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Multi carbs

Updated: Nov 3, 2023

I get asked how to do initial multicarb setup. Figured this is as good a time as any (2am), so here goes. It has been mentioned before that I often write these techs that are too long. Honestly, if spending 20 minutes reading this is too hard for you…. Well, I’m not sorry.

-Calibration change: Typically if using original factory 2x4 carbs, they are good to go as is and should need no changes. If any are required it will be minimal, like making idle slightly richer so the engine runs well with a large camshaft. With generic carburetors like 600s or 750s the changes are slightly more complicated. First to understand, signal and airflow is now split to 8 venturi and throttle plates, so yes you CAN run them as is, but to get closer to a good tune, two changes need to be made Richer idle and reduce WOT fuel. With a generic 600 idle feed is .026 and idle air bleed is .070, this often runs well as a single carb on a tame engine. It will be too lean for any decent camshaft when used 2x4. I would increase ifr to .028 and reduce idle air bleed to .063. If air bleed and idle feed relationship is correct, you should be very close to one turn out on the mixture screw when hot. This is true on 2x4 and 3x2 setups. Power valve channels also need to be measured. These are the tiny holes behind the power valve itself. A stock 600cfm will have a PVCR ranging in size from .046-.050, this is fine for single usage but when running two carbs, reduction to .036-.040 is ideal. Same goes with 715s or 750s. Original PVCR on a single 715 is .063 range. With factory 2x4 715s the pvcr is .028, so massive change. I would start at .028 and do tests to determine WOT A/F, drill pvcr to size. Failure to make these changes… well engine may run ok, but symptoms are like this: Engine idles but likes to shudder or seem weak when put in gear, or low speed issues. This is due to lean condition. Often A/F in the 15-16 range with a load in gear. Ideally you need 12.5:1 no load so when placed in gear engine will rise into the 13s. This is NOT a hard fast exact number, all engines and all cam designs have different requirements, as do O2 locations and whatnot. So you have fixed idle, nice and rich, idles great hot, transitions fine, cruises with no issues. Next issue with stock generics being used 2x4, WOT fuel. In the example of 715s, with the stock pvcr, wot often will be richer than 9:1, misfire territory. Reduction is needed. Also be aware that once you get over 715cfm, many modern carbs do not have a Vacuum signal amplifier to the secondary housing. Without one it may be impossible to get the secondaries to open. When in doubt, add one. It is easy. Again, not trying to seem condescending, but if measuring a hole or drilling an insert is too difficult, maybe let someone else do this. My mentality is if you are willing to spend $1,000+ on a set of carbs, spending $100 on drills, taps, inserts, and pin gauges should not be an issue. -Float height and fuel pressure First, I need to make my view clear here. Many people are under the impression that running high fuel pressure results in a greater amount of fuel available. This is incorrect. Here is why: By running high fuel pressure, you need to lower float level to maintain fuel height in the bowl. Go full throttle, fuel gets used and the bowls need to be refilled. If the float is already set very low, very little drop can occur when it comes to refilling the bowls. A better method (which I practice) is to run lower fuel pressure. This allows you to run the floats nearly to the top of the bowl, when full throttle occurs and fuel needs to refill the bowls, the float can drop a greater amount. This allows more of the needle and seat to open, which actually lets in a much greater amount of fuel. I like to set at 5psi, which is the original spec for most Holley carbs. With factory Ford multicarb setups, the carburetors mount backwards, which makes float adjustment via the sight plugs somewhat trickier. My experience has been that the primary bowl should run at the bottom of the sight plug, and the secondary bowls should be just below it. This is true for 2x4 and 3x2 setups. With 3x2, consider the center carb as primary bowl and treat the outboards as secondary bowls. Also of note on factory 3x2 setups, the outboard carb sight plugs are DRASTICALLY lower than the primary carb for a reason. Pay mind to it. Idle speed and Transfer Slot settings. I’m sure with 4v you know the rule of thumb, set primary slot .020 exposure and set idle speed with the secondaries. With 2x4s, you do kinda the same, but not exactly. First understand that area is exponential, so setting two carbs with .010 slot exposure is NOT really half of .020. Here is how I do it: Set both primary throttle bores to JUST cover the transfer slot. You want any touch of the throttle to expose it, but do not leave them open at all. Open secondary throttle bores on both carbs to reveal transfer slot, now close them to also just cover the slot. It is super critical to not have any secondary slot exposed. Almost all 4v carbs have a small .024-.026 leakhole below the secondary t slot, this keeps that circuit active and fuel moving. This helps not cause a lean bog when they do open. As such it isn’t as critical to have these set at any fixed value (compared to primary bores) This results in a 900rpm idle speed on most engines. If you require less rpm, shut the secondaries more. If you require more idle rpm, open the primary carb, primary throttle bore only. This is for progressive throttle action. The reason behind this is simple, with a typical performance camshaft, you will have 6-10inches of vacuum at idle. So that is what is drawing the fuel (technically pressure differential, but I digress). Put engine in gear and start to slowly accelerate, vacuum rises with rpm. With progressive linkage, vacuum can exceed 16in prior to secondary carb opening. If the secondary carb’s primary transfer slot is exposed, it will pull a considerable amount of fuel at this low speed cruise. With 1:1 linkage setup, this is not nearly as critical since both primary throttle bores will open at the same time. It is important to maintain these relationships as the increasing airflow needs to stay matched to the increased fuel flow and requirements. With 3x2 setups, again I think of the outboard carbs much like the secondaries of a 4v. As such, set t slot fully hidden on the outboards. Center carb will often require a good deal of t slot exposure to maintain a reasonable idle. On Ford factory 3x2 carbs, all 6 throttle bores have a leak slot constantly bleeding a small amount of fuel, so all circuits are active. Articles have been written in the past to close the mixture screws on the outboards. I have seen no benefit to this and only have seen driving issues caused.

