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MG MGB Technical - Cold Starting issues

I have a 1968 MGB that I have been bringing back after storage of about 15 years by the previous owner. I recently had my mechanic do a tune-up on the motor and it is running well when warm. However, I am having to use ether to get the car started and must nurse the accelerator to prevent stalling while the motor heats up. I have noticed that the motor will rev up to around 2800 RPM and then suddenly take a dive toward 1000 RPM and needs me to tap the accelerator repeatedly. I have an aftermarket fuel pump that is delivering 3 lbs of pressure (maybe it is 3.5 but in this realm). I did have the gas tank purged before the tune up. When I have the car warmed up it does accelerate well and it gets smoother as it gets hotter. I have noticed that it is backfiring on deceleration. If anyone has an idea where to begin sorting this out I would like to hear the opinion. I am listing the following work that has been done, so far. Thanks.

The car has new spark plugs, new plug wires, new air filters, carburetor overhaul,distributor minor overhaul with new P/C, new choke cable, valve adjustment, fuel filter, gas tank R/R and cleaned out (by my radiator shop).

Stephen Lofaro

Can you try a timing light to see if the advance(s) are faulty?

The sudden loss of power, plus the 'backfiring', suggests ignition worth a close look.

One other idea: when the father-in-law had the MG we now have, it suffered from this trait, and it was down to the connector between distributor and coil touching the engine block intermittently (under load in our case - when the engine tilted in its mounts) and shorting out!

R Kelly

Could be running too lean, also not getting enrichment on start up. Check that the jets are dropping when you give full choke (assuming HS4 SU's). Check that the pistons are rising and falling freely. Check the vacuum advance diaphragm is sound and also check for other air leaks. Pulling air in from manifold joints etc., on over-run, could cause the back-firing.
Allan Reeling

With respect to the ignition issues that may or may not have been resolved by doing a minor rebuild to the distributor and replacing the points and condenser, and having rebuilt the carburetors to satisfy the issue of the proper and free movement of the pistons therein, how would I repair the concerns about any additional air leaks beyond the vacuum advance mechanism replacement or repair? Should I remove and replace the manifold gasket? Where do other air leaks initiate?


Stephen Lofaro

Air leaks? Any joint, blanking plug, hose connection (servo), between the carb and head. Sometimes you can hear the whistle, otherwise if you can get the engine to tick over on a more or less closed throttle (high manifold vacuum) spray oil around the joints and watch the exhaust for smoke! But are you sure you have the carbs balanced, the mixture correct and the choke drops the jets? It's worth a couple of "flats" of enrichment, just to see if things improve!
Allan Reeling

Spraying carb cleaner or better still propane gas around all the joints will usually reveal a vacuum leak as a change in idle.

However if there *is* a vacuum leak, then it should be compensated for at idle when the jets are adjusted to give the correct response to the lifting pin if you are not aware of the leak. But that means at higher throttle openings the mixture will become over-rich, rather than weak.

Also HS chokes in particular should be capable of making the mixture way over-rich when fully pulled, so again weakness should not be an issue if the jets are moving correctly.

It could be that a weak spark is the problem, do you have the correct coil? Is the condenser functioning correctly? If you remove the cap and flick the points open the spark from the coil lead should be able to jump at least 1/4".

Incidentally 3.5 psi is a little high for HS carbs.
Paul Hunt

I found air leaks that made a big difference, so far. I am not sure on the cold start as yet because I had the engine hot at the time I looked over the vacuum line and also the air injection system. It turns out the vacuum advance line was broken and had hose on it that was cracked open at the inside bend. I fitted a new small piece of fuel hose for now. Long term I will replace that line. Also, the Gulp Valve hose was dislodged from the system. I refreshed those two hoses that connect the valve to the manifold. Those two 'fixes' took all the misfiring out and I did not hear backfiring in the warm up. I have not run the car out yet but I will report back about that as well as how the cold start goes next time. Thank you all for the heads up. I am used to looking at an MGA engine bay and I am getting used to seeing new things on the B. I have advice from one of my mechanics to remove the air injection system and the air pump and plug up those holes as I don't need to have this car smogged in the US any longer. So, that's an upcoming project. I want to turn my attention now to the suspension so wish me luck as I proceed. Again thank you for the's a great community of knowledge here!

Stephen Lofaro

This thread was discussed between 24/09/2014 and 26/09/2014

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