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MG Midget and Sprite Technical - Midget Misfire

Hi. My (1972) 1275 has developed a misfire over the winter. Initially, I found a problem with the float valve sticking, which i took apart, cleaned and now works fine. I have replaced the distributor and leads with one of those hall effect distributors. The engine seems to run OK but with an irregular misfire. I've checked the plugs and they seem fine. Any suggestions? I thought that I might strip the carbs completely and check for crud etc and rebuild, but not sure if I should look elsewhere first. I would add that I had the misfire before I changed the distributor and leads, so I'm sure that its a fault I haven't cured rather than a fault I've caused by trying to diagnose the problem.
Alan Drain

Alan,
thoughts only-
irregular – so it could be a very hard to find intermittent problem with supply of electric or fuel that only happens under certain circumstances

are there times when it doesn’t happen, engine cool/hot, not within a certain time of driving, dry day, whatever

you say the plugs seem fine, how old are they, what did they look like colour wise

have you reset the tappets, timing and mixture for you new dissy, is the whole dissy new or just the top part

coil faulty maybe

Guy I’m sure will asked if you noticed if the tacho twitched with the misfire
Nigel Atkins

Hi. When I said irregular misfire, I should have clarified it happens all the time whilst the engine is revved above idle, but there is no pattern to it - it is not regular.

Tappets have been reset and I have timed according to Haynes manual. I have had issues with oil burning, and I took the head off at the weekend, replaced the valve stem seals and refitted. I thought that the oil fouling might be the problem, but this hasn't had any effect.

I'm going to get a colourtune kit to check the mixture, but I don't believe it to be a mile out.

With regard to the plugs,they are a couple of years old, but the car is driven infrequently, so they are probably less than 2000 miles old.
Alan Drain

Rotor arm failure. Take it off, replace with a distributor doctor red one!

You may see 'tracking' or scorch/burnt marks underneath it when you take it off. They arc through the plastic to the distributor body but only do it under load and not all the time.
Robin Cohen

Alan,

Where are you in Gloucestershire ?

Richard.
richard boobier

rotor would have been replaced and dissy cap if the whole distributor and not just the igniter head, if you still have the old dissy body fitted did you grease and oil it (oiling is often forgotten)

unlikely I know, but always remembers Sod's Law about what you don't check - just because the plugs are only 2 years old and less than 2,000 miles you still may benefit from giving each a very close look

does tacho twitch with misfire?
Nigel Atkins

Richard, I'm in Gloucester.

Nigel, I fitted a complete new dissy, but the cap was not very good so kept the old one, which was in good condition. To be honest, I think that the problem ws there before I changed the dissy, but had ruled out the cap as there are no signs of breakdown or arcing. I take it from your comment about oiling refers to the old distributor?

I know what you mean about the plugs, so changed them for some others (oldish but good condition) to see if that was the problem, but this made no difference.

I have ordered a colourtune kit and will check the carbs just to be sure.

I haven't checked on the tacho twitching but will do so and re-post.
Alan Drain

if you got a new dissy the new cap should be a reasonable fit, check you're fitting it correctly to lugs and that the wire gromment isn't preventing it fitting well

yes oiling was for old dissy but oiling and greasing may well still be required at servicing with new dissy(?) - (not required on 123 dissy)

another test for old cap, have engine running and revving then mist spray with water in the dark and see if you get a light show

twitchy tacho helps pin thingsd down
Nigel Atkins

At the risk of jumping out of logical sequence for dianoises


Down the road when all else has been checked.. look at the points in. The fuel pump

The points in the SU fuel pump like to get filmy if they set for long over the winter where condenstion can be an issue
Prop and the Blackhole Midget

This thread was discussed between 05/06/2012 and 07/06/2012

MG Midget and Sprite Technical index

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