340 high idle

[crappy73cuda]

Hello all, I'm preparing to put this 1973 cuda back on the road and just trying to work out as many kinks as I can before I do put it on the road. I don't have a tach on the engine so I don't know how high it's idling but it's definitely idling high. I just replaced the thermoquad for an Edelbrock avs2 but the high idle still persists. I backed the idle screw all the way out but I'd guess it's idling somewhere around 1100-1200rpm but if you rev the engine hard it'll kick itself down to around what I'd guess 600-700. I checked for vacuum leaks and didn't find any. I was going to try tuning the carbuerator and checking timing on the engine but was wondering if anyone had ran into this and had some insight on something I'm overlooking.

 

[Xcudame]

There's a fast idle screw and idle screw that are separate on both the Thermoquad and AVS2. The fast idle screw is controlled by the choke and when it is engaged, the regular idle screw doesn't touch its stop. Once the engine warms up and the choke opens up (either by heat or electricity), the fast idle cam will move away from the fast idle screw and the regular idle screw will engage. Buy a tachometer to hook up under the hood. Less than $30 ones are readily available. Also consider purchasing Dave's book on Carter Carbs or something similar.

https://www.amazon.com/Super-Tuning-Modifying-Carter-Carburetors/dp/0931472113

 

[Chryco Psycho]

more likely that you have a vacuum leak or the advance weights in the dist have spring that are too light

 

[mrmopar340]

Spray some water around where the intake ports meet the head. See if there is any difference as the water will kind of seal a vacuum leak for just long enough gor you yo hear it. I'd try that first as I'm agreeing on the vacuum leak causing issue.

 

[Xcudame]

A vacuum gauge is one of the best tools you can buy! Here's my 64 Imperial's vacuum reading when I got it fired up today. I haven't even fine tuned it yet! Unless you have a really big cam, anything less than 14 inHg vacuum is an issue.

 

[crappy73cuda]

I'm currently running a stock distributor on the car. I sprayed around the carb with some starting fluid but I will try the intake manifold too. I recently bought a vacuum gauge, I was going to try tuning the carbuerator with it but I will check the current vacuum on the carb before doing such.

 

[cuda joe]

all very good suggestions check for vacuum leaks and see if the choke opened all the way when warmed up that controls the high idle when cold which is adjustable i wouldn;t have thought of dist weights but check it out maybe a spring rusted and broke

 

[Challenger RTA]

I just replaced the thermoquad for an Edelbrock avs2

All the above info is good. I would start with that. I the linkage lining up or binding up. Also is the chock closing and opening correctly. Reason I ask is the top chock linkage at same place as the as the other carb. I there a spacer under the carb now or before? Don't know if the there is enough travel in the choke. Vacuum Gauge Readings.

 

[Xcudame]

Interesting thing I just discovered this week on my 64 Imperial that I put a 74 440 Intake manifold and Thermoquad on. Fast idle was fine, but curb idle was about 1000 rpm. A little high for my taste. Realizing I had just the original AFB throttle return spring and not the double spring set up on most Thermoquads (including the Thermoquad on my 70 Challenger), I added a secondary spring and now the curb idle is 750rpm. So that's another thing to check!

RTA, thanks for posting the vacuum gauge info. Everyone should print that out and stick on the wall of their garage somewhere!!

 

[crappy73cuda]

So I ended up messing with the carb again and stopped being so delicate with the electronic choke which finally popped off so I was able to leave it fully open. That dropped my rpm down and I was able to get it at a lower idle finally. Thank you all for the help