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This is my second post on this topic. The problem is that the engines races when I decelerate while driving. The problem started suddenly a few months ago right after messing around under the dash to replace a motor wiper. I also replaced a few of the vacuum lines. Earlier folks suggested replacing the PCV -which I did to no avail. Another post suggested I've disconected vacuum lines controling the heater/AC unit switch. The truck has a 390 and an edelbrock carb. I wondered about switching the manifold vacuum to ported vacuum valve. Would this help? Im really stumped...
Inspect the gas pedal and linkage to the carb to make sure it's not binding up .. Is your truck running a manual trans ? If so While driving when you take your foot off the gas shift to neutral is the racing engine just going from load to no load ?
Sticking throttle is worth checking. Also, did you eliminate a vacuum delay valve on the spark advance vacuum line when you re-did your vacuum lines (or accidentally reinstall it backwards)?
I sprayed linkages with carb cleaner and everything appears to be moving fine. I have replaced several vacuum lines but I'm not sure where the " spark advance vacuum line" is located. Also the truck sounds fine when I tromp in the accelerator and while parking. It's only when I'm driving and shifting that I notice the jerking and racing of the engine. I read someplace this truck has a deceleration valve between the manifold and the exhaust. Could that be stuck? The PCV has been replaced and is connected to the front of the carb, the breather on the oil cap ( driver side) just hangs down and isn't connected to anything.
Edit to above..I did pull the spark advance out and replaced the contact mechanism. I suspected the vacuum mechanism but didn't replace it. Now considering getting new distributor. I don't recall a valve in the line however. Redroad asked about the change when shifting- yes it's a 4 speed and the engine is still racing even when I drop back to neutral. It seems like if I tromp on accelerator it tries to drop back down in RPMs. My daughters are with me for the summer so perhaps I'll get them out with me to watch and listen. Hard to do this by myself and never too soon to learn for teenagers.
Last edited by IdahoCS; Jul 14, 2015 at 12:14 AM.
Reason: Update
I sprayed linkages with carb cleaner and everything appears to be moving fine. I have replaced several vacuum lines but I'm not sure where the " spark advance vacuum line" is located.
It should run from ported vacuum on the carburetor to the vacuum advance on the distributor. Often, there's a vacuum delay valve (aka spark delay valve) installed in this line to prevent surging on throttle tip-out: it delays vacuum to the distributor advance during decels, but allows vacuum to release quickly in the other direction during accels. If it's not there at all, or especially if it's installed backwards (with the DIST side toward the carb and the CARB side towards the distributor), it could be a cause of the problem you're describing.
There's just one vacuum port on the vacuum advance. I don't recall seeing this vacuum relay valve prior to replacing the lines. I'll give it a shot. So I also noticed there are several vacuum ports on the rear of the carb ( passenger side). They come off of one stem so would it matter which port was used for the vacuum hose ?
