Throttle shaft wear symptoms?

[nismou20]

I've been trying to diagnose a intermittent stumble at off idle and replaced most ign related caps,rotors, cables. Carbs are dialed in pretty well, it runs super albeit the off idle jaunts from a light or hitting throttle at speed. Throughout the rpm range after off idle it's smooth with no hiccups. It seems worse when engine is fully warmed. The rear carb is pretty nice and tight. Carb cleaner test when sprayed almost kills the engine so it's a substantial play. But it almost seems to have manifested itself overnight. Is this a classic symptom many of you have experienced? Is it Ztherapy time?

[Linda]

Float valves worn or sticking? Try some spray cleaner or remove to examine. Also the carb filters might be plugged. They can rust too and drop bits in the fuel bowl etc
Seems like lot of play in throttle shaft
Linda

[Gregs672000]

Don't know, but I watched the video and that looks like a considerable amount of play in the shaft to me. It makes sense to me that when you first apply torque to the shaft it will shift its position, which may be the off idle stumble you experience. The carb fuel ratio is probably able to hide it as rpms increase, but at that slow speed I would imagine the leak is most pronounced, plus the vacuum or draw is highest so it pulls air rapidly but without the carb piston being able to rise immediately to compensate. Then as you go along the piston rises and the lean condition gets lost in the mix so to speak. Airflow down the throat of the carb is what causes the piston to rise; any other source is not acting upon the piston, so the "signal" gets lost and that carb goes lean until the piston rises to compensate for the overall increase in RPMs and air flow. That may account for your stumble at higher rpms too, as that carb is not getting a "clean signal" to the carb piston when you first accelerate, so it stumbles. That would be my best guess.

Regarding the timing of this... well, it has to happen some time! Just might be that things have worn to where they are making themselves known now.

[greydog]

Wow, that's visible movement. Gotta be at least .005". Roughly the diameter of a human hair.
You can pull a lot of air thru a "hairline" crack.
Z-Therapy can fix you up (and they'll be pretty too).
Keith Williams does a great rebuild job, not sure if he rebushes the carb bodies tho.
Dan

[Linda]

I think JT recently mentioned putting some 90 wt gear oil on the throttle end to temporarily seal the air out.
If that works, there's your problem.
Linda

[nismou20]

Yeah good idea Linda, I may even look for something thicker to try like grease.

[Gars68]

I'd try some higher weight dashpot oil. Regarding the throttle shaft play, for a very temporary fix, go to your local hardware store and look for a couple of end-plugs. They are made out of plastic and round in shape. You can use some adhesive and attach it to the left side of each carb without interfering with the movement of the shaft. They fit perfectly. This will create an air-tight seal on one side of each carb.
Gar

[nismou20]

Well, it seems my problem is electrical. I was noticing that battery VDC was only 12.7 when running and Alternator was only 12.9 when running. Upon disconnecting Regulator at connector and reseating, it was up to 13.5 and 13.7 while running. Driving it after this is normal no stumble. It seems there was insufficient battery voltage at off idle but recovering as I was going thru the gears. Now that I put a few miles on it the VDC is back to 12.5 while running again. It seems either my Alt or Reg is intermittent bad.

The Vacuum leak via throttle shaft wear is not the culprit. It seems you can have quite a leak and not really notice it. Now if it were newer cars it would definitely throw a code.