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MG MGB Technical - Early MGB Misses Under Load

Hello All,

It's literally been years since I've been on the forum (life got in the way, you know), and I'm glad to see it's still here.

I have a '63, G-HN3-L/6055 with the original 18G engine.

It's not my daily driver, but this past winter, in order to avoid renting a car (long story) I tried to drive it to work. I hadn't driven it for a while, and it was raining. It ran fine, until I came to my first uphill, then it started misfiring badly under load, so much so that I ended up in first gear with the throttle to the floor just to keep going.

We're having our first Fall weather here this weekend, so I thought I'd take it out this afternoon and see what would happen. The same thing. It starts up and idles just fine, and just puttering around only had a few misfires. But my first hill with a load on the engine, it started that bad misfire again.

When it first happened on that rainy day I thought that something in the ignition had got wet, but it's bone dry here now.

I know these things are difficult to diagnose like this, but I'm just hoping that this sounds really familiar to somebody who can offer a solution.

OK, Thanks and Cheers,
Greg Van Hook

Hi Greg,
You say you haven't been on the forum in years. Has it been a while since you drove the car? How old is the gas? Do you have points or electronic ignition? How old are your plug wires?
Bayard DeNoie

Greg welcome back...

When your car misfires, what does he tach do?? Does it "bounce" or does it drop to zero? If it "bounces" it's ignition. Check plugs, points, lead wires. Be sure the wires are "tight" especially coil to distributor.

How is the fuel? Did you put fresh fuel in it??

Give us some more scenarios. Does it run "OK" on a flat road or it is it miss firing all the time just more pronounced under load?


79 MGB
gary hansen

Hi Bayard and Gary,

Well I'm embarrassed to say that some of the gasoline in the tank could be as old as three years.

The ignition is stock. I don't really know how old the plug wires are. I got the car in 2009 (I think) and they're the wires that came with it. I've gone through and all of the connections are good.

I didn't notice what the tachometer was doing. I'll take it out again and see.

The miss is under load, when I depress the throttle pedal going up hill. Prior to my rainy day ride several months ago, that had never happened before.

I think I might have noticed a slight misfire on a hard left turn.

One thing I noticed yesterday, when I was first starting it up. When I switched on the ignition to let the fuel pump tick over to fill the float bowls (before starting the engine), it did it's usual fast ticking at first, then instead of stopping, would keep ticking, about once per 1 or 2 seconds. So I tapped on each float bowl with a leather mallet in case a float was sticking, and it stopped. That's never happened before. And it didn't happen again when I got back from my ride yesterday.

I hope this isn't too much information.

Greg Van Hook

The MGB had a rev counter until chassis number 48766, so without the diagnostic capability of the tachometer, unfortunately.

With such little use it could be anything to do with the state of tune, but given the symptoms the first thing I'd do is a fuel pump delivery check. Remove a delivery pipe from a carb (note that it may spurt if the ignition has been on recently) and direct it into a container. Turn on the ignition and it should deliver a minimum of one Imperial pint per minute (and in practice at least double that), in a continuous series of pulses with minimal bubbling.

fuelling has to be the first suspect but the ignition coil can also produce similar symptoms and sometimes get very hot in the process.

Greg, SU pumps work like that, an initial chatter till they fill the float chambers, then a tick every few seconds. Fuel in the States, I believe, has more ethanol than the UK. Ethanol not only attacks older rubber fuel lines (they decompose from within!) but it also attracts water. Two consequences here, 1) it rusts steel fuel lines and, of course, the tank. Rust isn't good sloshing around in the tank, and 2) water isn't combustable!!
Also check the fuel cap is venting properly. When the problem occurs, switch off and on, if the pump is chattering there could be a pump problem, but also it might be struggling against a partial vacuum in the tank.Get out quickly and remove the fuel cap. If there is an inrush of air the vent is blocked.
Allan Reeling

Pump shouldn't click more than once every 30 secs once the float chambers are full, and in practice it should be several minutes. If it clicks more frequently then possibly a float valve is seeping, but as long as that seepage rate is less than the consumption rate at idle it isn't a problem. I only found both mine were seeping when I had the ignition on with a stopped engine (coil disconnected) for several minutes - clicks more than 30 secs apart.

