2 x 4 Quick Fuel Carb Tuning

[Barry_R]

Joe is exactly correct.  Since you cannot "adjust" the idle circuit on the secondary side of the carb, you need to make certain that the secondary blades are properly centered and the stop/blade position/transfer slot exposure is correct before moving forward.  That sets the baseline for the more readily altered primary side.

[The Real McCoy]

Pulled the carbs back off this morning and set the transition slot to .020 exposure on the secondary throttle plates.  This did move the throttle plate quite a bit to obtain the .020 exposure.  The primary throttle plates speed adjusting screws on both carbs were turned in until they began to open and then backed off until they were to full closed. The transition slot is not exposed on the primary side of the carburetors.  Idle mixture screws for both carbs were set to 1/2 turn out.

Started the engine up and O2 sensor now was reading in the 15-16 range so that did make a big difference.  I might not fully understand what exposing the transition slot accomplishes but expect it is an additional air bleed for the idle circuit with the throttle plate almost closed.  Now the problem is the engine is running at 2000 RPM,  probably due to the secondary throttle plates being partially open.  I backed the each of the secondary throttle plate speed adjusting screws out about a half turn to get it down to 1500 but of course the O2 sensor is now going low again.  I would expect that the 1/2 turn back out pretty much covered the transition slot again......

Any suggestions?

[Joe-JDC]

Since you are using two carburetors, you will need less slot showing in the secondaries.  I thought I had in an earlier post to set them~.012-.016" each.  If not, then I should have for two carbs.  The wider the slot is exposed, the higher the idle speed, so if you want to bring the speed down, slowly take the same amount out of each secondary blade position, and you can bring the idle down to 700 rpms.  Every set of carbs is unique, so you may find to get the best idle quality your secondaries slot will only be .010" each for final adjustment.  However, both carb primary blades should be closed and then just tweaked enough open to keep them from sticking in the venturi, and the idles screws in the metering block may need to be adjusted out a turn, or less.  Joe-JDC

[The Real McCoy]

You are correct. You posted .012 to .016 on dual carbs.  I just had the .020 in my head when I set them. I backed it off about a half turn on each but only brought it to just under 1500!and O2 was going back down so I stopped. Was really afraid I'm getting back to where I started from. I'll take another run at it tomorrow. Probably will pull at least one carb off to see if it's covered up or not.

Thank you for the help.

[427LX]

Are you running any kind of PCV system? If so you could put a restriction in the vac. feed hose to cut back on idle air flow from it allowing you to correct the idle speed.
Initial Advance Timing...if say you are running 14 degrees you could retard it to 12 degrees,slowing idle speed,and add it back in the advance curve.

[Drew Pojedinec]

or just disconnect the pcv temporarily to check if it has a large impact.

I ended up putting a piece of hose into my pcv line to slow flow a lil.  Took a bit of fiddling, and a solid 3-4 times of taking the carbs on and off to get them dialed in, keep the faith, you'll get it.

I'd also make 110% sure you have NO vacuum leak (sure easy to do with 2 carbs).  Even a tiny leak will destroy your ability to get it tuned. 

good luck

[The Real McCoy]

427 LX....

Yes I am running a PCV system, valve cover to back of intake.  Will consider trying your suggestion.

Timing currently at 12 BTDC plus vacuum advance connected to manifold vacuum so it has a bit of advance at idle.  Starts good at 12 when hot and water temp stays down when idling for long periods with vacuum advance active so I would rather not make changes unless I have too.

Thanks for the ideas.

[Drew Pojedinec]

The vacuum advancing the timing at idle would raise the rpm, no?

[Barry_R]

Restricting PCV is a really valuable suggestion.  You can drop to a .060 restriction in the hose and still have enough flow to keep the benefits of the PCV.

[My427stang]

Are the primaries fully closed even with it at 1500 rpm?

If they are, then IMO, it's getting air from somewhere it shouldn't

[The Real McCoy]

I'm going to go back at it this morning to verify secondary throttle plate to transition slot location and set them to .010 so I know exactly where I'm starting from.

Drew
Think I'm good on no vacuum leaks.  I do have phenolic spacers that I had to machine down to get the hood/air cleaner/distributor to play together.  The machining of the phenolic was not really a perfect mirror finish so I used gasket sealant to glue both carb gaskets to the spacer.  Intake is an NOS low riser and of course carbs are new.  Carb ported fitting is plugged and carb manifold vacuum port goes to distributor and Snap On vacuum gauge that has kind of become a permanent fixture.  All new hoses and tight.  The wild card is the PCV which I will address this morning.  It has always maintained relatively good vacuum, little less than 15" when idle was 800-900 and probably 16-17" at 1000 as I remember.

Yes RPM did jump a bit when I switched to manifold vacuum advance.  I got caught up in a couple of traffic jams and water temp would go right up to the Hot mark in about 15 minutes.  Not really to the danger point but I really didn't like looking at it.  Changing to manifold vacuum really settled it down, at least on my car it seemed to make a big difference, so I would like to keep it that way.

Barry,
Yes, thought the same thing about the PCV, that is a really good idea to block off/restrict.  Will be the first thing I try when I get the carbs back on and I establish where I'm at with .010 on the secondary plates.  Is there a target vacuum/pressure that is desired for the crankcase?  It wouldn't be that hard to put port for a magnehelic gauge on the intake breather cover.

