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Old 11-04-2011, 08:19 PM
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1977 c6; how to adjust the modulator valve

I can hold my speed at 45-60 MPH and after about 2 minutes just holding the throttle there, my transmission will kick down a gear and back up, causing the truck to jerk. I have a 400 motor with a Holley 2 barrel carb. I have blocked off all unnecessary vacuum ports. I have the dizzy advance hooked up to the carb. My EGR is blocked off with a thick carb gasket. I'm just using the EGR plate as a spacer. I have the adjustable modulator valve, which I have turned the screw inside it to the left about 4 whole turns.
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Old 11-05-2011, 07:27 AM
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You could have a vacuum leak somewhere.


Also look at the KD linkage, there is supposed to be a small return spring at the carb.

You have adjusted the modulator to a dangerous level. That far out will produce early soft shifts.
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Old 11-05-2011, 11:22 AM
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the kick down return and the spring for it is fine, I checked them the first time it was doing it. I have checked all vacuum hoses. I think what is happening is there will be a sudden vacuum pressure drop and it shifts the transmission down, and back up to boost the pressure back up.
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Old 11-05-2011, 12:23 PM
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You must have full vacuum, not off a source that is metered.

From the back of the carb or brake booster is best.
You may need a canister to help retain vacuum.
Some look like a tomato juice can.
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Old 11-05-2011, 01:53 PM
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well... those hoses were taken off and never replaced when my motor was rebuilt, but all the vacuum ports on the motor that are not being used is plugged up or blocked off. what does not off a source that is metered mean? thanks for helping though.
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Old 11-05-2011, 04:00 PM
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I should have said ported vs nonported.


Ported is delivered to the distributor which has no vacuum at idle but increases as engine rpm is increased.

Nonported is the exact opposite. High vacum at idle and decreases as rpm's are increased.

You need nonported vacuum to the transmission.
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Old 11-05-2011, 05:00 PM
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so where do I find a non ported vacuum? I have the existing hose coming off the giant T connector right beside the oil pressure sending unit. which also has 1 line going to the power brakes. the rest of the plugs on it have been capped off.
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Old 11-05-2011, 06:21 PM
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My holly carb (4160) has a designated port for the auto tranny. Check out your carb specs.
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Old 11-05-2011, 09:57 PM
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ok. I will have to look up the manual for my carb, I accidentally left the paper copy outside, then it rained...
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Old 11-06-2011, 10:58 AM
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You are hooked up to the right vacuum source, so that’s not the problem. Check the vacuum lines very carefully. Look for cracks and excessive softness. The hose could be collapsing inside and cutting off the vacuum to the modulator. Replace the hoses with a good quality of vacuum line, also make sure the metal line running to the tranny is not kicked anywhere. Good luck.
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Old 11-06-2011, 02:09 PM
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ok, I will check it when the ground dries, thanks yall for the help.
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Old 11-06-2011, 02:09 PM
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1977, 1978, adjust, adjustable, adjustment, c6, f250, ford, howto, line, manual, modualtor, modulator, plug, ported, ranger, transmission, vacuum, valve

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