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MG MGB Technical - No. 3 plug fouling and not firing

I need help with a problem which is stumping me and several MGB mechanic friends. '70 MGB started running very rough in January. Could not drive it. Found problems with worn throttle shaft on SU carbs, major exhaust leak on exhaust header, and wobbly distributor shaft.Have replaced dist with new one, new HIF carbs incl shaft, and a new exhaust header. Also new coil. Car starts and then there is no fire from #3 plug. Car starts but cuts off, can restart but not drive without backfiring and smoking.(Smells like raw fuel. Have tried 4 different coils as they get very hot to touch. New plugs and wires. Timing is spot on. Checked for stuck valve - not one. Checked gaps and adjusted two. Still no better. p#3 plug is wet with fuel, but not oily.

Any thoughts?

Thanks.

Gary
G.L. Mills

Gary - Is the dwell on your new distributor set correctly? Also try swapping plug into a different hole if you haven't already and a different plug in the #3 hole. Cheers - Dave
David DuBois

Have you tried swapping the HT leads no 2 and 3 should be similar lengths. Just re-routng 3 to be clear of the heater valve mat cure it.
Stan Best

Thanks Dave and Stan. We have switched plug wires with each other, tried 3 different sets of wires, changed plugs (new and with each other), checked dwell, checked points (even though they were new). Nothing works. It is totally baffling. Last thing to check is compression and leak down. If nothing is wrong there, don't know where to go next.
G.L. Mills

Compression is the FIRST thing you check.
Sure you have the wires on in the correct order?
What does "new" distributor mean? Lot of crappy rebuilts and possibly new ones out there.
Otherwise, you either have no spark at that plug = bad distributor, or it is leaking out someplace = cap or wire.
Look up a huge long distributor discussion about a year ago on the MGA board - Steve Gyles.

FRM
Fletcher R Millmore

When you run the car with no 3 out of the cylinder and earthed to the block can you see a spark? No 3 is my favourite for this, you can just rest it on the heater valve. No need to even twirl the plug spanner just remove the lead and use a spare plug.
Have you tried a much hotter running plug? It may be fouling due to rich mixture and the carbs are the problem. 2 and 3 run richer on the B series. Can you get your hands on a gunson colourtune?
Stan Best

Is the plug fouling because it is not firing? Or not firing because it is fouled? You need to start with a clean set of plugs at least, and that will tell you which. What's No.4 plug look like? Did you properly set up the carbs for airflow balance and mixture?

If you have plug leads reversed two cylinders should be affected ...
PaulH Solihull

Good point Paul. I keep some hot running plugs and if I get sooted plugs when I'm adjusting my carbs in my amatuer way I can start the engine using them and they never get dirty. The Gunson Colourtune is a big help at times like this. Swapping the leads was not a firing order thing, just another substitution.
Stan Best

I was looking for the adaptor that blows up footballs and found one of the plugs that always stays clean. Its an NGK BKR6EKC. Its not important to be that part number, the key thing is that although total reach is the same the insulator nose extends beyond the outer threaded sleeve and so it runs hotter. I think this makes it a softer plug in the trade jargon. Garages used to have plug blasters that cleaned them with sand but in these computerised days I think you would have more luck with a softer plug. The football remains deflated.
Stan Best

I had an engine in a TF-1500 that would drop a cylinder like that. It was a worn bushing in the distributer that caused it. After replacing the bushing all was well.
Sandy
SANDY SANDERS

Gary. As Fletcher notes, the first step is a compression test. If you do not have at least 100 psi they cylinder will not fire properly. The next thing to check, as has been pointed out, is that the plug is actually firing. I have seen some Lucas distributors which had both a bent distributor shaft and a points cam where the lobes were of different heights. The net result was a wobble of .035". Depending on which lobe the points were set on the others either fired too soon or would not open sufficiently to fire the cylinder. Hook up a timing light to the number three (old Paul Hunt tip) and see if the light flashes. If it does, you are getting a firing impulse to the plug.

Check for air leaks around the intake manifold. Some of the exhaust headers do not have the same flange thickness as the factory intake manifolds and, unless that problem is corrected, air is sucked into the intake leading to a charge that is too weak to fire properly. This is most commonly found on number two and number three plugs which run slightly leaner than the number one and number four cylinders.

Les
Les Bengtson

"(old Paul Hunt tip)"

Erm, Paul Hunt old tip?
PaulH Solihull

Paul. Either would be correct.

Les
Les Bengtson

True, but I prefer the second one, I've only just begrudging accepted I might be middle-aged!
PaulH Solihull

Thanks for all the help. Put in an old Lucas coil (looked like it was on its last leg)and car runs great. New Unipart coil was maybe too hot???. Made no other changes after all the comments other than the coil. Go figure!
G.L. Mills

Maybe the wrong coil, you have to be very careful to match the coil to the ignition system i.e. ballasted or non-ballasted. The overall resistance in the ignition circuit has to be between about 2.4 ohms and 3 ohms, which can come entirely from the coil i.e. a 12v connection direction on the coil +ve or SW, or from a relatively equal combination of coil resistance and external ballast resistance, of about 1.5 ohms each. The only way you can reliably determine this is from measurement, not looking at labels on either coils or boxes.
PaulH Solihull

"I've only just begrudging accepted I might be middle-aged!"

Not to worry Paul, after extensive studies, it has been proven that birthdays are good for one - then more you have, the longer you live ;-) Cheers - Dave
David DuBois

I like it!
PaulH Solihull

This thread was discussed between 18/07/2010 and 31/07/2010

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