MG Car Repair: 1976 MGB Timing, manifold vacuum, vacuum advance

Question
I have a 1976 MGB that seems to miss fire @ > 2500 RPM. When I bought the car, it did not have a catalytic converter. It had an air pump, but it was disconnected. It has a vacuum hose running from the distributor to the Weber carb. When I set the timing, I disconnect the hose from the carb and plug it with a golf tee. I set the engine speed for 1500 RPM and adjust the Distributor for the timing to be  ~13° BTDC degrees. The engine seems to be running fine at that speed. I tighten everything, reinstall the hose and take it out. At @ > 2500 it starts to miss-fire. Since I’ve owned the car, since July '07, the hose has been connected between the carb & distributor. In the Bentley manual, it says for 1976 the "Vacuum advance - Unit fitted but not connected". The fuel filter and fuel pump are new last year, after market generic pump. The points seem to be set correctly ~.015 in. and the dwell is set to 60°. Am I doing something wrong with the vacuum hose or timing in general? Any help would be greatly appreciated.

Answer
Hi Harry,
Any time an engine is changed in any way the original timing advance curve is no longer correct for that engine. Without a lot of testing and some equipment it would be difficult to decide what advance curve to use. The only thing you can do at this point is to try different combinations and listen for detonation. (commonly called valve rattle or pinging)this is an indication it is advanced too far. Some tuners advance the timing until they get the ping and back the timing off until it is not heard. The factory is able to map this advance curve on the original engine and set the advance curve to a useable advance. If the advance unit is an "advance" not a "retard" unit and the port on the carb is "Ported Vacuum" not steady vacuum, I would use the vacuum. However if the unit on the distributor is a vacuum retard unit I would only connect it to straight manifold vacuum not ported vacuum. (ported vacuum has no vacuum on it until the throttle is slightly opened)

A misfire may not be the advance timing at all, You may still need to test the fuel pressure at misfire and check plugs etc and do a compression test on the engine.
Howard