Question about vacuum/diaphragm fuel pumps

[stinkwheel]

Do they allow fuel to pass through by gravity as normal when there is no vacuum applied?

I've noticed I get fuel flow issues on my 612 bullet when the tank is below half full (the engine is much closer to the tank than on a standard one so there is less head of pressure). I was thinking one of those little diaphragm pumps might give it an extra "push". It's not having to lift the fuel at all, just help it along.

It's especially an issue at idle, the idle circuit gets fussy if the carb isn't totally full and it can cut out. I think there isn't quite enough pressure to poke that last few mm of fuel in. It's am Amal MK1 concentric which are fankly a disaster area but I don't have the time and mental space to set up a Mikuni TM32 from scratch which is the ultimate aim.

My concern is it would need to allow fuel to flow when the engine isn't running because the carb needs to be tickled for it to start.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Robby]

I assume you mean normal vacuum-type fuel taps. If so, they should stop fuel flow entirely when there is no vacuum. Also a pain in the arse if you run an inline fuel filter below them meaning you have a big air bubble in the fuel supply side. They only seem happy with a constant supply of fuel on the wet side.

It sounds like you're dealing with the same problem as on the Honda NTV600/650 range, where the fuel tank isn't very far above the carbs. I would try an electric fuel pump and relay from one of them. Should give the correct fuel pressure, and if you copy Honda wiring then you won't cock it up.

You may also be able to solve the problem with a slightly weaker spring in the needle, or eliminating the inline filter (if fitted). They seem to make the fuel flow get a bit lazy.

[stinkwheel]

[oilyrag]

So it had a 500cc engine and you increased it to 612cc is that right? I would contact whoever you bought the upgrade kit from. This could be a common problem. They'll know what the best solution is.

[Tankie]

I use the same type of pump to regulate injection oil, works a treat , you can find them cheap in other places rather than off Allens
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Always learning, everyday is a school day

[Easy-X]

Might be worth looking at it the other way: are there any places that might offer a restriction? Petcock, fuel pipe diameter, tight pipe bends, air flow into the tank (i.e. very slight airlock) that sort of thing.
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[stinkwheel]

The issue is it's an amal carb and they are -and always were- shite but all the enfield folks insist on using them. The height of the engine just makes the problem worse. You daren't mention this to any brit bike enthusiasts because they go off on one.

I think the main problem is inherant to the design, the float bowl fills from a spigot on the bottom meaning the fuel HAS to go down then back up into the carb. The needle valve closes by dropping down into its seat, not by being pushed up into the fuel flow like on a japanese carb. (the float see-saws, the needle is the opposite side of the pivot to the float). There is no spring on the needle, it's a solid piece with a viton tip.

It has an OMG fuel tap and a straight drop of red-line karting fuel hose to the carb. No inline filter, just the gauze in the tap and I removed the internal filter in the carb. The tank is POR15 lined anyway so no sources of rust and crap.

I had similar problems with an amal on my 350 bullet and as soon as I put the mikuni copy back on, they all went away. It's just setting one up for this bike with no point of reference will be a ball-ache. Fitting a mechanical fuel pump would be very simple.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Tankie]

Put a pump on , yes it will free flow and will also provide increased head the Amal should have no problem with it
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Always learning, everyday is a school day

[Easy-X]

Sounds to me like someone wants to say "fuck you, purists!" and put a proper carb on

I put a flat-slide on the DT. No kittens were killed in the process
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Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter

[dynax]

[WD Forte]

I partially took one apart on a 125 scoot which sucked fuel up from the
underslung/belly tank to supply a carb
From what I recall it worked in similar way to some electrical ones in that
instead of electrical pulses, it used the varying depression between carb and cylinder
to work a diaphragm which sucked/pumped the fuel.
I assumed as the induction pulse frequency increased with engine speed, so to would the
amount of fuel being delivered.

Not sure this type of pump would be the best at priming a carbs float chamber pre start
especially on a larger engine.
If it was used between gravity fed tank and carb, the weight of fuel might be enough to push its way past the two one way valves for some priming.
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bikers smell of wee

[yen_powell]

I had an aftermarket Mikuni vacuum pump on one of my Africa Twins. It lasted longer then the Mikuni electric type they come with as standard which used to average about 30,000 miles before dying.

The Mikuni vacuum pump was doing well right up until we had some unseasonably super hot weather mixed with slow riding when it started to play up. I then bought a Facet electric pump and it lasted the rest of the bike's life.
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XT1200Z and a DR350 in bits