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MG MGB Technical - Hot starting...again.

G'day. I have read all I can about this problem but am now at my wits end.

'67B with standard carbies (except they have different numbers stamped on the body), new coil, two different distributors and extra heat shielding added to rear carb. The thing wont start when hot because the rear carb floods when it is left standing for more than 5 minutes. Both float chambers are the same relative height to the carb body (as per a previous problem), both fuel levels (measured in the chamber with the floats removed) are within 1mm of each other.

I have had this problem for about 6 weeks now and it is driving me crazy. Can anyone think of anything else I can try to stop this happening. I have noticed that the floats are different in size, apart from that everything is very close to being the same.

Thanks

Tony
Tony Oliver

I had a set of non matching carbs on my car when I got it and just couldnt get the car to run right. I eventually bit the bullet and replaced them with a second hand matching pair which I rebuilt using a rebuild kit. Not a bother now. They work perfectly. If yours were HIFs I would theorise that its something to do with the bimetallic strip in the float chamber but seeing as HS carbs dont have these then Im a bit stumped.
Ross Kelly

Tony. I would be interested in knowing why you believe the rear carb is flooding. What indications are you seeing? Is there fuel coming from the overflow pipe at the rear carb? Are you running a standard exhaust manifold or do you have a header?

You seem to be making the assumption that the rear carb is being heated, in some fashion, after the engine is shut off, forcing fuel up though the jet and into the intake manifold. This would be a logical assumption if the heat shield were ineffective or the exhaust system were putting out more heat than the head shield was designed to handle. How thick are the spacer blocks between the carbs and the heat shield?

A second possible scenario is that the rear carb is not shutting off the fuel flow (bad needle valve or bad float) and the rear carb is running rich all of the time but it does not show up in starting (where a richer mixture is an advantage) nor when running. Only on hot starting. In such a case, I would expect some fuel overflow from the rear vent and some sooting of the plugs in cylinders three and four.

But, mostly, I have no idea.

Les
Les Bengtson

Thanks Gents. The carbs are HS4 models. The manifold is standard.

I have looked down the throats of the carbs after a long run and having left it alone for a few minutes. I can clearly see fuel in the rear carb throat. I have also taken the dash pots off and can see the fuel in the rear carb sitting at the very top of the jet and slowly dribbling out. The front shows none of these symptoms.

I was assuming it was heat related as it is OK when cold. I re-routed the fuel pipes by taking them to the wing side of the carbs so they are not running over the manifold.

The needles and seats were replaced a few months back and I am thinking of getting new ones again in case it is pressure building up in the fuel pipes due to heat expansion and the rear needle and seat is faulty. I'll check the plugs today.

Tony
Tony Oliver

OK. All plugs are a nice tan colour. I have swapped the rear float chamber top for a spare I had. No difference.

Could someone tell me what the level of the fuel in the float chambers is without the top fitted. Mine are about 30mm from the rim of the chamber.

Tony
Tony Oliver

Whilst the vent on top of the float chamber cover is primarily a vent and not an overflow (if it gets blocked the float can't rise to shut off the fuel and so the pump pumps it straight up the jet and into the engine) in my experience fuel does come out of here when the float valve has failed, even though it might be above the top of the jet. If there is no fuel coming out of the overflow, maybe it's routing is too high which forces fuel out of the jet only in your case. Nevertheless it shouldn't do it, purely under 'normal' expansion with heat soak, it sounds like the fuel level is too high in that carb. But if the level in both of yours is the same with the lids off, even with different floats, then that is less likely to be the cause - *if* the floats were the same size. If the rear float is a different size, then even though the fuel levels might be the same with the lids off, a different float in the rear carb could result in a higher fuel level *with the lids fitted*.

There is another possibility and that is a very slightly leaking float valve. In normal use the flow rate is less than the leakage rate, so the mixture and fuel levels is correct. But with the ignition off the pump diaphragm is pressurising the fuel, and with a weeping valve it will eventually overflow until the pump diapjhragm has fully returned under spring pressure. But even that is less likely if it still does it with another float lid, but I'd be happier if you had swapped the front and rear and the problem stayed with the rear. You can eliminate that in any case by disconnecting the fuel line from the carbs immediately after switch off, and see if you get the same seepage from the jet.

If you do, then I'd say like Ross the carbs have to be suspect if they aren't matched, you can't tell what individual components might have been swapped as well as the carbs being basically different types.
Paul Hunt2

Thanks Paul. This is what I have done so far. New floats and pins, lowered fuel level in both carbs by about 3mm, I have a fuel pressure regulator on it's way just in case my pump is over pressure.

I tried disconnecting the pipes immediately after a long run with no result, but I haven't tried that yet with the new floats and fuel level. I am working on the theory that my fuel pump is higher than normal pressure and that the pressure is on the verge of opening the needle and as the fuel in the pipes expands it pushes the needle open, hence the regulator I have ordered.

I'll let you know how it all goes, but as luck would have it my head gasket has sprung a leak and when I removed the head I found a bent push rod. That plus the small hole in the pipe between the PCV valve and the inlet manifold goes a long way in explaining why it has been such a mongrel to tune up whilst chasing the fuel problem.

Cheers
Tony
Tony Oliver

"as the fuel in the pipes expands it pushes the needle open"

That's a very good point I hadn't considered. With the non-return valve in the SU pump any pressure increase due to heat expansion has to be taken up somewhere. With the well-documented cases of non-standard pumps overwhelming the float valves when running I wouldn't have thought it would take much excess pressure to overwhelm them from expansion. However this must happen in all MGBs to a certain extent, and the expansion volume is limited. Maybe it was a case of the float levels being too high, and the two float valves in the two lids you tried were both 'weaker' than the front float valve.
Paul Hunt2

Gents, the new floats and hinge pins seem to have alleviated a lot of the problems. The regulator is still on it's way and I hope that will be the end of the matter. My "spare" pump is the constant clicking type and when I have used it it regularly flooded the carbies.

By the way, when I replaced the pipe from the PCV valve to the manifold, replaced the bent push rod and replaced the leaky head gasket the car seemed to run just a little bit better!

Cheers

Tony
Tony Oliver

This thread was discussed between 03/05/2008 and 13/05/2008

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