[1966_L78]

I used to tune Holley for my buddies, but it’s been more than 20 years since I really played with a Holley.

I just picked up a new project with a 327 L79, 4-speed) and a 600 cfm, List 6619-1, 4160 vacuum secondary…. Carb looks newer, but car mostly sat for last 20 years. Ran, but rich, stalling, etc.

So I rebuilt it using a kit the previous owner provided.

put it back together and it runs better, but surges during a steady cruise…. The idle mixtures screws are almost all the way in. One side is 1/8 turn out before vacuum starts to drop, other side turns out much further…. Eyes burning…

Not very loping cam, I think factory L79 350 HP grind, but hard to get it to idle below 1000 RPM, and when reving, the rpms slowly drop back to idle (maybe 2-3 seconds) to drop from 2500 to idle.

64 jets primary, 31 squirted, (unknown plate in rear).

Initial timing is at 16 BTDC, I think the cam is the GM L79 hydraulic. All vacuum lines were disconnect and capped. During “cruise” vacuum advance was hooked up to ported ( in metering block

I am wondering if the Power Valve (new) is bad or incorrect. I think it was supposed to be a 6.5, and the factory L79 used a 6.5PV…

Any suggestions?

 

[WHT/73]

Maybe there's something wrong with the pv or it's gasket ?
Screws all the way in would give you the surge as it needs that fuel from that circuit at that throttle position.
I ran 2" below cruise vac for pv. So 15" would have a 12.5 or something very close.

 

[boyd66k20]

Are the transfer slots square?
Isn't the pv based on the vacuum at cruise?

 

[RifRaf]

What type of distributor are you using?... and does this distributor have an adjustable vacuum advance?

You could try this... Disconnect and plug the vacuum line going to the VA canister and go for another test drive.

If the engine stops surging while cruising, then the VA canister is providing too much advance (and you will need to install a VA stop plate to help solve this issue).

 

[WHT/73]

Fuel Idle screws should not be all the way or almost closed.

 

[japete92]

Timing advance?

Connect vacuum advance to 'full' manifold vacuum.

Recommend reading the 'ignition 101' sticky in the ignition section of this forum.

Recommend getting timing right before futzing with carb.

Pete

 

[vortec710]

FWIW...consider checking for possible torn diaphragm in the power valve and the correct gasket for your power valve, vacuum advance ..check if hooked to vacuum port BELOW the butterflies (full time) and limit the vacuum advance to around 10 degrees (more could contribute to your surging), has the rubber limiter fallen off your vacuum can arm ?? Verify TDC, then timing, first.

 

[Aml383]

I used to tune Holley for my buddies, but it’s been more than 20 years since I really played with a Holley.

I just picked up a new project with a 327 L79, 4-speed) and a 600 cfm, List 6619-1, 4160 vacuum secondary…. Carb looks newer, but car mostly sat for last 20 years. Ran, but rich, stalling, etc.

So I rebuilt it using a kit the previous owner provided.

put it back together and it runs better, but surges during a steady cruise…. The idle mixtures screws are almost all the way in. One side is 1/8 turn out before vacuum starts to drop, other side turns out much further…. Eyes burning…

Not very loping cam, I think factory L79 350 HP grind, but hard to get it to idle below 1000 RPM, and when reving, the rpms slowly drop back to idle (maybe 2-3 seconds) to drop from 2500 to idle.

64 jets primary, 31 squirted, (unknown plate in rear).

Initial timing is at 16 BTDC, I think the cam is the GM L79 hydraulic. All vacuum lines were disconnect and capped. During “cruise” vacuum advance was hooked up to ported ( in metering block

I am wondering if the Power Valve (new) is bad or incorrect. I think it was supposed to be a 6.5, and the factory L79 used a 6.5PV…

Any suggestions?

The size of the PV isn't causing your issue.
The PV comes into play when the throttle is mashed although if its faulty in any way then ofcourse it'll effect everything. You'll never get it tuned right.

but When you're at a light footed cruise (which would mean you're still running on the transfer slot and not into the primary yet unless ofcourse if there isn't enough transfer slot available . There absolutely should be enough transfer slot available)
then you're still in the idle/transition circuit.
Even at 60 mph you're in idle transition circuit IF your foot is light on the go pedal and not into the primary.
If its surging under these conditions the problem is within the idle circuit which btw does not at all sound like its currently tuned correct.

You need to start the tune with the ignition timing which also sounds suspect here if you're unable to get the motor to idle under 1000 rpm. You've more than likely got a few different issues here. That intimate relationship between carb and ignition is on the rocks here lol

This is what I would do with that:
Start over .

