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Q & A: Under Load Car Misfires

Hi John,

My '76B has:

1) 32/36 Weber Carb w/ manual choke
2) Euro-spec dist. w/ vac. advance
3) Rebuilt engine
4) Petronix Ignitor
5) A serious problem

This car runs so well around town and even on the open road at or around 55-60 mph. Once I get it on the highway, it runs well until I pin it/accerate though and once the speed reaches 65-70 mph, it will act as though it is running on 3 cylinders and the exhaust gets pretty choppy, occasionally with a mild backfire or two. Once I back off the accelerator for a bit, it clears itself out again, but it only does it under load. Will do the same thing climbing a light mountain with a steady incline.

I have changed the wires 3 times, plugs twice, and coil once. My shadetree mechanic friend set the timing at or around 16 deg. with vac. connected, which after doing it, he said was wrong.

I think the alternator may also be failing as the red ignition light remains on during my drive. Someone on the MGB Yahoo page said it sounded like a bad diode, or something. A mechanic I am not. The light no longers comes on when the key is turned to the 2nd position.

If you have a second, please help! I'm desperate. I'm ready to sell it at my wife's yard sale. If you'd prefer that I call, I would be happy to.

Thanks so much,

Bill

Bill,

Set the timing at 32 BTDC vacuum disconnected -- or, set it at 20 BTDC at idle, vac disconnected. Better to use the first figure with a dial-back timing light.

There are two vacuums -- ported, which gets g... read more