29 replies to this topic

[klivins]

Been thinking to use MJ to cut out electric fuel pump when engine not running. seems thats easy using relay.
have seen some info about fuel cut off on override - I think thats only poss with injection, carb's float chamber will be full anyway. Am i right in this thinking, or is there a way to mod carb so on override no fuel is wasted as in modern cars?

[klivins]

bumping this up again
Wired in a relay so fuel pump is not active when car has no vacuum in manifold.
Still thinking of having a electromagnetic valve in the SU 4 carbs small pipe from float chamber to the jet. Megajolt could operate this say when the vacuum is on the override levels? Going downhill no fuel waste? Someone have seen something like this?

[Bradley Gaunt]

Hi Klivins, why do you want the fuel pump to stop under those circumstances? The way I understand it you would run a low pressure fuel pump running 2-4 psi and once the fuel chambers are full the static pressure would stop fuel moving in the fuel line. Injection fuel pumps run in a circuit and continually pump fuel past the injectors under very high pressure. Im not sure what you mean about wasted fuel? Forgive me if I have misunderstood what you mean!

[icklemini]

Cut the fuel to the jet= engine stops :s

[jaydee]

You can connect the fuel overflow to a carchoal cannister and run a return pipe to the fuel tank.
I belive if you have a search on the net you can find some infos about that, it has been done in some US spec cars..

[icklemini]

but why??? - there really is no need as if you have fuel coming out of the overflow then the float is duff..

the charcoal cannister is to trap the petrol fumes and feed them into the engine to burn off... an emissions thing...

I really cant see what benefit the OP is expecting by having the pump controlled by the megajolt...

[klivins]

seems i need to explain this more. I mean two separate things.

Modern cars when braking, going downhill, etc, with engine revving waste no fuel for turning the engine, while carb still draws fuel from the float chamber. My thinking was with simple valve the flow to the jet would stop, and the flow would resume once the accelerator is depressed.

The fuel pump thing is another story. Its a safety feature, when no load in the type of manifold pressure = engine not turning, fuel pump is not needed.

[mini-luke]

I think the problem with that is in a modern car the injectors can be switched on or off instantly, with a carb there will be a delay whilst the fuel bowl empties and fills up again when you turn the pump back on.

[klivins]

thats why Im thinking to fit valve between the bowl and the jet - on a SU HS4 its easily done if I can find a 4 mm barbed electromagnetic valve!

[klivins]

like this!

any ideas, comments, criticism?

[liirge]

seems i need to explain this more. I mean two separate things.

Modern cars when braking, going downhill, etc, with engine revving waste no fuel for turning the engine, while carb still draws fuel from the float chamber. My thinking was with simple valve the flow to the jet would stop, and the flow would resume once the accelerator is depressed.

The fuel pump thing is another story. Its a safety feature, when no load in the type of manifold pressure = engine not turning, fuel pump is not needed.

So you would cut the fuel when the car is under deceleration, i.e. when the vacuum in the manifold is at its highest?

[klivins]

yes, since the MAP (manifold pressure) is the lowest, and vacuum thus the highest.

[liirge]

Just one little thing I can think of, lets take a scenario.

You're approaching a set of traffic lights a bit too quickly, so you slam your brakes and take your foot off the accelerator the fuel supply is cut, when you reach a stop I can see 1 of 2 things happening, either the engine will stall as it won't have the fuel to just idle and will cut off (there will be lag between fuel being supplied and the engine using it fire back up).

OR

if you have in gear still you suddenly get a jolt forward as the engine essentially restarts itself.

There's also the consideration that you will be running lean when decelerating.

If I'm honest I can't really see this being a practical or useful idea for a carburettor car...

Edited by liirge, 10 May 2012 - 12:30 PM.

[ian2000t]

This is something I have also thought about - not so much the switching, but the fuel-cut on deceleration.

Keep thinking about this every time I'm coming home, travelling down the one road that is about 3 miles of slight downhill following people doing 25-27mph (just low enough so I have to be in 3rd gear rather than 4th). I'm sure it's not massive amounts of fuel being wasted but every little helps.

I've got a HIF44 though, so can't even do what you were thinking.

One thought I had was a valve on the inlet manifold that would open allow more air to be sucked in resulting in leaner mixture (similar to cutting fuel). But, then it occurred that doing that would actually reduce the vacuum, and as youwould be triggering it by high vacuum (deceleration) it's not going to work....

[liirge]

Some SU carbs have this poppet valve installed on the throttle disk, in general they're usually removed due to a tendency to stick open and cause a high idle.

Edited by liirge, 10 May 2012 - 12:41 PM.