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73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: Johnbravado on August 21, 2017, 06:52:40 am
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I replaced the fuel pump and redid timing and adjusted carb for new pump and now i shift from 1st to 2nd at 3800RPM and it feels like i am going to rip back back end off. and it shifts from 2nd to 3rd at about 3000RPM. before i replaced fuel pump shifting would occur probably 2800RPM for each. What makes a transmission decide to shift from 1 to another? what did I inadvertently remove when i was replacing fuel pump? I have a 350 engine with carter AFB 4bbl 1707 and TH350 automatic transmission
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A TH350 determines its light-throttle shift point largely from engine vacuum. Higher vacuum (low throttle) = lower rpm shift, lower vacuum (more throttle) = higher rpm shift.
If it's shifting higher and harder than before (you didn't say, exactly), I would look to see that you didn't bump or damage the vacuum hose that goes to the vacuum modulator on the transmission. If it's disconnected, the trans won't see any vacuum and will think you're at wide open throttle = late & hard shifts.
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Agreed. Check the vacuum line running between the intake manifold and the modulator valve on the transmission and make sure it is connected at both ends. In addition, make sure the line is connected to a manifold vacuum source, not ported vacuum.
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That was it thanks. The vacuum line comes up near distributor and when I was doing the timing I knocked the pipe and they didn't use hose clamps on modulator so it just fell off. I like easy fixes. And I added hose clamls