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MG MGB Technical - Engine problems.

The car is a 1974 (chrome bumpered) mgb roadster with HIF4 SU carbs. She has a head modified to run on unleaded petrol but is otherwise standard.

She runs fine when cold (for of the order of 40 minutes) and then starts spluttering slightly when the throttle is open. The spluttering gets gradually worse over a period of a few minutes until the engine no longer has enough power to move the car at all.

The only problem that I know of is that the compression is slightly down on one of the cylinders.

Anyone got any ideas what's wrong?
Carl Kambites

Old VWs' were prone to vapor lock, where the fuel vaporized in the fuel lien before it got to the engine. This was caused by a line passing too close to something warm/hot. Do you have any way of testing whether your fuel delivery is still good when it starts sputtering?
Dave
David Steverson

Carl-
Simple things first: Check to see if your mixture enrichment system is functioning properly. Choke cable connected? Linkage moving when the cable moves?
Steve S.

OK, I have had another look at the car when it's hot. Fortunately it has a glass fuel filter casing so I can see into the fuel filter and I notice a few things which don't seem right.

1) The level of fuel in the outer core of the fuel filter seems to have dropped to almost nothing now that the engine is hot. Perhaps 1mm in the bottom of the thing, down from it being over half full with the engine running cold.

2) There is actually an air (well a colourless gas) bubble in the inner core of the fuel filter which seems to be being fed by small bubbles coming in from the carburettors.

Is this likely to be a problem with the carbs (getting to hot or something) or is it the fuel pump not providing enough pressure? The fuel pump is new (and hence it's condition unknown to me). The heat shield between the carbs and the exhaust manifold is slightly cracked and doesn't have insulating material in some places, is it meant to be insulated all over?
Carl Kambites

Carl-
Yes, the heatshield should be fully insulated on the engine side. SU HIF4s are more heat-sensitive than SU HS4s. If the heatshield is cracked and the insulation is shot, then it could well be that the bimetal adjuster for the fuel mixture is setting the mixture too lean. In extreme cases lack of insulation from exhaust manifold heat could cause the fuel to boil in the floatbowls, creating a lean running condition. It all makes sense in the light of what you describe. If your glass fuel filter is installed close to the carburetors then the heat from the exhaust manifold coul account for the presence of air in the filter after the engine is shut off.
Steve S.

Almost no chance of vapour lock at any time in the UK, much less in winter. Also any vapourisation in the fuel line is going to get pushed through the carb and out of the way double-quick as soon as the float valve opens. Vapourisation, unless the heat is really extreme and occurs faster the the pump can pump, is usually only a problem when it occurs inside the carb i.e. after the float chamber. Even with a broken heat shield I can't see that having much effect with the temps at freezing we have had for the last few days, particularly if the heat shield has been damaged for some time but the problem is recent. 40 mins is too long for a blocked fuel cap vent so that can be ignored. Air bubbles coming from the *carb*? Definitely not right. Air in the filter can expand when the filter body gets warm from the heat of the engine, but in your case I would suspect the pump is failing. Remove a feed pipe from a carb (watch out for a spurt if the ignition has recently been on) and direct it into a container. Switch on the ignition and time how fast it pumps. Should deliver at least 1pt per minute. Try it hot and cold and see if there is a difference.
Paul Hunt

I go with Paul. This is surely a fuel supply problem of some kind, and since the pump is new, I would start by replacing that see -through filter. Cheap, good, easy insurance. If it still happens, then it's a pump or tank problem. My car gets quite hot in summer, around town, and I took off all the asbestos from the heat shield, but I have never had a vapour lock problem, not a hint, and as Paul says, vapour locks tend to occur in or close to the carbs- remember those 1950's English Fords Zephyrs, Consuls ( and others!)with the loop in the copper supply pipe, which was right up close to the heat source, to counteract vapour locks? Fuel supply to the SU is better positioned. My 2C worth. Merry Christmas, Paul and Steve S, et al.
Ken R
Ken Rich

Have you changed the petrol tank filler cap lately??

Sounds to me like vacuum building in the tank as fuel is drawn out and that would account for the bubbles from the carb end and the low fuel level in the filter........
Chris Betson

A friend of mine had a similar problem. We did a lot of things, including putting insulating material on the back of the heat shield. His problem turned out to be the fuel pick-up line in the fuel tank, it was rusted and full of holes. If you car has the original fuel tank (or you suspect it is original), and you can't find any other cause for the problem, then drain the fuel tank, remove the fitting on the side where the fuel sending unit is installed, and look into the tank to inspect the fuel pick-up line. If it is rusted or you can see holes in it, replace the tank.

Good luck!

Greg Qiog;eu

This thread was discussed on 14/12/2002

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