SharkAttack

Couldn't get my stock 72 LT-1 running well with the original carb, so I temporarily installed a used Street Avenger 770 that I know and saw run/drive just fine (cold, idle, hot, acceleration, etc.). Once installed, car started right up. With the original carb, it would barely start then run very rough and rich, but that's for a different thread.

Verified the timing (4 degrees BTDC with distributor vac line removed and carb port plugged) and can't find any vac leaks, but the idle is too high @ about 1000-1100 rpms even with the idle screw backed all the way out. Backed out the fast idle screw about 1/4 turn and didn't notice any difference. Each of the air/fuel screws on the front metering block are backed out 1.5 turns (1 turn being 180 degrees, not 360 degrees).

The only vac lines going to the carb are to the distributor advance port and the PCV / charcoal/vapor canister (one large, one small). I completely removed those lines to eliminate them as a variable, but the issue persist. All other ports are plugged with good rubber caps, and the PCV and distributor advance ports are in the right places per the Holley manual.

Interestingly enough, if I remove the distributor vac line from the carb port while the engine is running, the idle lowers by 300-400 rpms and is close to where it should be (but - the idle screw is still backed all the way out and this should not be the case).

Experts - please chime-in with your suggested next steps or suspected causes. Thanks!

mvette76

did you make sure your throttle cable or rod is not holding the throttle blade open

commander_47

This may sound silly, but make sure the car is totally warmed up.

Your automatic choke on the carb will not want to come off until the car is totally warm.

You can than shut it down and adjust the fast idle screw under the choke.

Again, make sure your car is totally warmed up or you won't get a good setting.

SharkAttack

Throttle cable is fine with no interference, I checked that during installation. I did let it warm up because of the choke (original carb didn't have one). Choke is also set to open the fastest it can. May need to warm up longer, will try that next and am also searching the Holley website.

doorgunner

I had the same problem on my new Holley 600CFM carb. It idled and cruised perfectly, but the idle rpm was too fast.

Go to the Holley website and look up the tech instructions on the carb that you have---it will give detailed info about adjusting the carb.

Is there a hidden adjusting screw for the secondary butterflies on the carb that you have? They may be adjusted/opened too much at idle. The adjusting screw can be "backed-off" to make the butterflies close more which will slow the idle rpm down.

Also....as a last resort.......verify that the harmonic balancer is accurate by removing the #1 spark plug and rotating the engine to TDC on the compression stroke (making sure both valves are closing with the valve cover off helps confirm TDC also) The timing mark should point to ZERO at the tab.

My idle mixture screws are turned OUT 2&1/2 turns (360* per turn).

7t9l82

transfer slot adjustments are vital, they are almost never right.

Jeff_Keryk

If you backed out the idle speed screw all the way, the throttle blades should be shut. If the engine does not idle down and die, you have a vacuum leak. Air has to be getting in somehow.
Don't ask me how I know... It was the wrong gasket for me...

7t9l82

idle adjustment screw can be removed and either the primary or secondary transfer slot can keep the idle high.

augiedoggy

ditto...I just bought, rebuilt and installed a 670 street avenger on my 355 that I got from ebay... the PO said it ran too rich on his 327, first thing I noticed was his throttle plate adjustment was way into the transfer slots and the second thing was the float needle was somehow stuck for the secondary never allowing fuel in the float bowl from the looks of things... also his accelerator pump arm was adjusted all wrong.

the "hidden" screw to adjust the seconaries is under the carb on the passenger side... just back it all the way out to where the butterflies are closed then go 1/4 turn in and its set at default. on larger engines with more aggressive cams it needs to be opened more.... also make sure your fuel pressure is around 8psi or less...

doorgunner

Transfer slots........Ahaaaa! The little rectangular slots down in the throat of the carb where the lower edge of the butterflies rest.

SharkAttack

Removed the carb (finally), cause must be the transfer slots because they are far from being "square shaped".

What's the proper method to adjust the transfer slots, the fast idle screw or something else?

NOTE: If I back out the fast idle and curb idle screws all the way, the throttle plates just barely fully cover (conceal) the transfer slots. Apparently I'll have minimal adjustment range with the curb idle screw (which is OK, but want to confirm this is normal/typical).

