My transmission was rebuilt completely in January and by a very good mechanic. So this morning I noticed the transmission shifting weird when warmed up. When going from 1-2, it jerks back and forth like it doesnt know which gear to go in, then seems to jump to third. when I manually shift the truck, all is just fine. I noticed the idle to be a little higher and rough as well (vacuum leak?). When I squeezed the vacuum line going down to the modulator, the idle smoothed out.

Back in January, the mechanic did not replace the modulator valve - I had put one on a few years ago and told him not to do it.

Can the modulator be the cause of this?

 

check and see where the vacuum is located at if might be connected to the wrong port on the carb it should be connected behind the carb on the vacuum tree.

 

Found it. It was the small piece of vacuum line that connects the hard line to the modulator nipple. The intake manifold leaks so bad that it had softened / swelled the hose.

So another question - this has been this way (I think) since I got it rebuilt and am just figuring this out. My truck actually misses second gear. It goes from 1 to 3. If I manual shift, from 1-2-3, all gears are just fine.

Would any adjustment in the modulator valve allow for this, or is this more of a transmission band adjustment?

 

if you have a tach keep an eye on it to see if you can see when the trans shift. on my 71 from first to second is a hard felt shift which is a bad thing that needs adjusting but from second to third is smooth and can hardly be felt if at all but I can see when it shifts on the tach .

 

What happens is the 1-2 delays till about 20-25 mph, but at 25 it's time to shift to 3rd, which is what happens. It goes from 1-3 at about 25 mph.