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MG MGB Technical - ignition problem stumper

Hi all,
thanks in advance for any help!

My 70 18GH engine has developed a miss. It is worse under steady cruise-light to moderate load. It improves significantly, but does not go away under heavy load. It seems to run alright at idle, although I can detect some miss. It will rev in neutral easily to 5k+. I had this problem previously, but it went away when I replaced the cap, rotor, points, ignition cables, sparkplugs (NGK), and condensor. Unless noted otherwise, all parts were Lucas. Now it is back with only 3000 miles on the ignition components. I have filed the points and reset the gap to 0.015 (they were very slightly pitted), set the timing to spec, swapped out coils (although that didnt fix it the last time), replaced the condensor with one of unknown condition, and cleaned all connections including rotor and dist cap contacts. All this to no avail.
I doubt that it is the plugs or the ignition cables since I had the same exact symptoms prior to installing these, but I will admit that they COULD be the problem. I will check them once the engine cools.
I have also tried changing the timing to see if that has any affect on it since it does run better under load. Neither retarding nor advancing seems to affect it at all.
It may also be of interest that the first time I had this problem, I was cruising on the freeway at about 70mph and started with a slight miss that got steadily worse until it was quite apparent, but not enough to prevent me from continuing to cruise at 70+. The miss returned last week in exactly the same way--while freeway cruising.
Does anyone have any idea what my problem could be????
Thanks,
Ben
p.s. I disassembled/cleaned/lubed/reassembled the dist about 2k miles before the problem returned, and the vacuum advance does work properly.
**sorry for the long post**
Ben Pender

Ben,

Have you checked dashpot oil in carbs?

Cheers, Doug
Doug Keene

Doug,
A good thought, but the dashpots are exactly where they should be. When I first had this problem, the first thing I did was pull off the road and make sure that they were full. I also ended up swapping carbs thinking that it could be carb related, but the problem remained.

I suspect that the problem lies with the condensor. In fact, I removed it and the symptom not only remained, but got slightly worse.

thanks,
Ben
Ben Pender

Why would you think it was NOT the plugs? They have 3000 miles on them. Does it use ANY oil? Of course it does, it's an MGB. If you have even one cylinder pumping a TINY amount of oil, a plug could easily be fouled.
Chuck Asbury

Ben,
I had a simular experience and did all the things that you have done. It turned out to be the fuel pump points. The pump would run fine for long periods of time and then cause the engine to miss, then lose power. It turns out that the resitor that is wired accross the points failed and allowed the points to become pitted and to poorly flow electricity when hot. Ray
Ray

Problem solved!
Chuck wins the prize for having the closest answer. It turned out that #3 plug gap had closed to .009" (from .030"). I cleaned the plugs, regapped to a "loose" .029", reinstalled and it now runs great. Now the question is...Why did the gap close?????????????
Thanks for all the suggestions!

Ben
Ben Pender

Ben, when you dropped it on the floor!

Or, something foreign in the cylinder. Like a loose chunk of carbon floating around. Or, worse case scenario, the piston coming up waaaay tooo high.
My vote is that you dropped it, cause I'm guilty of that.
Safety Fast
Dwight
Dwight McCullough

Appears you have solved it. Went through something very similar and did everything you did before replacing the plugs. Bingo.

Also, what did you use for timing? I see a lot of info saying 14 Degrees BTDC but Tegler's site shows 20 degrees BTDC for 18 GH engine (same as mine and I have 41155 Dizzy which is critical as well) which when I finally used it has the car running fine. Haynes, etc. don't seem to be specific enough on that point and I had it set too low for years. I'm kinda slow mechanically.

JTB
J.T. Bamford

This thread was discussed between 31/08/2003 and 03/09/2003

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