Before I begin on timing, I may as well get this done first. Any preferred replacement for the tach gear on the distributor (L48 stock)? I've tested the tach itself with a drill and it's good. Any tips or tricks? The service manual isn't much help.

 

You need to pull the distributor apart and see if only the tach gear is bad, or if the main shaft gear is also chewed up.
You may need both.

 

Hello, I have the same problem with my 73´ C3.
I bought the car with a broken tach drive gear and some other issues.
I have replaced the distributor (because the main shaft gear was stripped) and replaced it with a aftermarket HEI dizzy (HEI with tach drive gear).
The car worked fine until I connected the tach drive gear cable again (which spined easily).
At the first engine start the tach drive gear broke immediately again. I removed the dizzy in order to check the small gear on the shaft which was stripped again!
Nobody needs rpm gauge, also not by law and I will not repair it again (maybe some day I will convert it to a electrical driven rpm gauge, but fro this I do not have time atm).
Now my biggest problem is that I cannot get the engine running after I re-installed the dizzy. Base timing is OK, but I cannot set proper ignition timing because the timing jumps very aggressively when I check it with my timing gun. Even with the vacuum hose disconnected it till jumps and the engine smokes like a steam engine and idles very rough!
The engine has lost all the power, when I push on the gas pedal slightly the engine revs a little bit higher, but when I push more on the gas the engine dies off!
I fear that the broken tach gear has *****d up something serious, maybe the timing chain jumped?
Could this be possible that the timing chain jumps just because of a small broken tach drive gear (milage: 80k miles)?

 

The quality of some of the gears has been bad for a long time

 

You may also need to do some machine work to repair the crossgear thrust surface inside the distributor housing and fabricate an insert to restore that surface. If you install a new gear on a hogged-out thrust surface, the gear will seize, stripping out the gear and the mainshaft.

This is a worn out & damaged crossgear thrust surface that must be repaired. This is done by machining the damage, and then fabricating an insert to restore the surface to its original condition. The brass thrust buttons provided in many "kits" are pure junk:



Lars

 

I bought a remanufactured Cardon distributor (HEI with tach drive) and I assume that the have replaced all bearings and repaired all trust surfaces which were bad.
However, the distributor tach drive gears (both bears) got damaged immediately after the first startup.

But the broken tach drive gear is not the problem anymore, because after cleaning the dizzy I grinded of all teeth on the small tach drive gear to make sure that there is no gear meshing anymore.
I filled the tach drive hole/housing with a lot of grease and installed the modified tach drive gear again which is now basically nothing more than a plug.
The dizzy is spinning very easily with the modified grinded tach drive "gear" (which is not a gear anymore, it now looks like a shaft without teeth, but I forgot to take pictures).

Maybe something else got damaged on my dizzy, maybe the vacuum advance mechanism?
However, I also checked the ign. timing with the vacuum hose disconnected, but the timing still jumps like crazy and the pattern is uneven!?

Maybe someone expert can listen to the engine sound and tell me what could be wrong...

idling before tach drive damage:

idling after tach drive damage:

I hope that not something serious has been f*cked up?

BTW, does anyone know what the maximum allowed timing chain slack is for 350 Corvette C3 engines (deg CRA)?

 

I do not have that capability. Should I just replace the distributor?

 

You could put a timing tape on your crank balancer, and then run a full check on the distributor operation, and it's timing advance, with a non-dial back timing light.
Idle = With & without vac advance, maybe 25-30* with, and then 10* w/o) tests vac adv can only
Idle to 3000 rpm or so = without vac adv (from ~10* to ~30*) tests centrifugal curve in the distrib only

That will quickly tell you if it is working, and if it is "right"