Tunning Dual Carbs

[67gt500]

Hi,  any initial setup and tuning tips when setting up the factory dual carbs on the 428.. i started with just cracking open the butterflies on the rare carb, then using the front carb to set the fast idle and set all idle mixture screws 1 3/4 turns out.. Car runs ok, but would like to know how others have done their initial setup.. 

There was some good information on the old forum, but that's long gone..

Thanks

[gt350hr]

   Both cars have idle circuits so both need to have the idle mixture screws dealt with. "I" try to set both throttle blades open the equal amount so the fuel distribution at idle is as consistent as possible. Same for the idle mixture screws.With the factory progressive linkage, the front carb is in use most of the time (obviously) so fine tuning it is important.
     Randy

[6s1640]

Hi 67gt500,

I am currently also trying to tune a SBF dual quads with the Holley carb's from June 1966, LIST 3360 and 3361.  They were concourse restore before I purchased and installed around eight years ago.  Up until last year, the car had been running great, even drove to SAAC 35, 2000 mile round trip with no issues.  Last July the car starting running bad, blowing black smoke at start up and not liking to start when hot.  Something changed inside the carbs and the Ethanol fuel I had to buy for the return trip home from the Seattle Historic Races I am sure did not help.  I try to run pure fuel as much as I can.

So I have since rebuilt and put back on the car.  If I understand how the systems is suppose to work, the secondary carb should not be working at all when at idle.  (If the secondary throttle blades are open, this will not be the case.)  You should be able to completely choke off the air horn with a towel and have no impact to the engine running.  The primary carb adjustment will be completely independent of the secondary.  Set the fuel mixture screw, float level and idle screw, etc.  Then set the secondary to match the primary and you should be good to go.

You may also want to run a vacuum gauge for the best setting for the fuel mixture screws.   If I understand correctly at idle you want the max vacuum.  With the gauge you can dial right on the mark.  You will also need to adjust the idle screw as needed to maintain your target idle RPM.

Now this is in conflict with what Randy said with the throttle blades open equal.  I found when the secondary carb throttle blades were open at all, I could not get the idle down because fuel was being drawn in from the secondary carb.  Soon as I got the blades closed tighter I was able get the idle down and adjust the carbs.

Bottom line, fine tune primary carb and then match settings in secondary carb.

That is my experience.  I hope this helps

Cory

[shelbydoug]

I run the 2x4 Holley set up on two of my cars. One is my 67 GT500. The other is my 68 GT350 (347).
I can tell you what I do with them. Others may disagree. Power to them? 



What I do is get the engine to only idle on ONE carb. The idle throttle positions do not need to be the same on the two carbs. You want the secondary carb throttle as close to fully closed as you can get it.

There will always be some idle flow through both carbs and that includes the secondaries of each carb.

In my view you MUST use a vacuum gauge procedure to adjust the idle. The idle mixture screws do not lean the mixture. They increase or decrease the volume of atomized fuel traveling through the idle slots.

In order to be atomized, fuel must be mixed with air. Thus an air jet and a fuel jet. Those control the mixture ratio.

The a/f mixture is set by the size of the idle air restriction and the size of the idle fuel slot. Those are the "jets". You can't change that mixture ratio.

You can in fact lean the idle by closing down the idle mixture screw to about 1/2 turn, then increase the throttle screw to open a little more.

The reason that you are leaning the idle by doing that is that when you open the throttle you are idling on the mains, which have a leaner mixture then the idle circuit does.

So you are mixing down the idle mixture by using the idle throttle screw.



As far as adjusting the throttle linkage, all that matters is that you are not holding open the secondary/primary throttle with it AND that you want the secondaries to start to open at about 50% of the opening of the primary throttle AND that when the primary carb is 100% open, so is the primary throttle on the secondary carb AND you are not over centering either carb to get that relationship.

In this manner, the two carbs act like a 600 double pumper Holley with a HUGE secondary vacuum operated throttle/booster.

Since the secondaries are going to be opened by engine vacuum/load demand, they really can't flood/bog the engine.

The reason that the vacuum secondary 2x4's were taken out of the road racing cars was that the vacuum secondaries under race conditions  had the TENDENCY of not closing the secondaries fast enough.



If you notice, the carbs are mounted backwards. The main reason for that was this is a "street" set up and the primary throttles are centered in the intake as much as can be done.

Some of the small block 2x4's had the carbs mounted forward with the primary carb in the front.
Depending on who you talk to, some think (including me) that this makes a fuel distribution problem in the manifold until the secondary primaries are fully opened.

