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MG MGB Technical - Runs again, but still 'spits'

Well, this started out as a suspected anti run-on issue. With some great recommendations and Paul's spot-on procedure to check for ignition electrical problems, I found a broken ground wire in the distributor. Ordered a new base plate that included points, condenser, and wiring. Started right up, didn't even have to adjust the points.
However, it still seems to "spit" or missfire occasionally usually when I have to let off the accelerator, and then re-apply (no jump or dive on tach).
I have tried two fuel pumps, and both showed the same symptom. Have not changed the plugs or plug wires yet.
Idle it good, carb balance is good, dwell is on the high side of recommended range, but within the limits. Have not checked timing yet.
Frustrated, but it is running again, so that is major progress.
Any further ideas most welcome.

Patrick Callan

I hope you bought those from a good supplier rather than the usual suppliers, if you check the Archives you'll see that new made parts like condenser, CB points and rotor arms can be crap - I'm saying yours necessarily are

never change a condenser unless you have to, clean CB points rather than replace

you can either do a full set up - tappets, CB points, plugs, timing and carb mixture, in that order, if you adjust any item in that chain then you'll need to check/adjust all the items that follow it in the chain - then go out for a good test drive

or check the new CB points and if they're OK and you want and they worked well before put the old CB points back on and the old condenser and take it for a good test drive and only do the tappets/points/plugs/timing/mixture if you need to

also with both above check your rotor arm, dissy cap inside and out, all ten connections on the HT leads as well as the leads themselves - clean all as required
Nigel Atkins

Thanks. The only things on your list that I haven't checked yet are the tappets and timing. I guess now that I have replaced a number of parts, it's time to re-test in the order you mention.
Really thought I had dealt with the total problem. It started, idled well, plugs looked great after a run. The annoying part was two very weak "spits" during about a half hour run. Idle speed does drop a little bit when warm. Temp gauge looks very slightly above the "N".
Anyway, back to the diagnostics and tests. At least I can get it out of the garage now.
Thanks again,
Patrick Callan

I think this is more likely to be an ignition problem, and not just because of the old saw "90% of SU carb problems are caused by Lucas electrics".

I know points problems would normally affect the tach but maybe just one hiccup at a time is not enough. Try cleaning the points. Turn the engine until they are closed, then open them manually and draw a strip of clean paper through them. Don't drag the edge through the closed points or they can snag strands of paper fibre which can cause a similar problem.
Paul Hunt

the important thing is that if you adjust anything in that chain then you'll need to check/adjust the items that follow (perhaps not the plugs but it's good to see what they look like so you might as clean and check gap at same time)

if you adjust the tappets then as they're at the start of the chain you need to check/adjust all the items that follow in the chain

as I put before and as Paul has put it might just be the CB points but I'd do the whole lot then you know you've checked them and everything is set
Nigel Atkins

New plugs. Two runs without spits. Hope that's it.
Also cleaned the oil pressure switch that controls the anti run-on unit. Was able to flush a lot of crud out. Didn't "click" as indicated on the Moss web site after cleaning, but there was movement inside the switch. Switching the engine to off seemed to be much quicker and smoother. Again, hope that's it.

Thanks to all for the comments and advice.
Patrick Callan

This thread was discussed between 09/06/2014 and 17/06/2014

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