I'm not quite through with the fast idle enigma on my '79 Catalina like I thought I was. I've noticed it takes a good hour of driving in town, 20 minutes sitting at the drive thru, or 25-30 min on the highway when it's as warm as it is outside to get it to stay off fast idle. That doesn't make sense to me.
How are rebuilt carburetors set up from the factory? Are they set to various "factory" set points that then need fine tuning for the engine, or are they good enough for plug and play applications? I suspect the fast idle cam spring is set too tight and backed it off a quarter turn to see what effect that has. The choke itself doesn't seem to be the issue anymore.
Since old cars like that are always running "open loop" the settings on a rebuilt or new carb out of the box are just a starting point. It's expected that the carb will need to be fine-tuned for your individual engine.
What made me think of this today is the fact my girlfriend's dad put the new carb on himself a bit before he parked it. I don't think he ever had anybody actually set it. From some conversation he and I had about that Corvair, he seemed to imply they're good enough from the factory to be installed and run out of the box
They're set at the factory good enough that the car will run but it still needs to be fine-tuned to your engine since there are no sensors or computers to fine-tune the mixture and idle speed on the fly.
That's what I figured. The mixture seems great, it'll do 85 on the highway, so it's running excellent. It's this idle thing that's weird. I backed the fast idle screw off a quarter turn to see if that has an effect.
If the mixture is good - smooth idle, no black smoke, plugs not carbon fouled, runs great - yeah, I'd just leave it as is.
Best way I found to do idle mixture is to hook up a tachometer and with a warmed up engine, turn the idle mixture screw in until the engine just starts to miss a little, then unscrew it until the engine is running at highest and best idle.
With fast idle settings, you just set the choke, start the car then turn the screw on the fast idle cam to the specification. Usually, carbs have three settings with screws:
1. idle mixture
2. idle speed
3. fast idle (when engine is cold)
I've not messed with the mixture screws, that engine runs like there's no tomorrow, so I'm leary of ever touching them, lol. The choke is loose enough that it just shuts the plate like it should. I suspected the fast idle cam might be too tight, because there's really nothing else putting tension on the choke linkages when it's hot.
There has to be some kind of fast idle adjustment somewhere on the carb.
I found the fast idle adjusting screw when I was fooling around with with alternator belt today, and backed it off a quarter turn to see if it changes anything. I was going to go a half turn but decided to go small in case I overdo it, lol.
Yes, small adjustments make big differences.
Also, remember, the performance of the engine on the highway is a function of a lot of fixed size jets - mains and air correction as well as the accelerator pump and other systems.
I couldn't tell whether or not there was a difference in the time it takes to fall off fast idle after doing the quarter turn change while it sat at work, so I waited until it sat overnight, then started fiddling with it this morning. Starting from cold I could really tell how much a quarter of a turn changes the fast idle speed. I went back to the way it was set originally and slowly backed it off while messing with the choke linkage to get a feel for how it affects releasing the choke to drop the idle. I set it around 1/3 of a turn and let it run for 20 min. Also backed off the base idle screw about a quarter turn, and set it to where it just starts to idle smooth when the tranny's engaged.
I've noticed the choke plate wiggles a bit when I set it to the slow idle and touch the throttle. I assume it shouldn't move at all?