Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Car Develops Terrible Misfire when it rains.

I'm at wits end in trying to figure out the problem. Hopefully, someone has had this same problem and can tell me what needs to be done. 1972 MGB-GT. Everytime I am out driving and it begins to rain the car starts to misfire so bad that it is all I can do to keep it from stalling and get back home. This has happened about 7 or 8 times now over the last year. The harder it rains or the more water that is on the road or the faster I drive through the rain the worse it gets. Misfire, rough runnig,backfireing on decelleration, stalling. The only way to keep it from dying is to pump the gas peddle, on and off to keep the revs up, also to stay in a lower gear.

When this first started I just figured I got some water up into the dstributer cap, but upon inspection things looked dry there.
More recently, I thought I figured out the problem, and came up with what I thought was the solution. Up under the both front wheel wells on the flat top boxes I found the sheleves to be completely rusted out. and there were rust openings into the engine compartment. So I completely sealed those areas so water could not be thrown up into the engine bay by the turing wheels. Then again when I drove the car yesterday in the rain the same problem occured. Barley made it home opened the hood and everything is very dry in the engine bay. The only water I was able to find was up front by the oil cooler and the radiator (rain water not coolant or any other fluids). However, I have noticed that water somehow is getting into the cab. It leaks down under the dash on the passenger side. It gets onto some of the wiring there. I could see it on the side of the fresh air vent corregated hose/tube. I looked into the fresh air opening on the cowling in front of the windshield and there is no water in there and the drain pipe is not plugged.

The way the car runs when it rains, I would have thought for sure it had to be water in the dizzy. But again it looks nice and dry around there, around the fuses and around the coil. Could this be happening because the wries under the dash are getting wet? Please I need some advice. I love driving my MG's on daily basis. But right now both of my MG cars are not very capable.

I really hope someone has an answer to this problem, my wife will not go along with me on drives because she fears the car will die.
John Fraioli


Just because it looks dry doesn't mean it's not damp enough to cause a problem. Have you tried isolating the problem area using a water dispersant spray (WD40)? Spray one item at a time to see if it helps. If the air is very humid that could be enough to cause problems, for example if you have bad HT leads with poor insulation. For the price of an HT service (leads, plugs, cap, points and rotor arm) I would change the lot to see if substition helps.

In my experience poor running in the wet is usually the HT side of things, but I suppose you could have a problem from the wiring under the dash. In particular you have the switched LT feed to the rev counter and, of course, the ignition switch wires under there. Sounds like the leak is from the windscreen seal?

T Jenner

By any chance during it'e past has the distributer cap been exposed to antifreeze?
Those caps are porous and the antifreeze becomes impregnated in the pores. Then in high humidity it is hydroscopic and absorbs water leading to misfire.
The only cure for that is to replace. I have not found any way to clean them.

Run it into a dark garage, open the hood and look for sparks anywhere around the distributor or spark plug wires. In addition to possibly being a bad distributor cap, deteriorated spark plug wires could be the culprit. Cheers - Dave
David DuBois

Almost has to be HT somewhere, highly unlikely for rain to affect the 12v side of things. To stop the ignition it would have to be shorting to ground, but it is unfused so any current flowing is more likely to boil the water off if anything. Could still be other things in the ignition LT thoiugh, is the tach bucking around when this happens or steady? If steady then even less likely to be LT.
Paul Hunt 2010

Thanks for the comments.

The spark plug wires were replaced about a year ago along with the cap points and rotor. I did run the engine in the dark about a month ago and did not see any sparks anywhere. Also about 2 months ago the heater valve did leak onto the distributor. So yes the cap did have antifreeze all over it. (But this problem with misfiring during the rain first began over a year ago). I did swap the Cap and rotor out about 3 weeks ago while participating in a rainy rallye weekend. Of course the cap and rotor were both used. They were loaned to me by a fellow MGB entusiast. I have not tried spraying any WD40 on any of the components although this was also reccommended to me during the rallye. What all should I be spraying? Just the cap and wires?

Paul, I did watch the tach while this was happening and it did not bounce around it held nice and steady.

I plan on getting some new parts today from our local British car shop. I also have a different dizzy I think I will try it. Currently the car has a 25D4, I have a 45D4 from my 79 MGB that I could try. It is fairly new but I will buy a new cap, rotor and points for it also today.

I really need to see if I can find where the source of the water under the passneger side dash is coming from. I guess I could take the garden hose and spray the top of the car and watch inside. I don't think it is the windscreen seal as this is a GT, and I would suspect I would notice any leak on the top of the dash.

Please, keep some more thoughts coming.
John Fraioli

I had water flooding behind the starboard side dash, then running down and pooling on the floor.

Look along the top edge gutter of the fender (where all of the mounting bolts are). Follow along to where the fender meets
the cowl. There should be a dollop of putty in the corner of that
gutter that seals the gap to where the fender meets the cowl.

Sealing that gap kept the water away in my case.

As for your misfiring...have you checked the connections at
the fuel pump? I figure that it's mounted low to the ground
and any water splash could reduce voltage from getting to it.
Daniel Wong

John - the water on the passenger side under the dash is probably coming from the vent box for the heater. There is a drain tube for this box on the passenger side of the box that goes down along side the transmission. The drain tube has a rubber end shaped like a ball with with a flat end sticking down. This tube gets filled up with debris and the water in the box doesn't drain out, instead it finds an opening in the side of the box and leaks into the passenger side of the car's interior. I have my sincere doubts that this water has anything to do with your problem. Cheers - Dave
David DuBois

With the engine running dry and well, take a spritz bottle and spray water on components in the engine compartment until something happens. This may locate the vulnerable component.

Wayne Pearson

If you do that anywhere near HT it is almost certainly going to cause a misfire, regardless of whether that is the cause in the rain. I had a bottom hose perforate on the V8, in the rain, and the only way I knew was a light misfire when accelerating. Oil and temp gauges were normal, even though by the time I investigated it (on a motorway so chose to wait for a service area) there was virtually no coolant left.

Looking in the heater air intake should show whether it is retaining water, you should be able to see the bottom. If so, pour some in anyway, just to check it *is* running out at a reasonable rate. But you would need to be in a helluva downpour for a partial blockage to overflow into the cabin, and that in't going to be anywhere near any electrics anyway. Faulty pedal cover seal or missing screws, or any missing or 'open' grommets on the bulkhead shelf will also let water in.

Paul Hunt 2010

Don't be so certain the GT windscreen seal won't leak. Mine did - badly, was due to the metal under the seal rusting out.

T Jenner

This thread was discussed between 05/07/2010 and 07/07/2010

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.