1979 GMC C1500 Hard Shifting

PREACHERDJW

MEMBER
1979 GMC C1500
V8
2WD
AUTOMATIC
169,000 MILES

My 16 year old son has above pick-up. The person he bought it from has put a new carburator, but am unsure of size, but I think it is a 2-barrel. He also has over-sized knobby/mud tires (possibly 17 inch). When he places transmission into drive, he must accelerate to approximately 40 mph before transmission will shift into 2nd gear. Once he gets up to this speed, shifting into other gears is fine. In order to keep from riding around town in first gear all the time, he places transmission into low 1 or low 2. When he gets to about 10-15 mph, he shifts to next low setting. Then when he reaches about 20-25 mph, is able to shift into drive as normal. Transmission does shift smoothly when speed decreases. Someone advised to check for an unconnected vacuum line, but all appear to be attached. Transmission seems to be good and does not seem to slip. Any ideas? Didn't know if there is an adjustment on the carburator I could make to help the situtation. Thank you.

1 Reply

HMAC300

MECHANIC
48,601 POSTS

In 1979 there were 3 transmissions one was a 350 a 400 and a 200. If you have a 200 it is critical for throttle press ure to be activated when the gas pedal is depressed. There should be either a metal rod or cable going from the accelerator linkage to the trans. If it's a 200 it will say METRIC on the pan.
You really should take this to a trans guy as he will find the proble quickly and probably cheaper than you would think. It could be other problems in the trans like a vacuum modulater- if equipped or a govenor is no good. Alot of them had plastic gears that would strip and not allow the govenor to turn. If it's not the throttle cable on the 200 it certainly sounds like a govenor problem.