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MG MGB Technical - Fuel pump pressure

I have a 1973 mgb roadster with HIF 4 carbs and have an intermittant fuel overflow problem through the overflow tube. It can happen most anytime whether the engine is cold, hot etc.

I did some reading in the archives and found the issue of aftermarket fuel pumps that put out too much pressure, so did a pressure test. I did not open things up yet to see if my fuel pump was Lucas or not though.

First a ran a 15 lb pressure guage to the fuel hose end that I had removed from the carb. This gave me 4 psi.

Given that this was a dead end test without the car running I next put a "T" in the fuel line and ran the guage off the "T" with the car running. The results were the same at 4 psi.

I tried adding a second fuel filter to see if that would add any resistance to feul pressure. It did not and the pressure remained at 4 psi.

The question is whether 4 psi would be enough pressure to cause the intermittant oveflow. I had a British car mechanic take the carbs apart and clean, minor rebuild and adjust them when I first started using the car last year. He was unable to stop the intermittant overflow though and it's something I've been living with. He's the one who suggested I investigate the fuel pump pressure when we spoke about the problem again yesterday.

Thanks all!

BH Davis
BH Davis

BH Davis,

does not sound as to be a problem with the pump.

It is not uncommon that the floats star sinking or that the nedle valve of the float chamber has to be renewed and the dry setting of the floats be checked.

Ralph
Ralph

BH. David DuBois has a tech article on the fuel pump on my website at www.custompistols.com/ which provides the fuel flow rate and pressure information. His e-mail address is hot linked in the article and you can e-mail him with any questions.

As to the overflow problem, I believe that 4 psi is slightly higher than normal fuel pump pressure (but, check the article as I am working from memory). The second thing to check would be the needle valves on the carbs. The Gross Jet has been known to have some problems and, if present in your cars, might be replaced with the standard needle valve. You might also want to check out the float height and make sure the floats are in good condition.

Les
Les Bengtson

Is it overflowing from both carbs, or just one?
Dave O'Neill 2

BH - The standard AUF 300 or AZX 1300 series pumps for the MGB is 2.7 PSI output pressure. Since the SU fuel pump is an impulse type pump and difficult to get a good pressure reading on as it pulses up and down. You say that you are getting a constant 4 PSI and you don't mention any pulsing of the reading so I assume that you have an aftermarket pump installed. 4 PSI is not sufficiently high to cause any problem with the needle valves in the SU carburetors unless the valves are worn or the floats have a leak and are begining to sink. Cheers - Dave
David DuBois

Both carbs are hooked up to the same wrap around overflow metal tube which wraps over the tops of the carbs. The rubber dtrain line comes off the front end of that and goes down for draining. As such I can't be sure if it's one or both carbs leaking. However, when the mechanic worked on the carbs last spring I think he was having problems with both.

The 4 psi reading was not pulsing, but then I did something stupid and didn't get a reading with the guage "T"d off the fuel line like I thought. The reading was dead ended on the hose without fuel going to the carburetors with both my setups. As such, the 4 psi was based upon a dead end pressure buildup in the hose. I guess I have to redo the pressure test to see if the pressure drops a bit or I get pulsing when the car is running.

I'll also try and see if it's one or both carbs.

The biggest annoyance with this is that if I want the key turned on to the accessorie position while working on the car without it running the pump will sometimes run (clicking sound) and I'll get gas blowby.

Thanks all !
BH
BH Davis

Your test is fine - when the carbs are full, the floats shut off the fuel = dead end pressure. That's when you are getting your leak.

4psi is a bit high as Dave notes, but he's correct that it "should" work OK. Combine it with slightly high float settings, slightly worn float valves, etc - and you have intermittent problems.

This is a bad situation, as it results in very rich running when it is acting up, even if it's not visibly flooding; when it is visibly flooding while stopped, it's probably filling the crankcase with fuel = dead engine. It's also a fire hazard.

Furthermore, with the key in "accessory" position, the pump and ignition are supposed to be dead - that's what "accessory" position is for: allowing accessories to work without power to the pump & IGN.