With 3x2s I often run the outboard mixture screws more closed than the primary. The progressive nature of the linkage allows for cruising on just the center carb.

For 2x4 this is less critical and it is common to run all 4 mixture screws the same amount out. Much like 2x4 and 4v, I find one turn out on mixture screws to be pretty close to ideal. As with other setups, 1/8 turn one way or another may be required. I have never subscribed to adjust each screw individually. I prefer to adjust all a small amount and not overall changes. From personal experience, I will mention that some setups are touchier than others. 715 2x4s and all 3x2 setups seem fairly touchy to idle mixture screw settings. What I mean is, 1/8 of a turn has a far greater impact with these vs dual 600s or 750s. All of the above assumes even throttle plate settings. I cannot stress this enough. Both primary throttle plates need to be on the same plane and expose exactly as much t slot at the exact same position. Same with the secondaries but to a lesser extent. Take the time and get them all perfect by any means available. Far too often even from the factory a carb will have .020 more exposure on one slot than the other, this is inexcusable and impossible to tune around. Even more so with a factory dual plane intake where have the cylinders will be lean and half will be rich. Get it right the first time. Last mention is for 2x4 setups. Vacuum secondary spring selection is difficult at best. When I build carbs it is a “best guess” scenario. Again with 2 carbs, each carb is seeing half the airflow and so the VS diaphragm sees half the airflow. Easiest way for you to set at home is to start with too light of a spring setting. Look for the bog when they open and install the next stiffer size. Repeat until bog is eliminated. If worried about tearing the diaphragm, get two quick change housing and find the right spring tension that way. Once it is known, reinstall the original housing with the correct spring. I wish I could get this right, but without a customer car, and extensive tuning, it is literally impossible for me to get this perfect, at best I can get close based on experience. Well, I hope this was helpful. Feel free to share, or save this post for later usage, I would appreciate credit for this post tho, as it technically took four years to write. Good luck and happy motoring Drew Pojedinec AirFuelSpark employee of the month

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