In Trans-Am racing, the cars were run virtually with WOT all the time. Trust me. If you never actually saw this in person, it more closely resembled death wish racing and everyone involved seemed to be the first one that wanted to die in combat.

If I could post, I'd offer the pile up of cars at St.Jovite as evidence.  Death wish.



My small block runs the exact same set up as the 427/428's with the carbs backwards. It simply runs fantastic that way. It's WAY different then my Webers on my Pantera and each is definitely a treat to drive. They put you in a special place and the biggest problem is you can't find a big enough, wide open enough road to drive them like you want to AND you have to go home eventually.



Also on the tuning. You have to get the automatic choke set up operating correctly before you can set up the throttle linkage. If you don't, the choke will have you on too high of a setting.

Best to try to do this with (the choke) with the engine hot. That's where you set the choke to be wide open at full operating temperature. Then you set the choke on "index".

Now bear in mind, that you CAN NOT use the universal HOLLEY choke thermostat. You MUST use the FORD thermostat. These things are marked and they have different opening rates.

The original Ford Master Parts book showed that choke thermostat for the 2x4 FE setup. My SB is using it also. It takes a while for it to fully open. About the same amount of time to heat up the oil +/-.

IF you live in a southern climate of the US, then it probably doesn't matter too much which thermostat you use BUT here in NY, when it cools off, if you use the fast thermostat (HOLLEY being the fastest) you are going to stall the engine which will set off a syndrome of pumping the carbs to start a stalled engine and as a result, you WILL wind up fouling all of the plugs.

Again, trust me on this one, YOU DO NOT WANT TO FOUL up the plugs on the FE. If you were Elvis, you'd take your gun and shoot the car...and it would deserve it too! Mutha effin hore!



OH, almost forgot, it is IMPERATIVE to get the fuel level correct. IF you were an intellectual and did your dual diligence and went to HOLLEY to get the procedure, you're in trouble. You need to drop the level about 1/8" lower then the instructions will deliver you or else, YOU WILL FLOOD with gasoline to the level you will think you hear GOD and think he is telling you this is the "great flood" and you need to build an ARK.

It seems to be related to the ethanol in the fuel? Something about it expanding when warm?



So that's my story and I'm sticking to it. I hope you can find it of some redeeming value?

[Bigfoot]

I loved having dual quads on my CJ but opted for stock 735 cfm years later.
Acceleration a tad less but running much leaner and stock under the hood looks great!

[gt350hr]

 On a forward facing application ( small blocks) the primary carb should be in the rear as opposed to the big block's backward facing carbs and primary carb in the front. How you make them idle is your choice because there is more than one way.
   Randy

[1109RWHP]

It is interesting that this subject was brought up. I just got my 347 running on the stand with the Holman Moody 2x4 Boss 302 manifold and 600 Holley vacuum secondary carbs. It does not have progressive linkage. I noticed the idle mixture screws do not have much effect. It idles at about 1,000 rpm. The throttle plates do not seem like they are open too far but I did not take the carbs off and look to see how much the idle transfer slot is uncovered. Both carbs have the throttle plates open the same amount. I am not really using either one as a primary carb at this point.

[shelbydoug]

It is interesting that this subject was brought up. I just got my 347 running on the stand with the Holman Moody 2x4 Boss 302 manifold and 600 Holley vacuum secondary carbs. It does not have progressive linkage. I noticed the idle mixture screws do not have much effect. It idles at about 1,000 rpm. The throttle plates do not seem like they are open too far but I did not take the carbs off and look to see how much the idle transfer slot is uncovered. Both carbs have the throttle plates open the same amount. I am not really using either one as a primary carb at this point.

I need clarification on understanding this. Are you using Boss 302 heads or Windsor heads? Again, it would be great if we could post pictures in this thread to help illustrate what is going on.

I don't know what the Holman Moody Boss manifold looks like but I do know what the SHELBY version looks like.

Your idle rpm may be more related to the camshaft that you are running? What are the Holley carb list numbers you are using?

[gt350hr]

It is interesting that this subject was brought up. I just got my 347 running on the stand with the Holman Moody 2x4 Boss 302 manifold and 600 Holley vacuum secondary carbs. It does not have progressive linkage. I noticed the idle mixture screws do not have much effect. It idles at about 1,000 rpm. The throttle plates do not seem like they are open too far but I did not take the carbs off and look to see how much the idle transfer slot is uncovered. Both carbs have the throttle plates open the same amount. I am not really using either one as a primary carb at this point.

