350 always on fast idle

Reply Subscribe

Thread Tools

Search this Thread

Jul 24, 2017 | 10:53 PM

dyerk

Thread Starter

Instructor

Joined: Mar 2017

Posts: 111

Likes: 7

350 always on fast idle

my 69 350/350 is always on fast idle when I pull up to a light. I am able to rev it to knock it down. What could be the problem? I have a quadrajet

Jul 24, 2017 | 11:15 PM

Revi

Drifting

Joined: Oct 2005

Posts: 1,615

Likes: 150

From: Forth Worth TX

The choke isn't opening up as it should. Post some pics, so we can see what is going on.

Jul 30, 2017 | 08:18 PM

dyerk

Thread Starter

Instructor

Joined: Mar 2017

Posts: 111

Likes: 7

Quote:

Originally Posted by Revi

The choke isn't opening up as it should. Post some pics, so we can see what is going on.

Jul 30, 2017 | 10:35 PM

mardyn

Safety Car

Joined: Dec 2007

Posts: 4,229

Likes: 39

From: Beautiful East Texas

Adjustment screw for the fast idle cam is hidden down low on the right front behind the vacuum motor on the Q jet... turn it out(CCW) a round or two and see if that helps.

mardyn

Jul 31, 2017 | 08:13 AM

Revi

Drifting

Joined: Oct 2005

Posts: 1,615

Likes: 150

From: Forth Worth TX

Your first picture shows the choke fully open, this is how it is supposed to look when your engine is at operating temperature and your idle should be 750rpm.

When you start the car, pushing the gas pedal to the floor will set the choke (fully closed), once you actually start the car the choke should open up about a 1/4" or so. (That's what the vacuum pull-off does.)

If you touch nothing else, what does your car idle at cold?
The "fast idle cam screw" that mardyn talks about is what is used to adjust idle speed when the engine is cold and with the choke open 1/4". (I have mine set to idle at 1500rpm.)

As the car heats up the choke thermostat (the little coil spring under the metal box on the intake manifold) heats up and starts moving the choke to the fully open position and reducing your idle speed at the same time.

Some times the choke thermostats go bad/need changed or perhaps your choke is sticking closed a bit?

FYI - you shouldn't have a rubber fuel line running from the fuel pump/filter to the carburetor. It also looks like you are not using any vacuum advance going to the distributor.

Last edited by Revi; Jul 31, 2017 at 08:19 AM.

Jul 31, 2017 | 08:58 AM

mardyn

Safety Car

Joined: Dec 2007

Posts: 4,229

Likes: 39

From: Beautiful East Texas

^^ The Force is strong with this one^^

Good info.

mardyn

Jul 31, 2017 | 12:06 PM

dyerk

Thread Starter

Instructor

Joined: Mar 2017

Posts: 111

Likes: 7

The car at cold actually idles low, around 500 and need to rev it to get it going. Once warms up the idle increases and then while driving and coming to a light the idles is up around 1700 and then I can rev it to kick it down to 750 range.

Great catch on the fuel line. I did not realize this as I recently purchase this project car. I will make that update to steel lines. As far a s vacuum I have a Mallory Unilit distributor and it does not have vac advance.

Jul 31, 2017 | 01:37 PM

lars

Tech Contributor

Joined: Aug 1999

Posts: 14,380

Likes: 6,391

From: At my Bar drinking and wrenching in Lafayette Colorado

You probably have several issues going on: The carb is one of the dreaded "commercially rebuilt" carbs, and it has been pieced together with several mismatched components. The airhorn and the throttle plate are Pontiac parts, and only the float bowl is Chevy. If the builder did this to the carb, they also did not pay attention to proper operation and function of the choke system nor to fit and operation of the throttle plates to the throttle bores and shaft bushing slop. It would not be possible to find all the problems on that carb without an actual teardown and inspection - I'll bet you would find several odd and incorrect items that are contributing to the fast idle issue and other problems. I'd suggest you find a good, correct, rebuildable core and start from scratch.