Vibration/engine stalling when shifting into drive/reverse

[Lemur]

Howdy yall a little background,
I inherited my grandfathers 1977 bronco with a c4 and 302, I’ve resealed the transmission and put about 100 miles on it since reinstalling the transmission. This past weekend we took it camping and did about 20 miles of forest service road cruising. The last mile back to camp it acted like it was low on transmission fluid. I checked it and it was a little over serviced. It starting shaking/vibrating only when shifted into drive or reverse with the brake on just idling and kinda lurch/break the tires loose forward with just a tiny bit of acceleration, It runs and drives great otherwise. If you shift into drive with your foot on the brake the engine will vibrate and stall sometimes. Not sure what this could be? Band adjustment? Bad torque converter? It idles in neutral great.

 

[Broncobowsher]

Sounds like an engine issue. Vacuum hose come off? Plug wire fall off, or get burned through?

Why would an engine issue cause the transmission to go into gear hard? Low vacuum. The vacuum modulator is how the transmission tells how much torque should be going into it. Torque output is a factor of manifold pressure. High vacuum, low pressure (absolute) and low torque. Line pressure is lower, clutches engage softer, shifts are softer, shifts come sooner since low torque is a sign of low demand. Have an issue at idle that reduces engine vacuum, line pressure goes up, the transmission acts as if you are dropping it into gear at heavy throttle and applys clutches hard and fast. All that tells me the transmission is working perfect. You have an engine issue. Misfire causes engine shake, low manifold vacuum, engine stall. As for why the engine is idling poorly, hard to say. Could be a vacuum leak, ignition issue, fuel system issue, mechanical issue. Those are the common ones anyway.

 

[Lemur]

It does feel like a misfire when shifted into drive, I’m hoping just a fouled spark plug, it does burn oil and I have spark plugs and intake manifold gaskets coming. I just did spark plug wires. Also I know there’s a vaccum leak (most likely intake manifold gasket) as I have always got no vaccum at idle with a gauge hooked up to manifold pressure and the vaccum advanced plugged. I’ll get 20in once I give it a tickle of throttle. What’s the normal reading for a 302 at idle? She’s set at about 800 rpm around 15 degrees of initial timing.

 

[Lemur]

From what I was told/can visually see the engine is stock except Edelbrock intake, Edelbrock carb, and headers

 

[Chiclassics]

Perhaps vacuum leak, typically more prone when you put in load (D or R). Given it started OK then got the issue, make sure all vacuum lines are still intact. Also to confirm that you have vacuum leak try spraying carb cleaner around the intake manifold gasket to see changes in RPMs, it is an issue you need to resolve. Also make sure proper engine oil level.

 

[Chiclassics]

What’s the normal reading for a 302 at idle?

b/w 15-20

 

[RangerRob]

Another thing to check is any interference with your spark plug wires and shift mechanism (since you said they were just changed). I've seen the #8 boot or wire ground out if it is rubbing when you change gears.