Ross,
Good point about extra air...., hindsight is I remember when on the dyno it really would not idle down.  Time was limited so the issue was not pursued at that time but that's another whole story in itself.  In the car with open headers it was running fast I think we backed off the secondary's to get it to idle.  More hindsight is the O2 numbers on the dyno were not as rich as I am seeing now once I added the O2 sensor, probably because of closing the secondary's. The primary plates are fully closed.  I'm really hoping the PCV idea has a big effect on the problem.  I guess my big worry is the secondary throttle plates really start moving when uncovering the transition slot.

Again, thank you for all the ideas.

[The Real McCoy]

This is what I'm seeing after setting the transition slots to .010 exposure on the secondary's and getting the engine fully warmed up.  Ambient temperature 59F, elevation 1310 feet, humidity 91%, and barometric pressure 29.97 if anyone is wondering. 

RPM                O2 Sensor             Vacuum
2000                 12.9-13.2                20.5"
Pulled PCV from Valve Cover and covered with piece of stiff plastic.
2000                 13.0-13.2                20.5"    Really didn't seem to have much effect at this RPM, darn......
Back each secondary throttle stop set screw out about 1/8 turn
1900                 13.2-13.6                20.2"
Back out another 1/8 turn on throttle stops.
1775                 12.8-13.0                20"    I would guess the transition slots are covered by now.
Back out another 1/8 turn on throttle stops.
1625                 12.6-12.7                19.8"
Back out another 1/8 turn on throttle stops.
1400                 12.3-12.4                19.3"
Uncover PCV  (hanging out in the breeze)
1450                 13.3-13.4                19"
Plug PCV back into valve cover  (opposite valve cover has twist in vented oil fill cap without fitting to air clearer)
1450                 12.9-13.2                19"
Put it in gear and let the clutch out when I shut it off, pretty sure it would have made an attempt to keep going at 1450 RPM.....

Flaws in my data.....
1/8 turn out is kind of a judgment call, may not be exact but think I was close.  I tried very hard to turn each set screw the same amount.
Vacuum gauge readings are not exact, gauge is steady but It doesn't read in .1 increments, my best interpretation that there was a change.
O2 sensor moves around a bit and I tried to capture the general range it was mostly displaying.

I am sure if I continue to back out the secondary throttle stops I can get it down to 800 RPM with 13-14" vacuum, at a 1000+ it will do 16+".  Problem is I'm back to the rich condition I'm trying to address.  How do those vacuum readings sound at those RPM's?  I guess I thought they were reasonable.

I know Ross felt it was getting to much air at the 1500 RPM which I agree with, I think it is too much opening on the secondary's and not from some other leak point.  Primary throttle is definitely closed.  Any other thoughts on where it could be getting air from....   If I did have some other vacuum leak would it show up more on the vacuum gauge or do these numbers indicate no leaks?  I did take a close look at the carb gaskets, spacers and base plate today and didn't see no issues.  Intake gaskets I think can be ruled out as a cause, not burning oil or other symptoms typical of an intake leak.  The only other fitting on it is for the PCV at the back of the intake, I did look at the fitting and PCV hoses, no issues observed.

So, I guess I will go out and ponder what I might be missing and hope one of you will give me another idea to pursue. 

Thanks for bearing with me on this.

[My427stang]

So, I am in NO WAY saying Barry or Joe are incorrect, however, I do this a lot differently and haven't seen the issues you are seeing

I start with the rear barrels just off of touching, basically the only thing I care about is that the butterflies can't bind in the baseplate

Then I put the primaries at .020-.030 transfer slot showing.  To be honest, I start with them looking "square" with the choke either adjusted open, or verified not touching

Then I start it and see where I am.  If I can get idle high enough without getting into the transition slots much more, I adjust to best idle, equalizing the front and back sets of primaries

So, all of that it sounded like you did, and then it was running rich by the O2 sensor at idle, and that's what brought you to the original question

so............

1 - If you can bottom all 4 primary adjusters after doing what I said above, and it doesn't stall or it idles fine and still rich, then either the secondary idle circuit has something wrong with it (too large IFR on the secondaries, NO IFR, warped body, or something else, because at that point (primary adjusters closed and rear butterflies closed) you should not have enough fuel to idle, nevermind idle rich

2 - You are sitting on the choke mechanism and didn't realize it, so the primaries are really open.

3 - Is there any chance you have an ignition problem?

In the end, I have never had to chase the secondaries other than to allow me to close the primaries on a good set of carbs.

One last comment, I have seen bad carbs from every manufacturer out there, QFT included.  Odds that two carbs have issues are unlikely, but remember, you can always swap one or both with a loaner and see how the car does

[Joe-JDC]

If these are reverse idle carbs, then all this information is suspect.  They lean out the further you screw out the idle screws.  I have never had a carb that liked setting the idle with the primaries.  It goes against everything I have learned over the last 50+ years dealing with Holleys.  JMO, but I can be wrong on occasion.  Joe-JDC

[My427stang]

So only 30 years of experience here...and I can't gain on Joe, every year I get one, he gets another one too

However, just teasing Joe, but I am a bit stumped on the situation.  All 4 blades on each carb are closed, except secondaries are just open enough to show .010 of the rear transition slot and it won't idle under 1400?  Something is goofy, did someone drill holes in the butterflies? 

If yes, find that guy and kick him in the nads LOL then put a dab of JB weld over every hole, it'll last forever

If no, maybe the way you are measuring ".010" is wrong?  An engine needs air and fuel to run, and if you have all the blades shut down, it shouldn't be humping away that high.

I have to admit, I tend to be suspect of idle a/f  using an a/f meter/O2 sensor, does it run well?  Does the carb adjust normally?  Have you run it to get the exhaust good and hot and then looked at idle a/f?