Engine Stumbling Issue

[RICK LAKE]

Terry Stabley Terry Check the basics first. It sounds like you found the place where the carb is changing from the primary curcuits to the mains in the carb. 40 degrees of timing sounds a like high BUT if the motor is not knocking, and we don't know what cam,heads, compession, intake, and exhaust is on the car, Checking the plugs would need to be done to make sure about pre ignition. Some FE motors are runing as high as 44 degrees. You could drop the timing back 2 degrees and keep doing this until you feel a power drop off, then go back 2 degrees. This will be a save timing spot. MSD tells you with the springs about which ones and how fast you want the advance to work. If the car is running OK except for this narrow RPM range, there could be some other factors involved like temp of the motor, WEATHER ( this is a big one some times) Did this problem just start or have you had it all the time? The next thing would be to go to a Dyno shop or get an LM-2 meter and install this in both exhaust pipes and see what the A/F ratio is. This may be a lean spot. Make sure you have the CORRECT fuel pressure for your carb. You didn't say or I missed it about is this all the time or only doing certain things like racing or cruising? What color are your side or tail pipes? White is NO GOOD, black is rich and better than lean. Rick L.