
The amount of fuel in the tank doesn't seem to make any difference, no doubt it would help, but if it makes a difference then I still have a problem somewhere else anyway
This is the problem child, just had a rebuild, so still getting some kinks out. 1981 Ex South Australian police. See my post on spark problems as well.
CB750P 1.jpg
No that is wrong. If it had 10 pounds of fuel pressure it would flood because it would sink the floats. Think about it for a minute.
Floats "float". When the fuel level is correct the fuel stops entering the carb. Although you are right that there is not 10lb of pressure from 10lb of fuel, increasing the height of the fuel in the tank increases the delivery pressure to the carbs. I suspect the filter in the tank that surrounds the outlet pipe (and lower reserve fuel outlet) is plugged.
The difference in fluid pressure at the outlet of a reservoir, when adjusting for height, is measured in meters. That's 3 ft. Assuming the tank, from reserve to full, is 1 ft, the change in fluid pressure is minimal.
Despite that, the floats stop fuel by being pressed up from the fuel below it. We all know this. But the key is, the fuel stops flowing into the carb when the pressure underneath the float is equal to the pressure of the fuel coming in. You can increase the fuel height in the tank all you like, even add a taller tank, but once the fuel in the bowls exerts equal upward pressure on the floats, they seat and fuel stops. If the carb overflows, adding fuel height won't help -- you have to get the float valve to seat.
Floats "float". When the fuel level is correct the fuel stops entering the carb.
Yes I know how a float works. When I said 10 lbs pressure would sink the floats that was meant to describe how the pressure would overcome the float and hold the float valve open. Those little floats will not hold back 10 lbs pressure.
This is getting a bit off-topic, but when working on a go-kart for a customer once I had direct experience with how much pressure it took to unseat a particular carb float. It used a low-pressure electric fuel pump to pull fuel up and into its small carburetor and the customer had purchased an adjustable fuel pressure regulator that ranged from 1 to 5 pounds per square inch.
From what I recall, anything 3 PSI and above would overcome the float and cause the carb to overflow. 2.5PSI seemed fine but I set it to 2PSI to be safe and the carb seemed perfectly happy. It was a small single-barrel sidedraft carb on a Harbor Freight 212cc Predator engine, probably no more than a 25mm venturi, with a fairly small round float and float needle.

I do know that fuel pressure on some carbureted sportbikes, such as a 1988 Honda CBR600F Hurricane, is a maximum of 5psi. Those carbs were meant to take a touch of fuel pressure and their floats and float needles can therefore hold back that 5PSI.
Makes me wonder if those Keihin CR's are made to take a low-pressure fuel pump to even out fuel supply, like the carbureted sportbikes of the 90's were.

Or perhaps their orientation when mounted isn't quite correct for their set float height?
I had a weird one on my ‘79 f.

When cruising at 40mph or so, after maybe 20 minutes, the bike would start falling in on itself.

I’d pull over, check to see if brakes were dragging, the petcock was on, if the small clear in-line filter was clogged or had bubbles.

Everything was good.

Well, after sitting on the side of the road for the 10 minutes checking stuff took, the bike would start.

If I went back to cruising speed, the issue would return.

If I rode it back home slowly, babying it, I could make it home. Just barely.
I’d park it and the next day it rode great
up and down the street in front of my house.

This is in the dead of summer.
It’s hot outside.

Back on the highway it would happen again.


So I have it out one afternoon, on the interstate.
60, 70 mph right after noon, bike is running strong, it’s busy, traffic is heavy, but life is great.

Then the bike starts losing power.

I’m pumping the throttle, hitting reserve, trying anything to keep it running.

Then, baawwwwaaa.

The engine dies.

Freaking cars blasting past me at 70, tractor trailers bout running me over,
I find neutral and coast off the concrete and on to the gravel shoulder.


I jump off the bike, holding it upright with one hand and look the thing over.

Traffic is blasting by, close, a car blows his horn, I’m thinking dude, you better do something.

I check for fuel running anywhere, thinkin maybe a float or 2 is getting stuck and flooding it out. All the plug and coil wires are tight. Tank cap comes off, plenty of petrol.

Looking under the tank again I notice a weird, I dunno what you’d call it, movement
on the fuel line that goes from the petcock, to the inline filter, from that to the carb rack.

Looking closer, I realize, the black fuel line is completely compressed in on itself.
Like it’s collapsed or something.

As I’m looking at it, it starts returning to its normal shape. I actually watched it kind of open up.

Reaching in, I touch the fuel line.
It’s hot.
And rubbery.

I jump back on the bike, turn key, hit button
and va voom it lights right up.

I grab the first exit just as it starts starving for fuel again.

The fuel line was old.
At speed, the heat coming off the motor would heat the rubber fuel line and it would get soft.

I run maybe 2 sizes up on jetting.

The combination of weak, hot rubber hose, along with the slight vacuum in the fuel line from constant fuel demand, was just sucking the line in on itself.

And stopping fuel flow, which killed the motor.

New fuel line, el fixo probmemo
It sounded to me like your petcock sleeve is clogging up (if your filter was good) or your tank wasnt venting. I wouldn't have thought a fuel line could collapse without tremendous heat and/or vacuum.