    Your idle characteristics are common for that intake/cylinder head combination and idle speed. Non progressive linkage ( in your case) is a plus. The only way to get back into the idle transfer slot is to drill a hole in the primary plates OR adjust the secondary stop to keep the blades open enough ( air wise)  so the primary can be closed some. My race car doesn't idle for long periods so I just set the idle speed and leave it at that..
   Randy

[1109RWHP]

It is a Windsor style block with Trick Flow 225cc aluminum 351 C style heads. I will have to look at the list# to see what they are.

[1109RWHP]

Here is a video of it I took with a potato phone.
https://www.youtube.com/watch?v=zb9bLhDpni4&feature=youtu.be

[shelbydoug]

I think that is the cam rather then the carbs. I have Webers on my Pantera with Ford A3 aluminum high port heads. The heads came off of a circle track race car as the guy was upgrading to the C302 versions. Those are 225+ ports so it is similar to your set up.

With the Webers though, it is set at about 825 but it does sound like a marching band with the 180 headers at idle with that cam. That's ok because the racket is behind me and is quiet in the cabin.

Those heads were shaved .030 by the original race engine builder (Danbury Competition) but the ports and pockets were left alone. They show 630 330@ 600, so why look for more?

I've got a lot of cam in it for a street car and it idles similarly to yours. Compcams solid 246 @ 50, 73 overlap, 606 lift.



I got tired of messing with the 1850 Holleys on the Shelby. Idle was too heavy. Almost like a double pumper.

What I did was take two 85-6 Mustang GT carbs and switched them over to look like 1850's. The benefit to that is that the idle is so clean, but that  hazes the ceramic coating on the headers because of the heat BUT it got rid of that, smarting your eyes at idle just about everything else did.

Those carbs are factory set to idle at 14.6 and they do but that is JUST idle.

I'm still screwing with them on the idle air bleeds to fatten that up a little so that I will still have some ceramic coating left on the headers. The dam things cost enough from Bittle and it took them two years to get them to fit the chassis right.

That engine idles right at 800 hot but it doesn't have as much cam in it. Compcams solid, 236 @ 50, 63 overlap, 528 @ 1.6, 561 @ 1.7.

The iron heads just crapped out so is in transition to AFR 185's. Idle will probably be a little different when that set up is finalized?



If those are the headers you are going to run, stainless with no coating, I'd suggest that you try them. They are worth the time to convert them.  I definitely have the 347 idling on the one carb.


As far as the linkage goes, it depends on you. On how you feel about it but for me the 427/8 backwards stile works for me particularly since it is using all of the Ford linkage. The Ford linkage is progressive and in reality runs on the front 2 barrels to about 3,000 rpm.

The progressive linkage definitely helps the feel.  With throttles just tied together all that means is you are running on the full 600 cfm all the time. That's just a torque killer.



Ironically, the issue I am having with the 2x4's is the single choke. If it doesn't fire up at one pump I have the tendency of holding open the throttle enough to cause the engine to backfire through the non-choked carb. That has made issues on more then one occasion.

I'm considering putting a choke on both carbs maybe with just the Holley choke thermostat on the secondary carb to make it open very fast?



Seems like everything is ALWAYS in development around here.

Just my perspective

[2112]

Those heads were shaved .030 by the original race engine builder (Danbury Competition) but the ports and pockets were left alone. They show 630 @ 600, so why look for more?

I hope this isn't suggesting 630 CFM @ .600" lift. That is Blue Thunder Big Block Thor head intake flow.

[shelbydoug]

Those heads were shaved .030 by the original race engine builder (Danbury Competition) but the ports and pockets were left alone. They show 630 @ 600, so why look for more?

I hope this isn't suggesting 630 CFM @ .600" lift. That is Blue Thunder Big Block Thor head intake flow.

It isn't suggesting anything. It is stating the facts from the flow bench. It's actually at .630 lift.

Iron 4v Cleveland heads with just a nice 3 angle valve job showed just under 300 (299) @ 600.

Look at the flow numbers out of the box on the current Windsor heads. The AFR 185's are 296 @ .550. They flow more at .200 then the stock iron heads do at 500. Amazing stuff but to give credit where credit is due, Ford Motorsports was making the Cleveland aluminum "high port heads" in 1981 and if I show you the "hot CHI" Aussie heads, they are just copies to a large extent.

The Windsor heads took a while to catch up.

[2112]

Those heads were shaved .030 by the original race engine builder (Danbury Competition) but the ports and pockets were left alone. They show 630 @ 600, so why look for more?

I hope this isn't suggesting 630 CFM @ .600" lift. That is Blue Thunder Big Block Thor head intake flow.

It isn't suggesting anything. It is stating the facts from the flow bench. It's actually at .630 lift.

Your flow bench is defective