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Q & A: Acceleration Flat Spot.

I have a basically stock 1979 MGB. Several years ago I purchased a HS6 manifold and carb kit from Moss Motors and recently got around to installing it on my car. At the same time I installed an exhaust header (1 3/4 inch stainless with heat shield wrap), ram horns and a manual choke kit. When I started the car up an took it for test ride I got a nasty flat spot. Back to the garage. Pulled everything apart. Noticed that the lands on the manifold were slightly thinner than the lands on the exhaust manifold. Also noticed that the HS6's were a late model that also had a port for crank case vent tubes. Blocked off the vent tubes, doubled up the gasket under the carbs (all new gaskets), disconnected and blocked off the distributor vacuum line, reset the timing and installed new plugs. Got out my SU Carburetor Tuning book and went to work setting up the carbs. Checked everything. float level correct, needles centered and no't loose , jets initial setting 2 turns down after verifying jet level with carb base.

Started up and let the engine warm up thoroughly. Set the initial idle with vacuum synchronize, set mixture at idle and at 3000 rpm with color tune (no load). Flat spot gone while carb linkage is loose. As soon as everything is snugged up the flat spot is back only now its worse. Running up the jets (leaning mixture) results in backfiring. Running down the jets (richening mixture) makes the flat spot worse. Do either of you have any suggestions?

First of all, connect the front tappet inspection cover to the carb vents -- use a 90 degree hose and the plastic Y piece. Use 5/16' hose from the Y piece to the carbs and you won't need clamps (it looks better without clamps).

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