390 with C6 Question Vacuum Modulator

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Mar 8, 2017 | 08:15 AM

Don Vito

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From: Phoenix Arizona

390 with C6 Question Vacuum Modulator

Question for you guys, I'm posting here as probably a lot more of you have 390's with C6's than the 48-56 Forum, so I have a 56 F100 with a 390fe & C6, I have a vacuum line from an intake Manifold port that also splits off to the PB Booster that runs to the vacuum modulator, I took it out for it's second test ride just down the road and it seems like it shifts into 2nd at about 3000 rpm, which I don't think is normal, now I don't know if this trans has a shift kit in it out not but it seems a little harsh. Did I hook up the vacuum to the modulator correct? FYI, before it dawned on me that I didn't have the vacuum line to the modulator, and test drove it, it was the same so the vacuum line appears to have made no difference, any idea's or suggestions would be appreciated.
Don

Mar 8, 2017 | 09:52 AM

jakeharp

Fleet Mechanic

Joined: Aug 2004

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From: Nevada

Rod

You should have the little rod that sticks out of the modulator ? Did it get put back in ? Also if this is a Pre-72 C-6 should be a threaded modulator ? Im replacing mine this weekend....... Sounds like the modulator isn't doing its job.... My vacuum line is hooked up to the port 'tree' on the rear of the intake along with the brake booster. I would start with a new section of vacuum hose, double check all connections, new modulator and start fresh $20 will get that job done....

Mar 8, 2017 | 10:13 AM

Don Vito

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From: Phoenix Arizona

there is no little rod sticking out the modulator only the port to push on the vacuum line, unless you mean the little rod is in the transmission (not visible) also read something about being able to adjust the modulator?

Mar 8, 2017 | 10:24 AM

jakeharp

Fleet Mechanic

Joined: Aug 2004

Posts: 1,312

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From: Nevada

Dunno

That sounds like it may be the model before mine, prolly need ND or someone with some knowledge on 50's series.... .

Mar 8, 2017 | 12:16 PM

lasermike

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Joined: Jun 2016

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From: Des Moines, Washington

Some modulators are adjustable, look into the vacuum hose fitting on the modular for a slotted tube that looks like it'd take a small screwdriver.

I know not which direction does what so you'll have to google it.

Also and just as important is the throttle position linkage. Check out the Edlbrock linkage question that's just popped up today for pictures. It's correct adjustment is measured in thousandths so seems like it might be important. ;-)

Michael

Mar 8, 2017 | 01:37 PM

DB429SCJ

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Joined: Apr 2016

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From: Buffalo NY

Check the kickdown rod to make sure it is not stuck. Push it down,it should spring back up. If that works ok, try adjusting the modulator. Remove the vacuum hose,and use a small screwdriver inside the hose nipple. To make it shift at a lower rpm,turn the screw out(left). You have it connected right.(manifold vacuum)

Mar 9, 2017 | 08:31 AM

Don Vito

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From: Phoenix Arizona

Thanks guys I'll check out all of above, I am not using a kickdown, rod, everythink is LoKar cables and adjusted properly. I'll search for the edlebrock linkage question to see what thats about..