1) take the carb off and adjust idle speed screw untill transfer slot is showing about a square or even a little less .
2) put the carb back on and run the mixture screws all the way in and then back them out 1 full turn 360 . Leave the idle screw alone for the time being
3) loosen distributor base enough to where there is quite a bit of resistance yet still can be turned
4) unhook hose from vacuum advance on distributor and plug the end of the hose
5) put a vacuum gauge on manifold vacuum
6) fire up the engine. If it won't fire up then turn the distributor a couple of degrees one way or the other untill it fires up
7) with engine running advance or retard the distributor untill you see best vacuum on the vacuum gauge (For now)
9) note the rpm the engine is running at under these conditions. If your idle is higher than 700 then see if you can reduce idle by running mixture screws in or out and or perhaps backing the idle speed screw out a bit .
Or you may have to move the distributor just a little.
The idea is to be pulling as much vacuum as is available given your engine combination AT LOW IDLE and i emphasize that because higher idle =higher rpm = more vacuum and that isn't what we're looking for here.
Its a balancing act and a fine line with the carberation and ignition timing.
And without getting into all about what your distributor is advancing and when and at what rpm you just get it running good under all the driving conditions and if you end up total timing been 33 and 38 degrees you just leave it alone .
This will take care of idle AND when your cruising 60 mph still in the idle .
Then you can deal with the other circuits as needed .

 

[Aml383]

Timing advance?

Connect vacuum advance to 'full' manifold vacuum.

Recommend reading the 'ignition 101' sticky in the ignition section of this forum.

Recommend getting timing right before futzing with carb.

Pete

Awesome article. I love the way he hammers the point home about using full manifold vacuum versus ported. Anyone who's anybody knows this

 

[68Chevele]

I can tell you from experience the 6619 is one screwy carburetor, it has the idle screws that turn backwards and what I mean by that is like most Holley's that you turn them in and the car wants to die the 6619 does the opposite, at least mine did. My 6619 even had a 2 stage power valve. I would get rid of that one and put the 1850 Holley 600 on the engine.

 

[1966_L78]

Timing advance?

Connect vacuum advance to 'full' manifold vacuum.

Recommend reading the 'ignition 101' sticky in the ignition section of this forum.

Recommend getting timing right before futzing with carb.

Pete

Awesome article. I love the way he hammers the point home about using full manifold vacuum versus ported. Anyone who's anybody knows this

yeah, I did forget that article…

as for the vacuum, I’ll move it. The metering block is where the originals went for this engine (67 L79). I am not going for a full l resto anyway.

 

[Aml383]

I can tell you from experience the 6619 is one screwy carburetor, it has the idle screws that turn backwards and what I mean by that is like most Holley's that you turn them in and the car wants to die the 6619 does the opposite, at least mine did. My 6619 even had a 2 stage power valve. I would get rid of that one and put the 1850 Holley 600 on the engine.

You're saying when you turn the idle screw in on those carbs it makes the engine want to die because the idle screws "turn backwards "
As in turning the idle speed screw in CLOSES the butterfly and in doing so decreases rpm ??
If that thing does that its gotta be the only one on the planet that does. Id throw it in the dumpster lol. Or no sell it

 

[68Chevele]

You're saying when you turn the idle screw in on those carbs it makes the engine want to die because the idle screws "turn backwards "
As in turning the idle speed screw in CLOSES the butterfly and in doing so decreases rpm ??
If that thing does that its gotta be the only one on the planet that does. Id throw it in the dumpster lol. Or no sell it

Ya Holley was trying the emission thing with that carb, did not tune well.

 

[Aml383]

Wow a carb where idle screw is turned in to close the butterflys. What will they think of next ?

 

[Ericnova72]

Wow a carb where idle screw is turned in to close the butterflys. What will they think of next ?

That's not how it works...idle speed stop screw is normal.

The difference is in the metering blocks....they have what was called "reverse idle", air and fuel bleed passages are routed different ....the idle mix screws control idle bleed air in these carbs, not the air and fuel idle emulsion mix like a normal Holley.
Reverse idle carbs have mixture screws with a relatively blunt and rounded tip, rather than the pointed needle tip of a normal Holley carb. Reason is the air adjustment needs to be courser to have any noticeable change in 1/8 turn increments.
mixture crew run in full tight = full rich since you are cutting off all idle bleed air....exactly opposite of normal Holley's

They are a complete dud in a performance application, you can't get them to behave very well at all.
Totally an emissions carb.

And no, you can't just swap standard Holley metering blocks in place of reverse idle blocks....those different air bleed passage routing in the main body prevent it....just in case someone thinks of that idea.

Those reverse idle emissions Holleys are much like factory TBI....they do not like performance parts deviations from stock, especially cams.

 

[1966_L78]

Thanks everybody…. Eventually I’ll replace it with an 1850 or similar…

but for now, moving the vacuum advance line to full manifold cured the ills…

the only reason I had hooked to” [email protected] was because that’s where the OEM carb is supposed to hood to…


once switched, it ran well, but idled really high, about 1800- 2000 rpm…

backed the idle speed screw out, and it wouldn’t drop any further…. Then I notice the throttle arm at the firewall was hitting the manifold…. Disconnected the rod from the carb, set the idle at. 750 rpm, and adjusted the rod to fit….

muxtures screws were able to come out about 1.5 turns each…


fid some more research on the screwy 6619 carb…

 

[oldguy67]

As already said,6619's were emission carbs,never really worked out on a performance type application.