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MG MGB Technical - Bad misfire under Acceleration


I have an MGA with a 3 bearing MGB 1800 18G engine which seemed to run fine until I had to replace the clutch which I did no problems. Now however when I try and accelerate hard the engine misfires and spits through the exhaust like it is lean, when I ease off the problem stops. I can accelerate under light acceleration but any touch on the throttle and the problem returns.
So far I have done the following:

Replaced the distributer with new.
Set the timing with a strobe.
New plugs
New ignition leads
New rotor
Set rockers
Checked valve timing
Renewed all inlet /exhaust gaskets
Checked drop of needle assembly
Changed oil in the dash pot (20W)
New floats and valves
New fuel pump
Checked fuel flow no blockages back to tank
Run with the filler cap open
Removed the breather control valve
Checked compression on all 4 cylinder cold 150psi hot 146 to 150psi
Checked engine earth
The car ticks over perfectly and I can adjust mixture ok so lifting the piston with the plunger checks out ok.
I can rev the car staionary no problem with the occasional misfire at high revs.

Before I go and buy new carbs and waste more money has anyone any ideas? I realise it is an MGA but the engine is MGB.


B Bridgens

Shame it doesn't have an electronic tachometer, as that would tell you whether it was ignition LT or not. If the spitting you refer to is a back-fire i.e. explosion in the exhaust, that also indicates ignition, but can also be HT as well as LT. Weak mixture is normally flat spots and hesitation, not backfiring i.e. when the carb damper is not functioning. Test those by unscrewing them and lifting them up, then pressing them down. You should feel the pressure of the oil in the damper before the cap reaches the piston cover.

You don't mention the distributor cap. When you floor the throttle you increase the cylinder pressures even before the plug fires, and that makes it harder for the spark to jump the gap. That means the HT voltage rises, and any weakness in the HT circuit can break down under those conditions.

The other thing is that vacuum advance changes the relationship between rotor and cap contact, and under certain circumstances the rotor can move away from the cap before the spark occurs. Try disconnecting the vacuum pipe and plugging the carb port.

What ignition system do you have - points and condenser or some electronic system? Condensers can fail, although usually they stay failed once gone. New ones can only last minutes. Ditto new rotors, unless you get red ones from a reputable source, which in the UK would be Distributor Doctor or SimonBBC. A pal also had the centre contact in a new cap come out after only a few miles.
Paul Hunt

Hi Paul

Thanks for the reply.
It is not a backfire in the exhaust is more sputtering and hesitation. I have checked the pistons rise and fall without the damper and that there is pressure when replacing the damper. I bought a complete electronic distributor from SimonBBC with red rotor and distributor cap. I clipped and closed off the vacuum tube from the distributor to isolate that but no change. I am feeling suicidal as the weather here today is 27 degrees with bright blue sky, I should be out there.

B Bridgens

A secondhand electronic tacho is cheaper than a set of carbs! Jury rigged in a shoebox on the floor would confirm the LT. Apart from the old adage that "Most carb problems are ignition", by simply removing /replacing carbs it would be hard to change much. Your mixture test seem to indicate they are OK

Some of the imported dizzies were/are a bit suspect- but you could use your strobe to find the timing (curve) at various rev points. Might show if something was miles out. Whilst it won't solve the problem you might have some clues by moving the dizzy a tad to advance /retard the ignition

Depending on your type of ignition there was a case where (under test) an under cap electronic module was firing out of synch with the rotor /plug contact. It was a Petronix but it has to be said there are millions out there there work fine so this was probably an "odd" one!
Michael Beswick

I agree that there may be a misalignment between the rotor and cap. I experienced this with a semi-custom distributor that I had built for my supercharged GB engine.The rotor was so far away from the cap's terminal that it caused arcing in the cap and actually burned a hole in the cap. I also had a new pertronix distributor do the same thing. I was told by the semi-custom distributor manufacturer to cut a hole, in the side of the cap, and observe the rotor's movement with a timing light. I was amazed at how far off it was. The manufacturer offered to fix it for free, but I was able to slightly modify the housing to bring the rotor and cap terminals into line. Also, try using 20W50 oil in your piston dampers. This will slow down their rising and enrich your mixture on hard acceleration. RAY
rjm RAY


Thanks Michael and Ray for your comments I'll give it another go tomorrow.

