Inconsistent idle speed?

[Harlow]

Alright, I did some minor tuning on the new Edelbrock 750 and did a few test runs, ran really good. The next day I start it up and let it idle for about 6-7 minutes, the car was idling at 650. I then took it up the street and pulled it back in the garage. The car would still idle at 650. I made no changes. When I shut it off the day before It would idle nice and smooth at 850. I've been reading, and it sounds like this can be a symptom of long duration/high lift cams. What adjustments should I make so that I'm not having to re-tune every time I want to go out and drive it.

[Harlow]

I guess I'll throw my theory out there. Tell me what you guys think. At idle with the car in gear, I'm pulling between 4" and 5" hg. The step up springs in my edelbrock are stock, operating at 5" and lower. Is it possible that some days I'm pulling more vacuum than others (I.E. sometimes the metering rods are non operational at idle and sometimes they are). What do you think?

- Scott

[histoy]

If you only have 4 to 5 inches of vacuum at idle, I'd first check the carb gasket and intake gaskets to verify whether you have a vacuum leak.    If they check OK, then my guess is that you have the idle speed screw cranked in so far that the carb is not running totally on the idle circuit.   There is a slot in these carbs for part throttle operation, and it's probably partially uncovered.   One way to correct that condition is to drill a small diameter hole (0.030") in each primary throttle plate.   This will bleed a small amount of air through the plate and allow you to back off on the idle speed screw to close the throttle plate and return to the idle circuit.   I've had to do this to several of these carbs when running long duration cams.    I'd suggest that you get the input of others on the site to see if they've had similar experiences. 

[Ghoste]

I had to change to a lighter spring in my Eddycarter for inconsistent idle and off idle performance.

[Harlow]

Alright, I'll check the gaskets for vacuum leaks. What should I use? Carb cleaner? Also I get about 6"-7" with the car in park, when I put it in gear the rpms drop to about 650. I'm also going to try and re-tune the carb and get the idle speed up higher and see what happens with more vacuum. When I first got it, I set the idle mix once and checked timing and took it out. I was reading and it sounds like I need to turn the rpms as low as they can go, set the idle mix, then turn the idle speed back down and do it again, raise the idle speed to where I want it, set timing, then do the idle mix again. Sound right?

[histoy]

Yes, you can spray carb cleaner around the carb gasket and intake manifold gasket to see if the rpm increases.  If it does, then there's a leak at that point.  There is also the possibility that the intake manifold gaskets are leaking on the valley side.  If so, you'll burn oil and foul the plugs.  Sometimes this happens if the heads are milled and the intake manifold isn't milled down to match the ports.

As far as adjusting the timing & carb, I warm the engine to operating temp and set the timing. Then I hook up a vacuum gauge to the intake manifold to monitor the vacuum as I adjust the carb.   I set the idle speed where I want it to be while in park, then adjust the mixture screws to give the highest & steadiest vacuum reading.   If the idle speed increases, I set it back to my desired speed, and then return to the mixture screws.  Once I have it where I want it to idle with the highest & steadiest vacuum reading, I put it in park, then back in drive 2 or 3 times.  You might have to fine tune the settings a little, but not much.   If the idle speed varies a lot, then you have other problems.    Changing the springs in the carb as Ghoste suggested might be a good thing to try.

[Chryco Psycho]

with the vacuum that low you need to change the springs under the metering rods for sure , try to get 1-2 " under the vacuum you have in gear at idle to keep the rods down