My 600cfm Holley vac sec carb seems to run rich at idle and lean at wot I have 68 jets in the primary. The fuelbowls are at the proper level. How does one go about adjusting this. This is on the car in my sig

 

do you have jets in the secondaries or the stupid metering block thing?

 

Sorry have the metering plate still

 

Buy a Secondary metering block conversion from Holley and go from there. Converts from a 4160 series to a 4150 series. Then you can easily change both the Primary and Secondary jets. You can likely lean out the Primaries two sizes and go up on the Secondary jetting two sizes.

May as well get the Quick change Bowl kits as well if that's in your budget.

Get the List # of your carb....usually on the airhorn. Then you can look up the stock specs for jetting on the secondary side.

 

I wouldnt go as far as buying the conversion kit just yet. One thing you should definately buy is a Quick Secondary Spring Change Kit along with the assortment of springs. Try jetting up in the primaries and using a lighter spring (so the secondaries open quicker)

To solve your idle problem, have you adjusted the mixture screws?

 

Yeah I have adjusted the idle mix screws using the vaccum guage and I do have the secondary quick change kit. On thing I think I need to do is stepup my main jets. I run a 8.5 powervalve with 17" of vacuum. But thi shouldnt cause me to be rich at idle and lean at wot

 

Rob the secondary metering plate out of a 750VS 3310
It has a#21 stamped on it. It is equal to using a #76 holley jet on the secondary side. These are as common a part as you're likely to find in someones holley parts collection. You can buy them new too. You can also remove and drill out the jet orices in your metering plate. It should be a #9 or #39

Drill it out .003" bigger at a time and test the car in the quarter mile. Watch the MPH at the end of the quarter mile.
As long as it improves with larger jetting keep going up.
A #21 plate has a .081" jet hole. Shouldn;t need any richer than this.

A #9 has a .067" hole ='s a 64 jet. a little lean.

I find 600vacuum secondarys carbs seem to like 64 to 66 primary jetting and a #21 secondary plate. Seems to work well overall.
Try it.
If you still get a rich idle, remove the carb and flip it over and look at the throttle blades in relation to the transitions slots.
They should just expose the transition slots a little .030" on all 4 bbls. reset this setting and reinstall the carb. reset the mix screws for best warm idle. then adjust idle speed.
make sure you have enough ignition timiing at idle 10 to 14deg
and use a PCV valve system. If you don;t use the PCV, the carb will idle rich. After you have tried this setup and it works good overall you can experiment with leaner primary jetting to see how lean the motor will cruise. Go down in jetting 2 sizes at a time till the cruise (40 to 60 mph hiway) drivability just goes away ( lean surge),, then go back up 2 jet sizes. Usually you end up with around 59 to 64 jets.

Of course we're assumiong you are feeding the motor all the fuel it wants at high rpm. If not it will leanout in the top end reguardless of jetting. Check fuel pressure at WOT rpm.

If you're at a high altitude +3000 ft start 2 jets leaner front and back.

Use vaseline or white grease on the bowl and metering block gaskets so they don't stick while R and R'ing jets.

 

Thanks f-bird, I thought of drilling out the secondary orifice a little. I think I might have the transistion slots exposed to much. I know this is adjusted on the bottom of the carb. How much adjustment is too much? How and where do you measure this at? Would you happen to have a picture? I also figured out this particular year car 1987 had an electric fuel pump and a mechanical fuel pump. So I will hook the fuel pump up through an ignition source wired through a oil pressure switch, I hope this will solve my leaning out condition on WOT, provided the pump is still good. I was wondering.....what happens if I do not use the pressure switch, would I run the risk of the engine filling with fuel if the ignition is left in the on position. Is the electric pump strong enough to unseat the needle and seat valve? I seem to remember reading somewhere where they had this problem, except his car was parked on an incline though.

 

The factory electric/ mechanical paired pumps on carbed Fbody's are low perf stuff.

The problem is volume at WOT. The pumps just can't keep up at maximum horsepower with an engine that is more powerfull than stock. Get a fuel pressure guage and read it while at WOT and high rpm. If during a high speed run it drops below 3.0lbs you need more fuel delivery.
Either the pump(s) are not big enough or there are restrictions in the fule line. (Connections, filter, pickup,rubber hose connections,
line bends or a combination of all. Any rubber fuel line on the vacuum side of a pump can/will be sucked shut and restrict flow, especially when hot. Also check the tank vent. If it is restricted the fuel will not flow reguardless of the pumps ability to move fuel.

The transitions slots should just barely be exposed on all four barrels. about .025" to .030" Adjust this and reinstall the carb.
Once you reset the idle mixture screws, you should only need to adjust the idle speed screw a small amount to get the desired idle
speed. If after doing this the idle is too fast, then remove the carb and reset the throttle blades again a little farther closed ( less transition slot showing) But it should be pretty even on all four barrels. I don't have a pic. If you remove the set screw for setting the secondary idle stop and screw it back in from the top you can make this adjustment with the carb on. But you need to baseline the throttle blade setup visually the first time so the barrels are "insync" and positioned right, or the idle will be off. (rich and poor response).
What is the fuel pressure at idle?