B Bridgens


Today I went through the whole lot again, tappets, timing and carb tune. Went out for a run seemed fine then after about 1 mile the problem returned. I hobbled back and got into the carbs which had the air filters off and what do you know the back carb damper was stuck down and took quite a bit to lift it. I took out the needle and reassembled and the piston was as free as a bird replaced the needle same sticking. Could this cause the problem? Checked the plugs 1 and 2 OK but 3 and 4 were lean. I was going to re-centre the jet but thought I might as well replace the assembly in case of a bent needle or problem with the jet. I dont know how easy it would have been to knock the needle assembly during removal and replacement of the carbs before and after the engine lift. Anyway B & G will take a couple of days to ship the parts so will have to wait till then to find if that's the problem. Any thoughts if an intermittent stuck damper this could cause these sypmtoms?


B Bridgens

When you originally said that " I can accelerate under light acceleration but any touch on the throttle and the problem returns." my thoughts were fuel starvation. That's exactly what happens when you are about to run out of petrol. If the damper in one carb won't lift, that carb is effectively non-operational and the engine is running on just the other one. So I guess that could be your problem. Let's hope so.
Mike Howlett

whilst you're at it check the pistons are matched, see - 41 Matching SU Air Pistons -

and consider using 90 weight gear oil in the damper and adjust if required, see - 123 MG Carburetors & Doors (first part needles and seats, springs and carb oil)
Nigel Atkins

One piston not lifting will definitely cause running problems! However 90 wt gear oil (not diff oil) is the same viscosity as 20W/50, so it's not that different. Some people swear by 3-in-1, ATF, specialist damper fluid and all sorts. The factory said to use the same oil as in the engine, which for our climate could be 15W/40 or 10W/40 as well as 20W/50, the first two being thinner than 90wt. The thicker you go, the more piston rise is damped, and the more fuel you will use while it is still rising.
Paul Hunt

90 weight gear oil is not quite the same as 20w50 - see chart below

I've been using it for about 18 months now in my midget and it makes for very smooth operation

I can't say if it uses more petrol on hard acceleration as I've never measured but on one very boring (50mph) mpg check run the figures were very good, above any test figures I've seen from the 70s but my car is improved by modern more reliable parts and components such as the 123 fully electronic dissy

it's also well above book power output and normally driven in a spirited manner yet the general mpg seems reasonable, I can't be bothered to check again though, I once done a 100 mile mpg check run at 50mph on the M6 and even my wife wanted me to give it up and put my foot down more

Nigel Atkins

Hi Guys

I am loosing the will to live with this problem.

Recentred the jets, no change but damper piston now move freely.

Put in 20/50 damper oil, no change

Blew out with compressed air the fuel lines, no change.

The engine ticks over beautifully and when I raise the damper pistons with the plunger, slight rise in engine revs then settle down. Reving the engine stationary there is no problem but as soon as I try and accelerate!*!?*. It seems to go bad at about 2500 revs.
Should the outer casing of the coil be earthed?

Time for lunch

B Bridgens

"Should the outer casing of the coil be earthed?" It's not necessary. It doesn't matter if it is earthed or not. Your problem is certainly a strange one. I'm afraid I have no more ideas at the moment.
Mike Howlett

In practice it is likely to be but it's not required for the ignition. The 'other' end of the HT winding is connected to one of the LT terminals, not the case, which boosts the spark from the auto-transformer effect.

As Michael suggested either rig up an electronic tach, or even a timing light you can see while driving along. At the moment you have no confirmation as to whether it is ignition or fuel. Tach will only tell you if it's ignition LT though, timing light will tell you if it is LT or HT.
Paul Hunt

Hi Guys

Just to finish this thread.

I decided to take off both jets tube assemblies to check thoroughly if they were clear. When I took off the rear carb tube I found that through deterioration the jet tube rubber seal had formed a neat flap which would seal the tube under heavy fuel demand but open under light fuel demand. Needless to say I checked the front also, not as bad but clearly deteriorating (damn modern fuels). I had been blowing through the jet tube from the jet side so I was not seeing this problem as the flap would open in the reverse direction. I managed to get two new rubber seals and washers and refitted the carbs. I have just been for a 10k drive with no hint of the problem. I think it might have been coincidence that it happened after I put a new clutch in, which really sent me off on the wrong tack.
Anyway thanks for all your help and suggestions it is great to get the car back to normal running and I hope that something has been learnt from this experience.I tried to take a picture of the offending seal but it was not very clear so of little use.

B Bridgens

Don't necessarily blame modern fuel, decomposition of the the seal on the fitting that screws into the float chamber on HSs has been something to look out for for years.
Paul Hunt

This thread was discussed between 18/10/2014 and 05/11/2014

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