TH350 premature upshift from 1st to 2nd gear

[tonykim]

My 78 firebird has a mild built 355sbc and TH350 trans. The trans was rebuilt and I used a mild B&M converter and it has a B&M shift kit. The problem is that the trans will upshift very early from 1st to 2nd gear, even if I am at full throttle. It doesn't slip and if i put the gear shifter into L it will launch and accelerate in 1st gear with no problems.

Any suggestions of what to look for? My first thought was the modulator although it worked fine before the rebuild.

[MarkB]

I would look at the modulator adjustment or the spring / weight combination in the governor. The lighter the springs the later the shifts in the governor. I hope this helps.

Thanks,
Mark