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MG MGB Technical - Low speed misfire

I get an occasional misfire when I accelerate in 1st or 2nd gear, usually after being in slow traffic or pulling away at lights or on a roundabout. It runs perfectly at anything over 15 mph. When parked, the engine runs ok at all speeds, with no signs of misfire.

I tend to have the carbs set slightly lean, to make sure I get through the MOT. It was about 2.5% CO on my gunson tester the other day. I've checked all the obvious stuff - points, dwell angle, timing.
Brian Shaw

But have you replaced the cap, rotor, leads and plugs as that's certainly where I'd start. If you can kleep it misfiring for as long as possible you can then shut off the motor and remove the plugs to find out which cylinder is the culprit. From that point it's at least easier to swap components etc until you find out where the fault is coming from. This fault is unlikely to be carb orientated, neither is it related to timing but you say that you have checked the dwell angle... have you checked that the points are fairly evenly gapped on all four lobes just in case the spindle is either bent or severely worn.
Iain MacKintosh

It's only an occasional misfire. It misses once or twice, but only when the engine is under load, at low revs. Then it may be ok for a day or two till it happens again. Can't get it to misfire at all when it's at standstill. I renewed all the ignition electrics a couple of years ago.
Brian Shaw

Brian, I had a similar problem three years ago on our A. It would happen at random at all speeds. After a lot of hair pulling it turned out to be a faulty condensor that was only a year old.
John H

It's under greatest load conditions and I suspect is worse the further the accellerator is depressed without letting the revs rise. Two years is a fair time, change the plugs, leads and cap.
Iain MacKintosh

My daughter's Metro would only misfire under load, but fortunately also when I quickly and fully opened the throttle at a standstill and closed it again before it went into orbit. If that doesn't do it then as Iain suggests restraining the revs at a standstill with the clutch (in 4th gerar, not 1st!) should also reveal it. I used the timing light to diagnose a faulty cap or rotor and changed both.
Paul Hunt 2

Before you rip the engine down check these things. If you have Bosch platnum plugs remove them and install standard plugs. We have had a problem with valves sticking when cold because of deposits from the new fuels. you can try top engine cleaner which is a cumbustion chamber cleaner. just tap into the intake manifold with a small hose and run it through. If you have chrome rings on those new pistons your oil consumtion is normal. It sounds like you baby that engine just drive it normally and the rings will seat if the machine work was properly done, hope this helps
Jim Pelletterie

I think the clue lies in your statement -

"I tend to have the carbs set slightly lean, to make sure I get through the MOT. It was about 2.5% CO on my Gunson tester the other day."

These engines run best slightly rich (5 or 6% CO) - the hesitation on acceleration from idle is almost certainly caused by the weak mixture.

I would start by just richening the mixture a flat or two (or a quarter turn on HIF carbs)
Chris at Octarine Services

A prior thread contributor mentioned inserting a short length of wire inside the plug wire at the distributor end. I tried it, it worked.

My car was occasionally misfiring for no apparent reason. I have an electronic ignition, aftermarket hot coil and almost new hi quality carbon fiber spark plug wires. The car was misfiring under hi load - particularly one plug, and on throttle transition it would occasionally stutter.

My distributor cap is the horizontal slide in style with an internal screw that punctures the cable and touches the carbon fiber conducter.

I checked the resistance between plug and cap and found only 1 of the 4 wires showed any resistance - the other 3 were open.

I shoved a stranded piece of wire (after tinning it with a solder gun) about an inch long up the center of the plug wire and reinserted the wires in the cap.

Viola! No more misfiring. Smoother driving starting acceleration, etc.

I think the play and vibration at the exit from the cap can break the fiber and make a hi resistance point that enables the spark to go everywhere, except to the plug where it is supposed to go.

This is, in my opinion, is one of those problems that can be hard to solve -- you have newer high quality plug wires -- they are obviously not the problem -- wrong!!

Barry.
Barry Parkinson

I don't like HT cables with carbon conductors as they tend to cause problems but only after a few years of service. It is for this reason that I always buy copper cored ignition cable off the roll which is a bit difficult to find these days. I usually crimp the dist cap ends on and use screw in connectors at the plug ends which are almost identical to those fitted originally. They incorporate a resistor so radio supression is not an issue.
Iain MacKintosh

Hi all.

Like Iain, I prefer copper leads.

It is important to fit resistive connectors (or resistive plugs) as the resistance beneficially modifies the characteristics of the spark.

Don
Don

"Viola! No more misfiring" Sweet-sounding, then (I think you meant 'voila').

