Varying Idle RPM

[Lord Sparky]

New Edelbrock 650 cfm AVS2 carb, “new” rebuilt 3690432 Mopar performance electronic ignition distributor, orange ignition box on 1969 Dodge Dart GTS 440.

When the engine is warm (cold 1300 rpm), the idle varies from 700-1200 rpm. When I started out today (engine warm), the rpm was 700 rising up to 1000 while driving. When I got home, it was 1200 rpm (touching the gas pedal did not lower it). Service manuals says regular idle should be 650.

Yesterday, the engine was running at 700, then dropped to 300 rpm after I got gas. It stalled in reverse so I gave the idle screw a tiny (hardly noticeable) clockwise turn which produced the above idle rpms. No more stalling, but why does the idle speed change/increase?.

 

[dano]

Most likely a vacuum leak. Did it do this before the carb? Did you plug or populate the vacuum ports on the cab? Did anything else change like intake swap?

Idle RPM is subjective. How built is your 440 or is it stock?

 

[Lord Sparky]

Most likely a vacuum leak. Did it do this before the carb? Did you plug or populate the vacuum ports on the cab? Did anything else change like intake swap?

Idle RPM is subjective. How built is your 440 or is it stock?

Same intake. 452 cubic inches. One carb vacuum line to power brakes, one to vac advance, others plugged. There were issues with starting with old carb and some idle issues, mostly when cold.

 

[dano]

Disconnect the vacuum booster and see if things change. A bad booster will cause a vacuum leak every time you hit the brakes.