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MG MGB Technical - Fuel system woes

Hi all, I have a 73 MGB with HIF4s, with AAA needles running K&N filters. Car has been running great up till recently (sorry for the long winded-ness of this):

I have been stranded twice in the past 3 weeks where the car would not start. Starter motor turns the engine over just fine, but the engine doesn't even sputter. This made me believe that I had no spark, so I had a person turn the engine while I held spark plug wire No.1 with a screwdriver near a ground, and saw a pretty nice spark. So, I then disconnected the fuel line to the front carburetor and had same person turn key to ignition, and had plenty of flow of fuel into a bottle we found lying nearby. Reconnected fuel line, engine still wouldn't start. The first time this happened, I happened to be at a car show, and one of the other car afficionados put his hand over the intake of one of the carburetors (filters removed) and the car sputtered but ran very rough. He then did the same over the other carb, and again it sputtered and ran rough. After some of this the car managed to run but poorly. I had about a quarter tank of gas in the car at the time, and filled it up at the nearest gas station (only about a half-mile away). Once filled, the car ran much better, although still stumbled at times. I picked up a new fuel filter and installed it that evening. The next day it ran very normally, and has since then.

The second time (3 weeks later) it would not fire at all, so again I checked spark and fuel flow (all good), and I tried pouring a small amount of fuel down the throats of the carburetors. The engine started and ran roughly for a short while, and then died. Repeated, and the car started and ran rough, but managed to stay running. I was about 20 miles from home, and the car ran a little rough on acceleration but was able to maintain highway speeds well. This time the car had about 3/8 of a tank of fuel, and again I filled up and now the car has been running quite well.

Some possibly relevant background...last year I had trouble with hard starting (it would fire sometimes but not run), so I replaced the fuel pump points, carburetor floats (there was a small amount of fuel in each), and needle valves. The new needle valves have a black tip on them whereas the ones that came out did not. I seem to remember they came in an SU bag. I store the car over winters under a deck barely higher than the car itself, so it is difficult to do any work under the hood once parked. It goes up a slope so it has to be driven into its spot under the deck, so I can't run the carbs dry of fuel before parking it. I have used Stabil in the fuel before storing.

I wonder if a needle valve is sticking closed, shutting off fuel to one carb. I am guessing one carb gets no fuel, and the air coming in that carb creates such a lean mixture it doesn't fire at all? I have been suspecting bad gas, but can gelled gas get through a fuel filter? I wonder if any of you can enlighten me as to what I am missing here?
Erick Vesterback

Eric - It is possible that your needle valves are sticking shut. It is also possible that the fuel tank vent line going to the charcoal canister is clogged, causing a vacuum to for in the fuel tank. Try removing the filler cap from the tank and see if this cures the problem. if it does, you will need clear the line from the vapor separator in the trunk to the charcoal canister. Cheers - Dave
D W DuBois

Ah yes Dave, thanks for reminding me about that possibility. Now, if that line were clogged, would there be little or no fuel flow from the fuel pump?

I forgot to mention that I gave the underside of the carbs a good whack to try to unseat the valves in they were stuck, to no avail. Not sure how hard a whack would be necessary to dislodge a stuck needle valve. I hear about trouble with Grose valves, are they also a type of rubber (or other material) tipped valve?
Erick Vesterback

Perhaps not fuel but the classic signs of a failing condenser ! try putting an old condenser back into the distributor.
dominic clancy

I've had a failing condenser years ago. Car ran but poorly, and only got worse and worse until I cleaned the points. Currently I have a Pertronix electronic ignition conversion so no condenser, and I believe when Pertronix fails, it fails permanently. For that reason I keep my old distributor in the boot in case I need that in an emergency. Given that I seem to have a nice spark, I really feel as though this is a fuel issue.
Erick Vesterback

if the car started when the intake was covered by hand (allowing less air in), could it be that the carbs are running very lean? It can run reasonably well when lean - so much that you might not know - but if so, could it be just on the verge of 'too lean' and adverse conditions have given you these issues?.

What colour are your plugs and what happens if you do a carb pin-lift test (please xcuse me if you don't have pin on HIF, I only have experience of HS). do you have points or electronic ignition?

have you tried a colortune kit to see how it burns?

stabbing in the dark a bit but hoping something might be a pointer.
Graham Moore

Erick- Sticking needle valves or vacuum would seem most probable. However, and as a bit of a last resort , check the fuel pump (again!) Mine worked fine with the fuel pipe off the carbs; it chattered away on the bench; but once connected up it ran for a bit then stopped. Usual whack got it to go again, but it was time to change it. I had already cleaned the points but it seems to be a combination of things. Whilst fiddling with the spare I discovered how easy it is to get it to pump, but it's quite a fine adjustment to get it to the necessary "chatter" to work properly. You might be able to replicate the on/off of the needle valves using a clamp on the rubber fuel pipe.

