Here is the straight scoop:
Under normal acceleration the tranny shifts fine. Now if you try accelerating harder (about half to full throttle)and watch the rpm gauge, when it goes to shift between first and second it slips and the gauge goes up about 300 RPM before it grabs and continues accelerating. This only happens between first and second under hard acceleration.
Tranny fluid is good, but since I got the car about 18 months ago it has not been serviced or modified.
Any thoughts?

 

I am not clear on what is happening. Instead of the RPM's dropping, does it go up 300 (flare) then drop? This would seem to indicate a Forward Clutch slip but then it should happen at other times too, not just 1-2. Please clarify.

 

I am having the exact same problem, I tried readjusting the TV cable and it appears to have helped some. What happens is shifting from 1-2 near WOT say at 4000 rpm, the rpm's will go up close to 4500 rpm for a split second and then come down, but this hesitation is definately hurting my 1/4 time.

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'86 IROC
T-TOPS, TINTED WINDOWS, BRAKE LIGHT BLACKOUTS
GM GOODWRENCH 350
EDELBROCK TES HEADERS
FLOWMASTER
EDELBROCK 600CFM CARB.
KN AIRFILTER
ACCEL HEI DISTRIBUTOR
160* Stat
2:73 GEARS
Rebuilt 700R4, with manual valve body
B&M Megashifter
Soon to be installed
Exhaust Cut-Out and different gears, either 3:73 or 3:42
"There is nothing more addicting than speed"

 

The situation is similar to Mark's, if I accelerate and say the RPMs are at about 3100 and just when it shifts the tranny slips some and the tach goes up to about 3300 before droping back down to about 2500 on 2nd. On the other gears it doesn't do it at all. As you can see we are not even talking WOT.
I took the car today to the shop and had the whole tranny flushed to include the converter. It seems to have help some but it still does slips some.
Hope this clarifies some.