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MG MGB Technical - Engine will not respond to throttle from idle

1978 B with HIF SU Carbs and 45D4 distributor.
When engine is upto normal working temperature it fails to respond to throttle from idle and stalls.
Engine was running perfectly prior to MOT Test when garage adjusted Carburettors on the strength that the C02 levels where too high.
Investigation highlighted that Timing had been wrongly set along with one carburettor set rich and the other lean.
Coil, points, condensor,rotor arm, distributor, plug and HT leads have all since been replaced. Timing set to 10 deg BTDC, Dwell 51 deg, CO2 3%. Ignition cct tested throughout including Ignition Relay and Ballast Cable.
Cylinder compressions check out No1 170, No2 165, No3 170 and No4 175 lbs/sq inch.
Any carburretor adjustment throught the range has very little effect
Any ideas please
B J Young

Do you have a see through fuel filter? If so look at the fuel flow as you rev (or try to rev) the engine.
I have heard of a case where a blocked filter would pass just enough to run the engine at idle, but if any more flow was needed it could not pass it, and the engine stalled. There may also be a blockage elsewhere in the fuel line. What is the behaviour of the fuel pump when this happens?
Andrew Blackley

At a "glance" the carbs sound lean. The fact that they were adjusted to "drop" your CO levels, probably means they leaned them out. Before = good. After = bad. (Sort of has that "cause and effect" feel, doesn't it?) You could try the "lift the piston" method to verify this (if you lift the piston and it immediately stalls, then the carb is too lean; if the idle increases, the carb is too rich...of course it needs to be done for both carbs...make mixture adjustments to both carbs, turning the adjustment screws by the same amount each time to keep them in synch). In any case, IF the only adjustment was the carb adjustment, stalling when taking off from idle tends to indicate lean. You might also check to ensure you have sufficient dashpot oil (although that's probably not it, but this is a good time to check it anyway). You should also check you timing, ignition condition, and valve clearances before ever touching the carbs, however you'll probably end up coming back to them and finding them to be lean. On the other hand, if you typically have these guys work this problem and you like working with them, then tell them what happened, let them test drive the results of their work, and give them the opportunity to sort it out (probably at no cost to you). Tell them now that you've passed the emissions test, you'd like to drive for a while ;-)

FWIW,

Barry
Barry Kindig

Unfortunately whilst the MOT people will tweak (I never let them touch mine, making sure it will pass CO beforehand) the carbs to get it through the test they do not drive it afterwards. If you take it back and complain they will want to charge you for investigating why it can only pass the test with an excessively weak mixture. The alternative is for them to fail it for excessive emissions. Also I'm assuming that they managed to drive it out of the test bay back to the car park.
Paul Hunt

There was no reason to expect the CO2 levels would be incorrect before MOT Test,I don't normally let the testers make adjustments, my mistake. Fuel pump and supply through to carbs is okay. One thing I haven't checked yet is the possibility of an air leak on the inlet manifold.
B J Young

Oh...I had no idea that the MOT people would actually apply a "correction" to bring a car into compliance. I thought he was referring to an independent garage.
Barry Kindig

The Hifs have bi metal strips to lean the mixture when hot

if the fuel is too hot, it will cause an overlean situation

if an emission test was run at MOT, then the engine is likely to have been run with car stationary for a long time, and the fuel in the llines wd be very hot

I shd recheck CO (not CO2) reading when the engine is just run up to temp, and remember to clear the carbs with a 1 min ute run at 2500 rpm between each test

Also check for stuck pistons etc

3% is very lean for cars of this era - 4 to 4.5% is much more likely to be satisfactory
chris storey

This thread was discussed between 10/06/2002 and 11/06/2002

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