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MG MGB Technical - Another cold car running bad question

OK, I have asked some of this before, but let me put a new twist on it. My car is a 1973 MGB and is basically stock, completely rebuilt about four years ago (3,000 miles). About two years ago I put in a new, later model distributor and a pertronix system. The car ran/runs great. I did this in the summer of 2005. All during the winter of 2005/2006 the car ran great, as well as all through 2006. Now it is the winter of 2006/2007. There have been no changes to anything on the car since the new distributor was put in and it only may have a total of 1000 miles since the update. During the winter the car mainly sits in my garage, under a cover. Every few weeks I go out and drive it to keep everything moving and charged. However, the last two times I have driven it, when it is cold, the car sputters, and coughs and chokes and acts like it is trying to back fire through the carburetors. However the car has never failed to start on the first try and it idles fine, but when you start driving it has no power and will not hardly go - that is until it is completely warmed up. Yesterday when I drove it - it was about 70 degrees here in Atlanta yesterday - it did the same thing. All this has only happen the last two times I have driven the car.

Yesterday, I realized that the gas tank was about half full - normally I keep it full during the winter. Could all these problems be caused by condensation/ water forming in my gas? Would water problems go away after a car is hot? I filled the car up and added some stuff to get rid any water in my fuel, so I will have to wait a few days and see if it makes any difference. But has anyone ever had a problem with their car forming moisture in the gas while sitting like this and causing similar problems?

Any other suggestions? As I said, nothing has changed and the car starts fine and runs great when it is hot.
Robert Browning

Robert. I would tend to suspect condensation inside the distributor cap. Forms over night, shorts out the HT circuit until the engine is warm enough to heat the distributor and cause the water to evaporate. Then, with the system not shorting out, the engine runs fine.

As to the gas, some "anti-freeze", alcohol, will mix with the water and allow it to be burned. We used to do this at the beginning of every winter when I lived in cold weather country. You might also consider some fuel stabilizer to keep it from breaking down when it is sitting.

Les Bengtson


Could it be you have a faulty choke system, or maybe a very small manifold leak? A manifold leak caused the same problems for me. Stupid me-I put the hose on the distributor without a clamp. Installed a clamp and tightened and problem solved.

Checking for moisture before you run the car could eliminate that.

James Huggins

Les and James,
If there was moisture in the distributor - which to me sound very likely - would the car still start and idle. I know from past experiences that if I ever had any water/moisture in the distributor the car would not start.

Also, will someone define faulty choke system? I am not sure what this means. To me, in the simplest form, a choke system is when I pull the choke cable the butterfly valves in the carburetors close and as the car warms up I gradually release the cable allowing more air into the carburetors. What would be faulty if this part works?

Also, I will check for a manifold leak, but wouldn't I still have the leak when the engine is hot? ..and wouldn't this cause the engine to run bad even when hot?

Thanks again for all your help.

Robert Browning

Les - what type fuel stabilizer would you recommend?

Robert Browning

Electric Choke. I like automatics.
James Huggins

SU carbs don't have choke butterflys, which are different than throttle b'flys. So it is not a "choke" or as prior terminology described it "strangler", rather it is a "cold start enrichment device" - the most generic term. It functions by increasing the effective size of the main (and only) jet on SU, making the mixture much richer for cold start.
Your symptoms are those of a failure to fire under load: either poor spark (moisture, cracked/tracked cap, rotor, bad wires, bad points, or coil); or lean mixture. Typical developing lean mixture causes are : Loose inlet manifold causing an airleak - extremely common on cars "recently" apart - 3000 miles and 2 to 4 years is about perfect. Loose carb mountings. Dirty fuel filter (dirty includes wet with water). Lean on acceleration only is low carb damper oil, or water in the bottom of the float bowl and jet tube.
If the engine was correct and hasn't been "adjusted", then you have one or a combination of "self created" these conditions. The slightly lean mixture will let the car idle and run OK, as long as it's not heavily loaded in acceleration. It's more sensitive to lean conditions when cold, which is why the trouble is more apparent then, and also why you have to use the "choke" to start. The "choke" richens it so much as to more than compensate for the fault. So the first place the problem shows is on cool engine accel with "choke" off. As it progresses, warm accel and then general running will deteriorate.
Water in the carb bottom can sit there with gas going right past it, but on heavy demand it sucks the water into the jet and blocks it, since the water "glob" won't go through the jet. When demand lessens, the water falls back to the bottom and unblocks the jet.
Lean is harder to light than rich, so spark problems show up under the same conditions.
The conditions that cause condensation in the tank cause the same in/on the ignition components, so having double trouble is normal.
Fuel dryer = isopropyl alcohol or commercial product.
Fuel stabiliser = Stabil or similar.

