My TCS suddenly stopped working properly. I have stock 1970 L34 with M21. The system will not advance the timing in 3rd or 4th gear. Car runs fine without it, but it ran fine with the system working and I'd like to make it operational again; for originality. What steps should I take to determine cause for malfunction? All vacuum hoses are new and fuse is OK.

No answer for you but this may get more attention in the engine or electrical forum.

I took the tcs solenoid apart, removed the spring and check ball, put it back together and put a bb in the vent hose to back of carb looks factory but has advance in all gears

What Jeff did is a nice mod that you could try because no vac adv in 1st couple gears was only for emmissions and the motor/car with perf better with vac adv working in all gears accept when on the throttle hard enough to momentarily de-activate the vac adv.

Nice suggestion Jeff.

Scott