1273 metering block questions
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1273 metering block questions
Hi yall I picked up two 1273 carbs for dual quad use on a 302sbf. My question is the metering blocks don't have the usual brass restrictions for the IFR. It's just an open hole. Is this how they came from the factory on this model? I ain't never handled these 1273 carbs before these 2 I bought so I can't tell. Also the metering block has additional passageways compared to the later blocks I familiar with can some one explain them for me? Last question the primary IFR air bleed is just an open hole, would make sense to balance the big IFR in the block but like I said first time I ever seen these 1273 carbs. I'm liking them gotta say. Thanks!
Re: 1273 metering block questions
The 1273 is the '57 Ford 312 carb. You must have some originals or relatively early manufactured replacement carbs because the 1273 metering block castings produced after the mid '60s the are the same layout as most common 1850, 3310, 4776 through 4781 double pumpers, etc. If the metering blocks you have are the castings with provision for additional passages those passages are not used in the 1273 or any other Holley I know of or have yet to see.
The original 1273 metering blocks are stamped 1584 primary and 1589 secondary.
The idle jets are located in the idle well below the brass cup plugs. Primary IJ is .028" and secondary is .031". Primary T-slot passage in the body has a .070" restriction pressed in the bottom of the casting.
The primary curb idle passage in the body has a.035: bleed orifice drilled horizontal into the bore. A bleed located downstream of the adjusting needle reduces idle fuel flow sensitivity to changes in intake vacuum and reduces the over-rich effect of high vacuum during deceleration.
The primary idle air bleed is a pressed in brass .076 in the outboard locations.
The main air bleed is the inboard location, near the bowl vent tube. The main bleed is open because there is only a single .026" emulsion bleed.
The 1273 is unique because it incorporates a .028" bleed between the top of the angle channel to the idle well above the idle jet. Air flow direction through that bleed depends on throttle position. The location of the lowest pressure (highest vacuum) in the carb moves from the edge of the butterfly up to the venturi as the throttle is progressively opened. At small throttle opening the flow through the bleed between the top of the angle channel (upstream end of the main nozzle through booster leg) and top of idle well is toward the idle circuit but at large throttle opening the flow reverses to flow toward the main nozzle when throttle opening is large enough the main nozzle has lower pressure (more vacuum = stronger signal) than the idle/transition circuit.
Original PMJ is #55, SMJ is #48. primary power valve 8.5, PVCR is .037", secondary power valve 10.5, PVCR is .052. Booster legs are .125" P and S. Angle channel is .125" primary and secondary is horizontal .125"
Original 1273 metering works pretty good on a T-Ram so start with that and tune accordingly from there. You'll excuse me now, I need to make another pot of coffee.
The original 1273 metering blocks are stamped 1584 primary and 1589 secondary.
The idle jets are located in the idle well below the brass cup plugs. Primary IJ is .028" and secondary is .031". Primary T-slot passage in the body has a .070" restriction pressed in the bottom of the casting.
The primary curb idle passage in the body has a.035: bleed orifice drilled horizontal into the bore. A bleed located downstream of the adjusting needle reduces idle fuel flow sensitivity to changes in intake vacuum and reduces the over-rich effect of high vacuum during deceleration.
The primary idle air bleed is a pressed in brass .076 in the outboard locations.
The main air bleed is the inboard location, near the bowl vent tube. The main bleed is open because there is only a single .026" emulsion bleed.
The 1273 is unique because it incorporates a .028" bleed between the top of the angle channel to the idle well above the idle jet. Air flow direction through that bleed depends on throttle position. The location of the lowest pressure (highest vacuum) in the carb moves from the edge of the butterfly up to the venturi as the throttle is progressively opened. At small throttle opening the flow through the bleed between the top of the angle channel (upstream end of the main nozzle through booster leg) and top of idle well is toward the idle circuit but at large throttle opening the flow reverses to flow toward the main nozzle when throttle opening is large enough the main nozzle has lower pressure (more vacuum = stronger signal) than the idle/transition circuit.
Original PMJ is #55, SMJ is #48. primary power valve 8.5, PVCR is .037", secondary power valve 10.5, PVCR is .052. Booster legs are .125" P and S. Angle channel is .125" primary and secondary is horizontal .125"
Original 1273 metering works pretty good on a T-Ram so start with that and tune accordingly from there. You'll excuse me now, I need to make another pot of coffee.