Tighten manifold, add fuel dryer, if there are signs of moisture on cap, dry it off inside and out. The real purpose of WD40 is to displace moisture, so it works on distributors, but is messy and I don't use it except in "emergency" situations.
FR Millmore

Great thread. I just learned about the possibility of condensation in the gas and distributor. I wish this thread cropped up a week ago.

J. Palgon

I have a feeling that your problem is related to the plugs where they are fouling due to not being heated enough. Under these conditions they will break down immediately you put any load on the engine and running will become a mess. Replace them with new just to prove a point.
Iain MacKintosh

My V8 used to need new plugs every few thousand miles, much less than the normal replacement period. I would suddenly find one morning it wouldn't start, whereas new plugs (I always kept a set ready gapped in the car) would fire it up straight away. Sometimes this would be preceeded by a few days of rough running when cold, but fine when hot. Eventually it happened one morning when I had time to properly diagnose it, instead of having to be somewhere so just changing the plugs. Clipped a timing light on the coil lead and cranked it - lots of nice regular flashes. Clipped it on any plug lead and very erratic flashing if at all. Changed the distributor cap (Happened to have a spare) and it hasn't needed new plugs since. A similar thing happened on my daughter's car, diagnosed the same way. I already had a newish rotor on so left that, otherwise I would change both cap and rotor.

What happens is that as the plugs age they get harder to fire, particularly when cold, and this gradually pushes up the HT voltage though the life of the plugs. The HT system shiuld be able to cope with this, but if anything like coil, leads cap or rotor are marginal they can start breaking down hance no spark much sooner than they should. The replacing of the plugs 'curing' the problem was actually just another symptom.

I doubt it is fuel, over winter my roadster (and V8 if the weather is bad enough can have a tankful last anything up to 5 months. I've never had a problem with bad fuel. If you have condensation in the distributor cap it is quite likely from an intermittently leaking heater valve, BT DT as well, although in my case even though it was 'full' of water and the first thing I knew of the valve problem was water running from under the car it didn't miss a beat. Just was well, we were several hundred miles from home in Ireland.
Paul Hunt 2

At 70F air temp, I doubt it has much to do with air mass, but as Paul says, much more to do with the things he and Les have mentioned. You do have a higher avg. humidity than where I live, so my guess is that condensation could be a suspect, but I also guess perhaps not as much so as where Paul lives in Britain. If you did get a slug of water in your gas, that could be from a fill up as not all gas stations do a good job of cleaning it out of their underground tanks and if one of theirs had corroded or ruptured (unbeknownst to them) and water got in/accumulated, you may have been treated to a slug of water along with the gas you pumped.

Clear up the guess work on the ignition and then if you still have the problem, get out the alcohol and fuel stabilizer. Another product, SeaFoam, can also be a good chaser for a slug of water in the tank.
Bob Muenchausen


I agree with wahat Paul mentions. Last year my mgb roadster stopped and i wasn't able to start it anymore.
visually checked everything but couldn't find it. Then replaced the rotor and distributor cap and the car ran. Then fitted the old rotor and the car refused to start. So it was the rotor. I couldn't see anything but there must have been a crack so that the sparks leaked to the distributor shaft.

I had a problem similar to that. I found that when anti freeze gets spilled on a dist. cap it is absorbed into the pores. Parking overnight in a high humidity area would leave condensation on the inside of the cap and just leaving it out in the sun would heat it up enough to dry off the moisture. Changing the cap solved that problem. Apparently the anti freeze was hygroscopic and this created my problem.
Sandy Sanders
Sandy Sanders

This thread was discussed between 23/02/2007 and 02/03/2007

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