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Hard jerk into drive and reverse

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ntgarwood:
Trying to diagnose what may be causing my 78 to jerk hard into gear when going from park or neutral into drive or reverse, or from reverse to drive. The car will feel like it lunges into drive and somewhat feels like it drops down or jumps back when going into reverse.  Once in drive it shifts smoothly.  I changed my transmission fluid and rear diff fluid this week and no change to how the car goes into gear.  I'm wondering if U-joints possibly?  But I don't have a vibration when driving. The car only has 33k miles.

My idle is around 8-900 in park, 550 in drive. Fast idle is at 1800. 
78 Y88 400 TH350

Thanks for any help!

Stringer:
Your fast idle seems high, 1200 or would probably be better. Simple adjustment on the carb. Having a high idle can cause the vehicle to lung when put in gear. The jerk could also be a broken transmission mount or motor mount. Is there any noise like a "clunk" sound or is it just a hard shift?

ntgarwood:
The fast idle is spot on to what the radiator sticker states, and it happens whether it's in high or low idle. There's not a definitive "clunk" sound, it's more just a solid jerking into gear hard. With such low miles on the car it makes me think of things that are more rubber based that could fail causing this.

Stringer:
I don't recall the specs for my 403, but my high idle is no were near that high. I don't live in a cold climate so it is not as much of an issue.

I would put the rear on stands and have someone put it gear; to see if you determine what happens when it is put in drive or reverse.

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