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Q & A: '72 MGB Tuning

I have '72 B that at some point a previous owner installed a '71 GK engine. It has the HIF4's and the distributor from the '72 engine. I had the carbs rebuilt by Joe Curto and just tuned and adjusted them. I readjusted the floats and blew out the fuel lines because I didn't think the delivery volume was ample.The car runs great, good acceleration, good idle, but when I get the car up over 40 and ease back on the throttle where I'm neither accelerating nor decelerating (neutral throttle, if you will), the car surges or stumbles, almost like fuel starvation, but when I hit the gas, it goes. If I leave the throttle where it is and climb a slight grade, it stops surging and pulls strong. It doesn't spit or backfire so I know it's not the timing, which I set at 20 BTDC. I put a '71 distributor, 41339 I believe, for that engine and it still does it, with or without the vacuum connected. Both distributors aren't frozen up and so the weights move freely. I've been messing around with this for a year now and I'm at a loss. It's been suggested that it could be a vacuum leak but I can't find one. Any suggestions would be greatly appreciated.

The problem you have is either timing or fuel. Timing is easier to attack first. Between 1971 and 1972 the distributors and the timing changes a lot. For the 1972 distributor, you should time the car about 15 BTDC at 1500 rpm vacuum disconnected... read more