Who's running 2x4 Holley BC/BD Carbs?

[jgkurz]

Hi All, I am planning to use my Holley BC/BD carbs on a new build. The carbs are in amazing shape considering their age. My intake is the original MR 2x4 aluminum intake. I'd appreciate collaborating on your experience (good and bad). Did you have to do anything special to get them to run well on the street? Obviously primary jetting, power valves, secondary springs, and idle mixture all have to be selected properly but is there more to consider?

Thank you, John

[jgkurz]

Please reply if you are running the BC/BD carbs.

[RJP]

Hi All, I am planning to use my Holley BC/BD carbs on a new build. The carbs are in amazing shape considering their age. My intake is the original MR 2x4 aluminum intake. I'd appreciate collaborating on your experience (good and bad). Did you have to do anything special to get them to run well on the street? Obviously primary jetting, power valves, secondary springs, and idle mixture all have to be selected properly but is there more to consider?

Thank you, John

I run the BC/BD carbs on my 67 Fairlane, 427/C6 on a orig. C7 Med Riser manifold. I recently rebuilt the carbs, milled the metering block surfaces of the main bodies and trued up the throttle body surfaces as they usually warp with years of heat cycles. Use orig. Holley kits as 'white box' kits usually have incorrect needle/seat assembles and unidentified power valves. Start with the factory jetting and work from there. Off hand I don't remember exactly what jets I used but IIRC they were a step or 2 richer on the primary and the sec. metering plates I used were 3-4 sizes larger than stock. One other thing I did was I drilled out the secondary bowl plugs, tapped them for a fuel fitting making each carb dual inlet. I believe these carbs are too big to rely on the small transfer tube to supply enough fuel to fully feed the secondaries at WOT, esp. if the secondaries are jetted up. I made a 4 outlet fuel log to supply the 4 fuel bowls. For adequate fuel to the carbs I run a 1/2" line from the fuel tank including a 1/2" tank pick up and a 120 gal. Holley mechanical fuel pump with 1/2" I/D fittings.   

[jgkurz]

I run the BC/BD carbs on my 67 Fairlane, 427/C6 on a orig. C7 Med Riser manifold. I recently rebuilt the carbs, milled the metering block surfaces of the main bodies and trued up the throttle body surfaces as they usually warp with years of heat cycles. Use orig. Holley kits as 'white box' kits usually have incorrect needle/seat assembles and unidentified power valves. Start with the factory jetting and work from there. Off hand I don't remember exactly what jets I used but IIRC they were a step or 2 richer on the primary and the sec. metering plates I used were 3-4 sizes larger than stock. One other thing I did was I drilled out the secondary bowl plugs, tapped them for a fuel fitting making each carb dual inlet. I believe these carbs are too big to rely on the small transfer tube to supply enough fuel to fully feed the secondaries at WOT, esp. if the secondaries are jetted up. I made a 4 outlet fuel log to supply the 4 fuel bowls. For adequate fuel to the carbs I run a 1/2" line from the fuel tank including a 1/2" tank pick up and a 120 gal. Holley mechanical fuel pump with 1/2" I/D fittings.

Thanks for the great reply. Your information is not what I expected. From the few posts I found on various forums, most had a problem with being too rich. Your setup benefited from more fuel. Regarding mods, have you ever considered trying some of the new style metering blocks from Quick Fuel or Proform?

[Falcon67]

If you change metering blocks, use Quick Fuel parts - my experience using both brands is that QF is way better even though they look identical except for the color of the anodizing.  QF sells good parts. 


Every project is going to be different.  The best way to jet your carbs would be to use a wide band O2 sensor and some software, like Innovate sells. When I did my tunnel ram project on the 302, i had to jet the 600 CFM (#1406) Edelbrock carbs up 4% primary and secondary on both units.  "Conventional wisdom" said that's too much carb on a 302, gonna be way too fat.  No so in real life.  But - always start at the factory settings on everything.  If your build is close to stock or a bit more, that will be close from the get-go.  Holley jets are usually 3% change but the tolerance in the regular jets is such that you might get 3 or a bit more than zero, so normally we change 2 sizes in any direction.  IF needed - build it and road test, then go from there. 

