351w Holley Street Avenger, duraspark distributor on regular non-ported vacuum advance.
It’s been bugging me for a little while now and at first I thought I might have a vacuum leak, but I just can’t pin it down. The problem is after warming up the engine and driving a few blocks, my idle goes from 650 to 800 rpm for no apparent reason. I have wired the electric choke open to take it out of the equation. I have sprayed MAF sensor cleaner around the carb base and intake to ensure there were no vacuum leaks as well.(no increase in rpm after spraying)
Why would my idle increase after driving, but not when it’s been warmed up and left at idle? After getting on the highway and doing 80 for my hour long commute into work, it stays at about 900 rpm. It’s got me stumped and it’s ticking me off.
My car is different: 351W 2bbl Motorcraft 2100 carb with stock hot air choke, but I got a very similar issue (same RPM same time) caused by a missing cap in the button of the piston chamber (choke not equipped with piston). Back to normal when new cap was installed.
I am running an electric headlight door conversion with no vacuum lines running to the headlight switch. Only 3 things with vacuum to them: dist. Vac advance, brake booster, and new vacuum can with line running to ac/heat selector switch in the dash. The ac switch does have a leak when I turn it to anything other than off. I am planning on taking that unit apart and get a rebuilt vac switch very soon. I’ll cap off the feed line for that and see if it makes a difference.
That’s a thought. I have disconnected the ac vac switch components from the carb and plugged the port for my hour drive to work tomorrow. I will see what that does first. I will check the mechanical weights and springs in the distributor after I get home tomorrow.
Being as you have to yank the trigger wheel and points plate to get at the weights/springs, how about checking the timing with a timing light to see if it’s sitting advanced at idle, vac disconnected?
Before and after driving would be the best way probably. Some one correct me if I’m not thinking right…
My thought process is that it should be at what ever base timing you set when you start it and let it warm up (Vac advance disconnected) and show some advance when you rev it showing the weights working (If I understand the above, the idle remains fine till after a drive) and then if you check it after your drive (with the problem present), if it’s the advance hanging up, you’d see advanced timing at idle.
That makes sense to me, except…wouldn’t the timing advance a little due to the increase in idle. Maybe I’m wrong, but I don’t know if I could tell why the idle went up. (From the mechanical advance due to idle being increased or advanced because of a weight issue)
I will check it out and report back. Probably this weekend if I can’t try it tomorrow night.
On another note, I did cap off the vacuum port that fed the ac can and rotary vacuum switch that has a minor leak. The results are a slight decrease in the idle symptoms. From appx 900 rpm to 800 or 850 rpm after an hour long 80mph drive. Normal idle with engine warmed up and sitting is between 650 and 700 rpm before any driving.
