(‘68 L68 427)
Been having an idle issue problem which I’m not sure what to attribute it too.
I go out to the garage to fire it over. I’ll tap the pedal to set the choke hit the key and it fires quickly and it’ll idle at 1,500rpm. Let it settle in and the rpm’s climb to 1,800. I tap the gas and it drops to 1,000. I back it out of the garage, turn around, tap it again and it settles to 750 with a nice mild lope. Perfect. I’ll take a drive down to Asbury about 7 miles away and it runs great stopping at lights and it idles fine. I get to Asbury and it low speed driving 15 to 25 mph and now the fun starts. The cars warmed up but not running hot, yet the idle drops to about 500rpm. The lope sounds more aggressive but as I roll in the idle drops more to around 400 and I’ve got to tap the gas as it sounds like it wants to stall.
If I were to leave the slow speed area and drive normal speeds the idle returns to normal.
What’s it sound like, vapor lock, a fuel pump starting to fail ?
Any ideas ?

 

Recommend getting a Chevrolet chassis service manual and follow the process for a basic tune up. Make sure you get a manual that has the Tri Power section. It really could be many different things and they only way to effectively trouble shoot is to get a baseline by doing a tune up. Otherwise you’re just shooting in the dark.

 

What is your timing set to?

 

could be idle setting, low fuel bowl float setting, might be a fuel pump but you would have issues all the time and worse at speed when it cant keep up. What quality idle is it when its down to 500hp. is it consistent or is it up and down and is it stalling at all. Has this been a recent issue or has it always had a low idle issue. If you have a consistent idle at 400 or 500 rpm without stalling I would say you have a great running issue with no vacuum leaks.

First change out the fuel filter if that hasnt been done in a long time. Then, without knowing your settings, as advised I would give it a performance timing adjustment so that its all in between 2500 and 3000 rpm to eliminate that issue. It will run its best and you and your motor will be happier as well. Increasing the timing will bring up your idle. Once thats done if the issue persists check you center carb float setting. it cant be lower than the bottom of the site hole. preferably a little in the hole. Too high and it spills into the vent. To low and the carrb will run lean as it struggles to pull fuel with no head pressure with such a small signal. Then readjust your idle mixture screws so you get best vacuum even if they run alittle rich, this will even out any wandering idle issues provided the carb is clean internally


What you dont want to do is adjust the idle set screw for the lower butterfly and increase the butterfly openning. Doing that opens up the transition slot too much and you will get idle and transition issues off of idle when accelerating. IF none of those settings arent it, try using a carb cleaner in the idle air restirctions on top of the carb. THe outside ones are for idle air. If none if that helps, you may want to have your carbs taken apart and cleaned. have the Idle air jets cleaned and make sure the idle fuel restrictions are cleaned. any crap internal to these carbs can play hell on your circuits. Unfortunately they do not have easily adjusted air bleeds on top and the internally restictions are all in your metering block. If you havent had them taken apart in a while its a good idea just to check for water corrosion as the outboards will collect water and the metering plates will dissolve

 

Ok I found “an issue” and now so far it’s looking like it could’ve been “the issue” because it’s running perfect now.
Turns out it was an issue caused by the distributor being reclocked. The factory installation has the tach drive facing the firewall and the vacuum advance can facing forward.
Once it’s been reclocked as this one was the tach drive faces the drivers side fender and the vacuum advance faces the passenger side fender.
This was done by the prior owner and what I wasn’t aware of was that the vacuum hose to the vacuum advance was contacting and rubbing on the ignition shielding box which wore a nice slice in the hose. That caused a vacuum leak and most definitely prevented the vacuum advance from operating properly. Pulled like a bear anyway.
Above where the hose was I modified that small area of the boxes bottom edge. I used a cutoff wheel and then a Dremel to round it out and finish it neatly making a cutout in the bottom of the box to give clearance for the vacuum line. It blends nicely into the box, doesn’t look butchered.
The car idles perfect at all times now.