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MG MGB Technical - Misfires under load, coil or timing?

My problem is a bit complex. My car started misfiring a little while back. It was real on and off, but normally happened on the freeway, it would misfire and lose power unless you backed off the throttle a lot. After I replaced my clutch the problem got a worse. Now it misfires going up hill, even in lower gears, second if you rev it high. The timing was about 20 degrees BTDC at 1000 rpm. Well I had ordered a new Eurospec dizzy with petronix installed, so I decided to swap that in and see if it changed anything, but no luck. At 20 degrees @ 1000rpm timing, the car still misfires and loses power under load. So I backed off the timing a bit more, and the results are the same. I have good fuel pressure, I have a new dizzy, rotor, cap, the wires are fairly new, the starter and solenoid are new, the alternator is fairly new, the fuses are new. Really the oldest piece of my system is the coil.
So what do you guys think, coil, or should I keep backing off the timing?
Is there anyway to test the coil with a multi-testor?
Thanks in advance (or retard)
Bill
Bill Mertz

The coil should produce 1/2" of blue spark. How are the spark plugs? Are you still using the same Petronix unit? It could be going bad. If the miss is electrical, generally retarding the timing will make it worse. When you retard the timing, the spark occurrs later and the camber pressure is higher. With the higher camber pressure, the voltage required to initialize the spark across the spark plug gap increases.
Leland Bradley

May want to dump the petronix and see if the problem goes away. If you still ahve the old parts that would not cost anything. There is a way to test coils in Bentley shop manual. Don't have one now, but maybe someone else could add. If you pull a plug and look at the spark it should be bright. Also make sure you have a good solid circuit in the ignition wiring. Maybe a coroded end could be your problem.
J Arthurs

Bill;
I had a coil go bad in my 76B, and it acted exactly as you describe.Mine actually had a pinhole corrosion hole, and some of the oil in the coil had leaked out. When the coil heated up, and it would get hot, the car lost power and misfired. New coil solved the problem.

Pete Haburt
Pete Haburt

FYI, the petronix is brand new, and the problem occured before the petronix was installed.
Thanks for the responses so far. Unless I hear otherwise I think I'll try a new coil, it can't hurt.
Bill
Bill Mertz

Bill; Are the plugs also new? If not you may want to try a new set. Good luck. Clifton
Clifton Gordon

Just for the heck of it, have you replaced the
fuel filter? (I realize you stated fuel pressure
is good -- was this tested at the filter outlet
however?)
Ronald

Are you running SU carbs

If so, check that the pistons are running free in the dashpots - sticky pistons give just the sort of symptoms you describe
chris

Oh man, this sucks. Ok so I tried a new coil, and no dice, same exact problem. Then I took the fuel filter out completely, no dice again, same exact problem. Then I fiddled with the timing. I retarded it all the way to 5 degrees BTDC at 1000 RPM and retarding it some helped a really tiny bit, now I could rev completely through second without losing power and missing, but once I hit third it happened again exactly the same way.
I'm truly at a loss.
By the way I'm not running SU's I'm running a side draft weber. I just seems so weird that it would run so well through two gears then start to mess up.
Could valve adjustment be the cause?
The only other thing I can think of is an over filled tranny, would that do it?
Please help, I'm getting desperate.
Bill
Bill Mertz

If you have an electronic tach, and that is rock-steady during the miss-fire, then nothing in the ignition LT circuit i.e. coil, points/pertronix etc is to blame. If you can rig up a timing light where you can see it when you get the missfire you may well be able to determine if it is HT, otherwise it must be fuel. Had a similar problem on my daughters car but fortunately I could reproduce it by hard revving while stationary. Clipping the timing light on the coil HT showed a regular flash all the time, clipping onto any plug lead showed an irregular flash during the missfire. Didn't bother diagnosing whether the rotor or the cap was to blame, just changed both - no further problems.
Paul Hunt

Bill; I don't think valve adjustment is your problem. Can you borrow another Weber from another
B and try it? The over filled tranny will not cause your problem. Clifton
Clifton Gordon

Bill,

Have you tried your old dizzy w/o the Pertronix with the new coil? Is the new one the stock coil? With which some people think the Pertronix doesn't work well.

What happens if you give it some choke when backfiring?

Are you checking and, if needed, cleaning your plugs between attempts?

Just some random thoughts here. - Tom
Tom Young

Ok guys, thanks for all the help,
I think I got it fixed, at least temporarily. I pulled apart the weber and thoroughly soaked parts in mineral spirits, then blue it all out with the air compressor. When I took it on the test run it started missing a bit under load but then it cleared up. Maybe just some junk that blew out. But I through new plugs and a new fuel filter in for good measure.
By the way, what coils came stock on MGB's in the US? I have a Bosch Blue coil, with no external ballast. I don't think its the stock coil.
Thanks again for all the help,
hopefully the gremlins will stay away.
Bill
Bill Mertz

Bill; The stock MG coil should be a Lucas. Clifton
Clifton Gordon

12v on a chrome bumper, 6v with loom ballast on a rubber.
Paul Hunt

This thread was discussed between 20/07/2002 and 25/07/2002

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