I bought a second-hand Moss supercharger kit over a year ago for my MG TD and have been very pleased with the perfomance improvement. I coupled it with a 4.10 gearset from an MGA TwinCam along with Alfin drums and wire wheels. It is hard to beat this combo!
My only problem is an occasional BANG that comes when I am running at speed and full boost. Using the supercharger guage, this happens at 5 pounds and approximately 3500 rpm. It does not happen when I am going through the gears, seeing the same (or higher) rpm and the same boost reading.
This BANG sounds very much like a backfire. There are no other such symptoms at any other opearting condition. No octane pings, the plugs are a nice light chocolate color, and the idle is always smooth. I believe I am ruling out mixture and timing issues when I make these observations.
The carb is an HS4 1-1/2 inch SU. The car does demonstrate some surging-like symptoms prior to the above-mentioned BANG. So I am wondering if there might be a fuel supply issue or perhaps there is a manifold pressure issue. There is some type of spring valve on every manifold that Moss uses for their superchargers, but I have not seen any reference to the function of this valve.
Is it possible what I am seeing is the valve blowing off? If so, is this normal? I have to say that it scares the daylights out of me and probably anyone who is within hearing distance!
Looking for help. Thanks!