[RJP]

I have no reason to change metering blocks as the orig. Holleys work fine. IMHO there is no need to look for a problem that doesn't exist. But with that said I have found that late model Holley metering blocks can have problems in the emulsion part of the main circuit but early, say 60s thru mid 80s seem to work fine as they have a more precise fuel curve and respond accordingly. I have cured some idle/main transition problems by using a early metering block and matching the IFR and PVCR sizes to the orig. carb's metering block. Seemed the problems with the fuel curve was more associated with Street Avenger type late style carbs. If I needed a metering block I don't already have or anything that had anything to do with precise fuel control Quick Fuel would be my 1st choice.  As far as the BC/BD carbs being too rich that can be something as simple as a warped main body's met. block surface and the gasket is not compressing evenly allowing cross channeling or leaking into the power valve vacuum chamber. I have also found OEM carbs have more precision circuitry than over the counter replacement type carbs as these carbs are subject to emission control and testing, they have to be correct. For example a 780 cfm 3310-1 4150 is the correct OEM carb for a 375 hp/396 Chevy whereas a 3310-2 thru 10 is a over the counter replacement 750 cfm straight leg booster 4160 [no sec. metering block] The 3310-1 is a excellent carb to work with, the others? not so much. I cringe any time some one says they are dumping their OEM BC/BD or BJ/BK carbs for a cheap pair of 1850s from Summit or Jegs just because they can't get the orig. carbs right. If yer gonna toss the old ones...toss 'em my way, I'll be happy to take them off your hands at no charge to you

[jgkurz]

I have no reason to change metering blocks as the orig. Holleys work fine. IMHO there is no need to look for a problem that doesn't exist. But with that said I have found that late model Holley metering blocks can have problems in the emulsion part of the main circuit but early, say 60s thru mid 80s seem to work fine as they have a more precise fuel curve and respond accordingly. I have cured some idle/main transition problems by using a early metering block and matching the IFR and PVCR sizes to the orig. carb's metering block. Seemed the problems with the fuel curve was more associated with Street Avenger type late style carbs. If I needed a metering block I don't already have or anything that had anything to do with precise fuel control Quick Fuel would be my 1st choice.  As far as the BC/BD carbs being too rich that can be something as simple as a warped main body's met. block surface and the gasket is not compressing evenly allowing cross channeling or leaking into the power valve vacuum chamber. I have also found OEM carbs have more precision circuitry than over the counter replacement type carbs as these carbs are subject to emission control and testing, they have to be correct. For example a 780 cfm 3310-1 4150 is the correct OEM carb for a 375 hp/396 Chevy whereas a 3310-2 thru 10 is a over the counter replacement 750 cfm straight leg booster 4160 [no sec. metering block] The 3310-1 is a excellent carb to work with, the others? not so much. I cringe any time some one says they are dumping their OEM BC/BD or BJ/BK carbs for a cheap pair of 1850s from Summit or Jegs just because they can't get the orig. carbs right. If yer gonna toss the old ones...toss 'em my way, I'll be happy to take them off your hands at no charge to you

Thanks again for reply RJP. You obviously have a ton of experience. At this point, I have no intention of running different carbs. You are not the first to tell that my OEM Holley carbs are better than many aftermarket carbs. The one concern I have is that one of my metering blocks is not the correct 4731. I had a shop in New York make a replacement clone for me but I am skeptical it will work as well as the OEM. I guess all I can do is try them out and go from there. Dyno time is expensive so I'd like to get all this figured out ahead of time.

[RJP]

My only concern about your cloned metering block is the core used will have the same emulsion configuration as the orig. metering block. The other variables is just the IFR and the PVCR and is easily matched to the orig.