Low speed misfire.

[Vic Butler]

My MGB GT has developed a low speed misfire. Compression pressures are OK so I'll check the connector between the Lumenition infrared eye and the control box, then the plug leads, and if all that fails the most likely cause is the ignition control box. The same thing happened to the Lumenition set up on my Land Rover and fitting a new control box cured it.
Once the misfiring has been cured it's a 4 mile trip to the Mota Lita factory for a steering wheel.

[Allan T]

I was mystified by uneven running some time ago, and none of the obvious routine things made any difference. Til in desperation I looked at the electronic ignition lens, and noticed it had a light coating of dust. A quick clean, and my troubles were over. Mightn't fix yours, but who knows? It's an absurdly simple thing to try, and good routine maintenance anyway, and certainly won't do any harm. Good luck!

[Vic Butler]

Thanks Allan, I'll check it out.

[Allan T]

One other thought - does your rotor arm have a rivet? Earthing through that would be more likely to be evident at lower revs. Again, even if it doesn't fix your uneven running this time, swapping your arm for a rivet-free one would be a good idea.

[Peter Cresswell]

Hi Vic
On a different tack entirely, have you checked that the carb piston in the dashpot is falling freely and completely onto the bridge in the venturi? I had this happen on my Mini Cooper, which had a new pair of carbs, but one didn't drop freely resulting in a misfire at low revs. Things to check are:
1. Does the misfire clear at higher rpm?
2. Does it only occur after the engine has been running for some time?
3. Does it start and run ok from cold?
4. Is the piston and the inside the dashpot nice and clean and with no signs of the piston rubbing on the dashpot?
I my case it happened on one of the Mini Cooper Register tours - Mini to Monte. Initially it occurred mainly after running for about 100 miles, but eventually it did after a it after a few hundred yards. The tour had a travelling mechanic who changed all the ignition components a couple of times, to no avail, and finally suggested that as it started at about 4,500 rpm, I drive it home (from 50 miles south of Lyon) above that engine speed. Once home it did it on the drive and whilst investigating I heard a soft 'plop'. I had heard that before many years ago when an MGA I had did the same.
Changing the dashpot for an old one cured the problem.

[Vic Butler]

Peter it's at tickover. When I blip the throttle the engine very quickly hits the red line. I'd removed what I thought was a faulty lead then the engine misfired all the time.
I fear it's something to do with the electronic ignition module. I changed distributors earlier and found that I hadn't allowed enough free movement in the wiring of the eye on the original distributor and the wiring was frayed inside the grommet. I got another eye from Lumenition but it was the latest version but they said it should be compatible with the module. I'm going to check the plug from the eye to the module, fit a spare set of leads and it's overdue for a plug change. However, during the first test run with the different distributor (no vacuum advance) power was down and it ran very hot leading me to believe the ignition was very retarded. Advancing the ignition by a small amount resulted in the engine kicking back against the starter. It's definitely ignition and the most likely cause is a failing module.
The original distributor with vacuum advance has been refitted.

[Ian F]

Hi Vic,
did you get this problem sorted out? Let us know please.
I think running a distributor without vacuum advance is mainly done to allow a lot of advance to be used at tickover - this helps very hot cams to run a little better. Absence of vacuum advance allows circa 20 degrees static advance and then only circa 10 degrees dynamic advance ( practically the opposite of the standard distributor with a vacuum advance).
Not sure whether this might be contributing to your running problems?
As usual, happy to be corrected on this!

Ian F

[Vic Butler]

Evening Ian.
The optical unit is back from Lumenition. I've yet to refit it, with new plugs and leads. I'll let you know in due course.
If that doesn't cure it then it can be only the control unit.

[Ian F]

Hi Vic, hope you got this sorted out by now? Was it the ignition unit or the ignition timing?

Ian F

[Vic Butler]

Evening Ian. I've been tied up with other things for a while. I've fitted the ignition module into the distributor and now it's a plug and plug lead change, fitting the plug to the module wires, fitting the distributor, connecting is up and praying. I must really get down to finishing it as it shouldn't take much more than half an hour or so.
I'll post the outcome hopefully soon.