If the HT wires don't have fitted brass caps with a free length of 'conductor' (carbon string, silicone or whatever) trapped between the outer insulation and the brass contact then the connection i.e. from a screw in the cap will be hit and miss. There shouldn't be any need for a bit of wire if the leads are fitted with push-in connectors.

The original carbon string type are still available for concourse use but in my experience are very expensive and of limited life as Iain says. I've been using silicone cored for 40 years, one set per car for as long as I've had any car which has been up to 15 years/75k miles and counting. I've only had one 'failure' in that time which was a curious bright blue coating which developed on the connector of the lead in the coil, which proved impervious to HT and scratching with a sharp implement. Rather than take a file or anything else to it and risk weakening it and a subsequent failure I bought a replacement set.
Paul Hunt 2

Chris at Octarine: are you sure about the 5 or 6%? My car is a 1979 UK spec - the Repair Operation Manual 1978 says "exhaust gas emission reading at engine idle speed 3.0% maximum". For a 1979 car, anything over 4.5% will fail the MOT.
Brian Shaw

I've seen this on other cars Brian. You are trying to set the emissions to lean burn standards which the B certainly is not. By all means set it to 3.0% for the purpose of the MOT but as soon as you leave the station put it back to 6%
Iain MacKintosh

I've never bought a Gunson tester as I gleaned from comments by others that they often end up overestimating the CO. Have you checked the readings against a professional system? You may have more room for adjustment than you know, even at MoT levels.
Steve Postins

I've seen these bad comments as well but I've used a Gunson tester for many years and seem to have got away with it.
Iain MacKintosh

My Gunsons 'accuracy' has gradually drifted out over the years, as indicated by the print-out I get from the MOT station each year. However it seems consistent, and I just compensate accordingly, and so far the V8 hasn't failed the test. The problem - well, the first problem - is that once it has warmed up I can't get the reading in air down to 2%. It is to begin with but gradually increases as it warms up. The 2nd problem is that the reading is *very* sensitive to any tilt of the unit. 3rdly I have noticed that if standing on hot tarmac it gives a higher reading than supported a couple of feet off it! I tried emailing Gunsons but got no response. I also had a Gunsons multi-meter which packed up after only a few years (I have analogue instruments going back to the 60's which work perfectly). The replacement would have been about 60 quid, whereas a very similar Draper unit (no 250v scale) only cost about 20 quid.

MOT requirements are:

First *used* before 1/8/75 is visual test only.
First *used* between 1/8/75 and 31/7/86 - CO <=4.5%, HC<=1200ppm.
First *used* between 1/8/86 and 31/7/92 - meter test CO<=3.5%, HC<=1200ppm.

Note that it is the first used date that is important, not the manufactured date like for free road tax. A car built before 1/8/86 but registered after it will have to meet the more stringent limit. Only a couple of years ago I saw an unregistered midget in Yorkshire, can't see that ever being street-legal.

My 72 built, 73 registered roadster is eligible for free road tax :o)

My May 75 built, November 75 registered V8 has to meet 4.5% :o(
Paul Hunt 2

Re Gunson tester:
Mine always reads a bit high, when compared with the testing station (if the Gunson reads 3.0, the MOT printout will show 2.2 to 2.5). Although not very accurate, the Gunson is fairly consistent - I always use it in the same spot on the drive and stick to the same procedure, particularly regarding the warming up of the car, and the Gunson unit.

Re my misfire:
The car's favourite spot for doing the misfire is just after the straight stretch of road just before I get home - there's 300 yards at a steady 40 mph
followed by 300 yards at 30 mph, then a sharp left turn which I have to take in second gear. As soon as I start to accelerate in second there is a misfire - just one - then I can accelerate through the gears smoothly without any further problem.
Brian Shaw

Sounds like it is the sharp left-turn that is the problem. If you have HS carbs it is easy to check you have no debris in the bottom of the float bowls, HSs are also more subject to disturbance from G-forces ...

Could also be loose wiring, either from the force of the turn or the tilting of the engine when accelerating. When I first had the roadster I found the ignition would cut out when *making* the turn. One of the wires on the coil was too long and wrapping round the steering column :o)
Paul Hunt 2

I thought at first that Paul Hunt was going to suggest you get the Council to fix it but no !!

I'm still sure that you have weaknesses in the ignition system. Firstly get the mixture adjusted richer and then actually replaye the ignition parts especially the leads and plugs until you find out what is wrong. If you can't definitely eliminate parts then it becomes difficult to narrow down the problem.
Iain MacKintosh

This thread was discussed between 05/01/2006 and 17/01/2006

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