Low speed misfire.

Technical MGB discussion
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Vic Butler
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Forename: Vic
Surname: Butler
Location: North West Hampshire

Low speed misfire.

Post by Vic Butler » Sun May 12, 2019 10:45 am

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.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Allan T
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Forename: Allan
Surname: Titmuss

Re: Low speed misfire.

Post by Allan T » Mon May 13, 2019 12:37 am

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
Posts: 275
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Low speed misfire.

Post by Vic Butler » Mon May 13, 2019 7:40 am

Thanks Allan, I'll check it out.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
Allan T
Posts: 5
Joined: Mon Apr 01, 2019 10:59 pm
Forename: Allan
Surname: Titmuss

Re: Low speed misfire.

Post by Allan T » Mon May 13, 2019 9:43 am

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.
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Peter Cresswell
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Surname: Cresswell
Location: Stone, Staffordshire

Re: Low speed misfire.

Post by Peter Cresswell » Mon May 13, 2019 11:22 am

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.
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965
Vic Butler
Posts: 275
Joined: Wed Dec 21, 2016 6:07 pm
Forename: Vic
Surname: Butler
Location: North West Hampshire

Re: Low speed misfire.

Post by Vic Butler » Mon May 13, 2019 7:05 pm

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.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine
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