Over to the expert- Dave
Michael Beswick

a poor condenser will often give a good spark when there is no load. There is an old saying that most SU carb problems are actually electrical, and it is very true. The symptoms you have are much more like a failing condenser than a fuel problem. In addition fuel problems don't usually appear unless you have done something to either the pump or the carbs. So I think you are on a red herring, and you actually Need to change the condenser before playing around with the carbs and actually causing more problems than you have right now.
dominic clancy

Your original problems were caused by the leaking floats sinking and causing too high a fuel level, hence flooding on start up.When you replaced the floats and valves did you check the float heights? The newer, white floats are slightly different from the old ones. The metal strip on the float arm is stainless as opposed to brass and is riveted on in a different way. Also it is possible to put them in upside down! Never actually seen a sticking needle valve resisting the pressure of a good pump, unless the float is holding it in!
Allan Reeling

Domonic he's on an electronic igniter head so no condenser


Erick,
I'd check that the delicate wires to/from the Pertronix aren't pinched or chaffed inside or outside the dissy cap

did you replace the rotor arm and dissy cap too? (lots of p*ss poorly modern made rotor arms and some dissy caps about)

often when these units play up the owner blames the unit rather than the installation of it

as far as fuel goes just check that a good constant supply gets to the carbs at least, if you had to replace the points on the pump then the pump might need checking

if you have fuel venting then it might be blocked

to answer one question you have Viton needles by the sound of it, see here for info on needles -

see the start of this video as it compares different types of needles - http://www.youtube.com/watch?v=82YNx-RkGNI&list=UU40j4KqUJPMVv4FQ29ro-xQ&index=11

this video shows possible disparities -
http://www.youtube.com/watch?v=pVX_JClyeDQ&list=FLr4Udxf9e9Cq7fxH4V23MWQ&feature=mh_lolz



Nigel Atkins

Thanks guys for the pointers. I have checked the Pertronix wiring and it was correct and the wires appear to be in good condition. I replaced the rotor and cap recently, the replacement rotor being the red type with no rivet. I would be really surprised if this is an ignition issue. Also, should I not expect some fuel smell from all the attempts to start if it were bad ignition? In both cases there is no hint of fuel smell from the carbs.

When I reassembled the carbs I did have to adjust the float height. That is something I suppose I can check when I recheck the needle/seat. I'll also have a look around inside the float chambers for gelled fuel also. Probably worth another look at the fuel pump also, but when I have these problems the fuel pump seems to do its normal chatter for a second or two before I turn the key. I always listen for that when I am about to start it.
Erick Vesterback

Leaving old fuel in your carburetors is a recipe for disaster. I mounted a switch, under my dash, so that I can turn off the fuel pump when I intend to let my B sit unused for any length of time. I had to replace the float in my HIF44 carburetor due to the ingress of fuel. If you check your wiring diagram, you'll see that there's a white wire coming off of your fuse box. No cutting of any wires is required to install a switch. This is where I broke the fuel pump circuit and wired in my fuel pump cutoff switch. I've had no problems since installing the on/off switch. RAY
rjm RAY

Ray, My cut off switch is mounted in the boot area and also wired in to a flashing LED on the arm rest. That way it looks like an anti theft ( also stops you setting off with no pump!!!) and in the boot it's locked!
Allan Reeling

"Also, should I not expect some fuel smell from all the attempts to start if it were bad ignition?"

You should, also the plugs should be wet i.e. flooded.

Take them out after cranking for a few moments, and they should smell strongly of fuel, if not be wet. If they are dry no fuel is getting through, but for both to 'suddenly' fail like this would be unusual.

If the float chambers are full, by removing the air cleaners, lifting the piston, and blowing in the overflow port, you should see fuel bubble up from the jet.
Paul Hunt

Ray and Allan - Great idea about the fuel pump shutoff switches. I'll get to work on that. Ironically, I probably don't have to store the car over winter anymore since I am moving to a climate where I can drive it year-round.

Paul, I will check the plugs next time I have this problem. Now, is the overflow port the one that has piping running to the adsorption canister on the passenger side rear of the engine compartment? I HIF4 carbs on 1973 NA-spec MGB.
Erick Vesterback

"is the overflow port the one that has piping running to the adsorption canister"

Yes.
Paul Hunt

This thread was discussed between 14/07/2014 and 19/07/2014

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