Regarding water and ethanol some time ago I found a USA EPA document that said the following:

"Water vapor, however, dissolves in gasoline very slowly, even at very high humidity. For example, at a constant temperature of 100 degrees F and relative humidity of 100%, it would take well over 200 days to saturate one gallon of gasoline in an open gasoline can (assuming the only source of water is water vapor from the air). Water absorption from the air is far slower at lower temperatures and humidities. (At a temperature of 70 degrees and relative humidity of 70%, it would take over two years to saturate one gallon of conventional gasoline in the same gasoline can.)
Water phase separation in any gasoline is most likely to occur when liquid water comes in contact with the fuel. (Water in the form of moisture in the air will generally not cause phase separation.) Water which is in solution with gasoline is not a problem in any engine, but as a separate phase it can prevent an engine from running or even cause damage. Since oxygenated gasolines, however, can hold more water than conventional gasoline, phase separation is less likely to occur with oxygenates present."

This all came about from a discussion on brimming the tank over winter, about which the EPA document says:
"... gasoline should not be stored for long periods of time, especially during seasonal changes which usually have large temperature changes associated with them. ... If it is unavoidable to store gasoline for a long period of time, one should be sure that the tank (is) full to prevent condensation of water from the air ..."

But quantifies the risks:
For example, assume a tank containing conventional gasoline contains only one gallon of fuel. Assume also that it is closed while the outside temperature is 100 degrees F with a relative humidity of 100 percent. If this tank is left sealed and the temperature drops to 40 degrees F, water will likely condense on the inside of the tank, and dissolve in the fuel. In order for enough water to condense from the air to cause gasoline-water phase separation, however, there must be approximately 200 gallons of air per gallon of fuel over this temperature drop (100 to 40 degrees). Since oxygenated fuels can hold even more water than conventional gasoline, it is even more unlikely that enough water will condense from the air to cause gasoline-water phase separation."

In other words phase separation is less likely in fuels containing ethanol than without! As far as absorbed water goes it says:
"Water in solution operates as no more than an inert diluent in the combustion process. Since water is a natural product of combustion, any water in solution is removed with the product water in the exhaust system. The only effect water in solution with gasoline can have on an engine is decreased fuel economy. For example, assuming a high water concentration of 0.5 volume percent, one would see a 0.5 percent decrease in fuel economy. This fuel economy decrease is too low for an engine operator to notice, since many other factors (such as ambient temperature changes, wind and road conditions, etc.) affect fuel economy to a much larger extent."

This is all significantly different to what Moss claims.

Hell Paul that's some thesis. Does it mean ethanol "enriched" fuel is just as hygroscopic or less hygroscopic, than plain petrol? I was told that in a stored car's tank, the fuel evaporated and hence the water concentration rose. But then again some stuff you read about ethanol addition is scare mongering.
Allan Reeling

"Since oxygenated gasolines, however, can hold more water than conventional gasoline"

I would take that to mean it is more hygroscopic, although how the rates of absorption compare isn't indicated.

I would have thought that under normal circumstances you would have to store the car for very many months to get any noticeable fuel evaporation at all. During which time water density would increase, until it reached the maximum capacity of the fuel, then it would start to separate out and lie at the bottom.

I would think there is more chance of moisture from the air above fuel level condensing on the roof of the tank, and dropping into the fuel, which is why people talk about brimming the tank over winter. But even that is overdoing it in my opinion, especially in the UK.

I've never had a problem, and I will usually have the same tank of fuel typically from October to March or April.

That makes sense Paul, can't say i have had a problem either! But there again I have copper fuel lines and newish 'tinned" tanks.
Allan Reeling

This thread was discussed between 25/09/2016 and 30/09/2016

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