7t9l82

Little Allen head screw in the baseplate if you look on the drivers side of the carb its slightly offset to the shaft.

gerry72

Don't get too wrapped around the axle over the adjustment. The important point is if the transfer slot exposure goes beyond square when adjusting the idle speed, that is when you enter the intermediate enrichment. What you want to try to do is have some of the air metering controlled by the secondary throttle blades to the point you can control the hot idle speed with the primary speed adjustment without going into the intermediate circuit. You could end up where the secondary was just barely cracked open, leaving the intermediate circuit barely visible and not square at all with the primary adjustment having good control of the idle speed and on the idle circuit only. The recommendations to adjust the throttle blades to where the intermediate circuit slots in the primary and secondary appear square is a "usually works to get the carb in the ballpark" adjustment. If done properly you would normally have a good idle speed adjustment range in the screw...a couple degrees of rotation of the screw one way or the other should make a 50 rpm difference.

To start, set the secondary blade opening as 7t9l82 noted. Then adjust the primary blade angle to mirror the secondary. Bolt on the carburetor and see if you have the adjustment range you need in the primary idle speed screw. It's good practice to make sure the fast idle screw isn't making contact with the choke cam when making adjustments. You can always reset the fast idle once you have the throttle properly sorted.

I doubt you have one but just throwing this out there; One good use of an O2 meter is it will show you precisely when you enter the intermediate circuit. O2 reading will immediately drop nearly ratio 2 points as you increase idle speed to the point of overexposing the circuit slot.

SharkAttack

Thanks, great info, but how exactly are the primary shaft butterflies adjusted to change the transfer slot sizes? The only way I see to do this is the fast idle screw. For the secondaries, I know about the screw in the base plate and they're adjusted properly.

Here's where I'm at right now, and I want to get this right before putting the carb back on the car:
Secondary transfer slots adjusted properly (showing a square). Primary transfer slots adjusted properly to show a square with the choke cam engaged. Curb idle screw completely backed out (no contact). Fast idle screw back out to where its plastic boss makes contact with the choke cam, but the screw does not (full out position).
If I disengage the choke cam, the primary butterflies close a little more and completely (but barely) hide the transfer slots.
If the transfer slots need to show a square with the choke cam disengaged (as if the engine is hot and choke is fully off), how do I adjust the primary shaft/butterflies?

These carbs have a lifetime warranty, so backup plan is to call Holley on Monday if necessary.

SuperBuickGuy

I hate to be the bearer of bad news, but it's a rare circumstance where an aftermarket carb bolts onto a car and runs correctly.

You need to learn how to adjust a Holley carb - there's a ton of really good books out there to help you with this so I won't even begin to tell you the testing steps; what I will do is give you some of the reasons it's idling high:
- improper idle position screw
- improper adjusted mixture screws
- bad or wrong power valve
- internal leak (vacuum or fuel)
- wrong jets
- intake manifold leak (my bet given you had trouble with the prior carb)
- cracked intake
- bad baseplate or intake gasket
- improper tightening of the carb to the gasket
- wrong gasket
- plugged port
- improper float level
- improper fuel
- overpressuring from the fuel pump

and the list goes on

SharkAttack

Thanks, good info, but as mentioned in the first post, this Street Avenger carb ran perfectly fine on a different engine right before it was swapped to the LT-1 engine. And the LT-1 engine ran perfectly fine until its original carb sat for a long time. The Street Avenger carb on the LT-1 engine runs just fine except for a high idle and the current cause is suspected as the transfer slots (confirmed much larger/longer than they should have been). Just trying to confirm how to properly set the transfer slots. Once done, if that's not the issue, I'll move on to other suspected causes.

OMF

You adjust the butterflys with the curb idle screw.

7t9l82

turn the carb upside down and look at the baseplate, there is a tiny allen head screw recessed into the baseplate turn that little allen screw and you will see the butterflies open or close. that is the adjustment for the secondaries. quick fuel makes a secondary adjustment device that takes about a minute to install and allows adjustment in seconds. your just going to have to pull it off and look.
it is possible to adjust the screw with the carb mounted on the intake but you won't visually be able to see the slot

SharkAttack

Thanks, I know about that screw and the secondaries were already set. I adjusted the primaries (transfer slots), installed the carb, and the idle is in the correct range now. Running great so far! Thanks everyone!

Kacyc3

Was the car high idling before?