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MG MGB Technical - Misfire when Hot

My 1980 MGB has been running well until recently- when after about an hour of running the engine starts to misfire under load and then backfires which seems to clear it for a few minutes before the misfire starts again. I have replaced all of the ignition parts-Coil,plugs,points,rotor arm,distributor cap,ignition leads and condensor. The car runs beautifully for 30 -40 miles when hot,and then the misfire starts again-The timing and dwell all seem OK-Im baffled any ideas?
Paul
P D Camp

Its a long shot, but try removing the fuel tank filler cap the next time this problem occurs. If this clear the problem, find and remove the clog in the vent line from the tank to the charcoal canister. Cheers - Dave
David DuBois

See my post about dud new condensors. These were our symptoms, flowing the joints with solder fixed it and i had to set the tickover back 300 rpm. I have a capaicatnce meter but a continuity check showed this up.
Stan Best

Paul,
I have got exactly the same problem. (76 MGB)
Cold engine runs smoothly, no problems with performance under load.
But after about 30-45 minutes it start in exactly the way as you mentioned.
I did the same replacement of all the parts you mentioned.
I am thinking, the problem is in the Carbs (HIF4), There is a bi-metalic regulator, this adjust fuel supply when engine is hot. If I am right fuel supply is less when engine is hot.
Do not know what to do about it. maybe give some more fuel? ???

David, I did the fueltank filler cap test, no result.

Stan, I have had recently 3 differant condensors on. must be a good one among these I think, also no result.

At this moment I go with Paul, Im baffled as well and hoping for some good ideas to solve this problem.

Jacob
J Halma

Gents,

Has no one of you ever thought about a crack in the cap of the ignition?

Frank
F Navest

Jacob
I am thinking it might be the carbs Thats my next test after I double check the timing
Thanks to all for the interest
Paul
P D Camp

Paul,
Looks like I solved my problem.
On my car where big K&N air filters fitted, looks great but....
After I've seen the original air filters, makes me decide to tape of the K&N filters with ducttape to less then 1/2 size for a test.
Now the problems have allmost gone, so I go for the originals.

Jacob
Jacob Halma

Those original air filters suck. If you can't richen the mixture enough with the adjustment screws, put some richer needles in the carbs and keep your K&N's.
Mark

The original air filters suck just the right amount, which is why you have to make the needle richer when fitting K&Ns. A classic example of an 'improvement' which makes things worse.
Paul Hunt 2

The original air filters also do a great job of using air swirl to take advantage of the entire air filter element, and prevent larger bits from impinging on the filter element. Probably the best engineered air filter can and air inlet horn ever designed for a production car.

dave
Dave Braun

It is funny, today i talked to a friend and he told me of the same problem. After everything was checked and nothing could be found, he screwed in the sparks again, using new ones and the missfirung was gone.
As it seems to be the easiest way to cure the missfiring, just try it. Hope this helps on your cars too.

BTW, we are both not just unexperianced with this engine, not even with race prepared versions of the B-Series but have never had this before.

Ralph
Ralph

This thread was discussed between 10/06/2007 and 15/06/2007

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