The type of issue that happens every once in a while, not systematically (apparently) so it is hard to pin point the cause.

Well, not systematically “apparently”: Apparently, in fact, it looks like it only happens after reaching some fairly high speed – today was 110 mph (I just floored it for a few seconds and oooops!)

Then I can’t shift the gears anymore. As I downshifted to help slow down, I was able to disengage the 4^th gear but not to engage any other gear, not even the 4^th gear back. Not the first time it happens.

As I had enough speed built, I was able to get off at the next ramp and stop on restaurant’s parking.

Impossible to engage any gear at idle either. I had to shut the engine down. Then I could shift gears at will, with the engine stopped. Start the engine: Can’t engage any gear. Shut it down and gearbox seems to work fine. Next things, I start the engine with the 1^st gear engaged – and the clutch fully depressed, of course. While moving the car in 1^st gear, I was able to shift to 2^nd gear, then to 3^rd, etc. Gearbox went back to normal operation.

As this is not the first time this happens, I expect it to happen again. I may not be that lucky next time and be able to drive back home.

Any clue as to what exactly I should look at?

Might check your neutral gate alignment. That's really just a shot in the dark though.

1973 L-82 4 spd

I would suspect the clutch assy is sticking at high RPM.

Joel Adams
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If there's no load on the transmission and the engine is running, you can normally shift gears as long as the car is moving.

1973 L-82 4 spd