FRM
FR Millmore

FRM,

Thanks for the input. First, I'll make sure I'm not putting the key in the ON postion as versus ACCESSORY. That could be the issue there.

Next, the car accellerates just great, but has been running a little ragged at all run speeds. Not quite hesitating or sputtering, but sort of sluggish. Is this what you mean by how it could run if the mixture is too rich?

Thanks,
BH Davis
BH Davis

Perzactly that! And crappy gas mileage too.
FRM
FR Millmore

FRM and BH Davis: I'm having a similar problem with overflow but an opposite problem of what seems like too low pressure to the filter. I adjusted the float levels and that seemed to cure the overflow. Now my question is - when the carb bowls are full and the pump stops pumping, should the fuel filter be nearly full before the pump completely stops? Sometimes I end up with a very low level in the filter, but when I pull the hose off the front carb and turn on the key, gas will flow out great. Could any condition inside the carbs cause the pump to be unable to flow gas into the filter?

Thanks for your comments.
willieL

It's quite normal for the fuel level in the filter to be low when the pump stops. Nothing to worry about
Iain MacKintosh

The filter can be anything from full to empty, and change from one to the other (and anywhere in between) from time to time, and in itself that is fine. I don't know why they are so variable, but they are.

2psi in the fuel line is the same for fuel as air. If any air gets into the the line then when the float valve opens it will be pushed along the system and out through the float valve and vent, to be replaced by fuel which will cause the float to rise again and shut off the flow. This is why vapour-lock anywhere other than in the jet pipe of HS carbs is virtually impossible.

I had a couple of bouts of overflowing on the V8 - from the same carb - despite changing the float valve each time. I'd also noticed fuel dripping from the overflow after being switched off on very hot days as the fuel in the float chamber heated up and expanded, but at that time it wasn't overflowing with the engine running.

It was only on the third occasion that I noticed the float was partially filled with fuel, and probably had been the problem all along. Squeezing it and heating it showed no fuel leakage from it no matter which way I orientated it, so the crack must have been minute and sucking in fuel during heat/cool cycles. I did drill a pin-hole to drain it out, and attempted to plug to afterwards, but it sunk again in pretty short order, so replaced it. Since then it has been fine.
Paul Hunt 2

If having a filter that is only partially full really bothers you, either get one that you can't see through, or mount it such that the inlet is lower than the outlet. Cheers - Dave
David DuBois

For whatever reason, Dave, mounting it like that make no difference, if my V8 is anything to go by.
Paul Hunt 2

Paul - In that case, my first suggestion would be the best solution ;) Cheers - Dave
David DuBois

I don't know about HIFs, but HS carbs are not that sensitive to fuel pressure. I run a regulator but even fully open the Facet red top doesn't challenge the float valves.
Neil
Neil Lock

Well, back to the original issue here....the overflowing carbs.

I have the car in the garage this morning working on the turn signal trip system (see other post). I have to have the key on in order to be able to turn the wheel to test any changes.

I now have the overflow outlets on both carbs to individual hoses going below the car. I walked out of the garage with the key still in the on position so the wheel could turn and came back 15 minutes later to a puddle of gas on the floor.

I took the hoses back off the carbs and had a drip out of the rear (key in ON position). I started the car for a moment or two and got a spitting flow out of the front. Then with the car off and the key in the on position I got dripping out of both front and rear. So, if it's a carb problem it seems it is on both of them.

The engine was cold during this process.

Could it be that the pump isn't turning off at the pressure point at which it should?

I've got an Ebay set of carbs coming (same HIF 4 model) that I'm going to experiment with on the bench to become more familiar with the things. Maybe I'll rebuild those and do a swap out (although I really like have as much truly original equipment on my car as possible).

So, seesm the possibilities are two bad floats, two bad needles/seats or a pump not shutting off at pressure.

Thanks,
BH
BH Davis

The carbs are the only remaining 'original' (ie not replaced/fixed) component, aren't they? Probably the most likely, possibly just two generally worn units playing-up in tandem.

Regards
Roger
Roger T

This thread was discussed between 15/05/2007 and 27/05/2007

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