Re: 1273 metering block questions
Thank you kindly for that detailed and satiating reply Sir. They sure dont make em like they use too, the quality of workmanship and materials in these carbs is another level compared to the modern days carbs im familiar with. Yes, both carbs have those metering block numbers you mentioned and both sets of blocks have those passage ways. One is date coded 1956, the other seems like a service replacement but must be late 50's early 60's. (no date code i can find). That additional bleed you mentioned is that the same in fucntion as a modification i have been making on those metering plates that take jets? i have been adding a seperate idle jet and divorcing the two channels, with the only connection between the idle and main jet channel at the very top corner of the plate with a small .021 hole. does that make sense? because like you mentioned air flow direction will depend on which is getting the stronger signal.
Re: 1273 metering block questions
Sorry, not exactly sure what you mean by "metering plates that take jets." Is this it?
Can you post a pic of the other side and point out what you are doing?
Can you post a pic of the other side and point out what you are doing?
Re: 1273 metering block questions
Sorry Tuner Sir for the delay, I was unable to attend to posting the photos earlier. Here are the modifications I am referring to. The Idle circuit and main circuit are divorced except for the one .021 hole that is pictured in one of the attached photos. Is this the same in function as to what you refered to about 1273's?
Thank You Tuner
Thank You Tuner
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Re: 1273 metering block questions
I should add that in photo 2 beneath my thumb the idle jet i added is visible (6/32 thread), and i drilled a channel from the idle jet upwards connecting with the existing channel to form the new idle circuit. the drill mark is visible in photo number 2 above my thumb where the drill bit exited the body of the plate to connect with the pre existing passage. I used epoxy putty with copper wire to divorce the main jet channel form the idle channel. I hope this conveys what I have done.
Re: 1273 metering block questions
This queston has casued my mental flow bench to have some unsteady flow, so clifsnotes first, then the ponderment details.
The two obvious differences between your secondary plate with this added bleed arrangement and similarly located bleed in the primary of the 1273 is the much smaller secondary idle air bleed and the divorced idle jet. Less obvious is the effects of those details on pressure difference across the bleed.
The smaller SIAB results in much stronger signal (lower pressure) in the secondary idle circuit between the IJ and T-slot discharge than a primary idle circuit with a much larger PIAB. (.026” vs. .076” = 9 X larger area)
The idle jet divorced from the main circuit sees float bowl pressure which is always higher than the T-slot discharge signal so fuel will always flow through the idle jet toward the low pressure of the T-slot.
Because the divorced SIJ is not downstream of the main jet, the pressure difference across the SIJ is not affected by the main nozzle depression acting on the restriction of the SMJ as it does with the normal location of SIJ downstream of SMJ.
At WOT the main nozzle depression acting in the main well downstream of the main jet is the lowest pressure in the carb. In the common Holley arrangement, like that of the 1273 primary (and nearly all other carbs) with the idle jet located downstream of the main jet, the idle circuit reverses at WOT when venturi booster nozzle vacuum becomes stronger than the vacuum acting on the T-slot and air from the large primary IAB flows through the IJ into the main well and then the IJ becomes a main well air bleed which has significant correction effect on WOT high airflow A/F.
In the 1273 primary with the bleed between the main nozzle and idle well, that bleed flows air in either direction depending which has strongest signal, toward idle well at small throttle opening and toward main nozzle at large throttle opening and WOT.
In your secondary plate divorced IJ and additional bleed arrangement, because the bowl pressure remains nearly constant and is always higher than T-slot signal, at WOT fuel flow through SIJ will continue toward the T-slot, instead of reverse through the idle jet at WOT, as in the case of common circuit configuration like the normal Holley secondary metering plate and primary metering blocks.
Because at WOT the main nozzle (main well) pressure is lowest in the carb the question then becomes what will flow through the additional .021” bleed in your jet plate, air or fuel?
The idle circuit signal, even though less than venturi signal, will be more than the 1/2” or so needed lift fuel from float level to the height of the top of the circuit. Because the SIAB is so near the size of the .021” restriction you drilled, I think the flow through the additional bleed between idle and main wells will be fuel or possibly a mix of fuel and air, depending on the ratio of the bleed areas, not just plain air from the bleed.
Without testing I don't see how to confirm that. I guess if it makes the carb do what your want it to do, so be it.
What is the reason you did this and what is the result? Have you looked at this with a WBO2? Divorcing the idle jet in the primary of a gasoline carb nearly always results in an extremely rich area in the throttle position where T-slot and main fuel flow overlap, but I suppose in the secondary at mid power level air flow that may not be necessarily so or cause drivability problems like it does in the primary.
P.S. I about made myself delirious proofreading this, so if anybody sees a need for correction be my guest.
The two obvious differences between your secondary plate with this added bleed arrangement and similarly located bleed in the primary of the 1273 is the much smaller secondary idle air bleed and the divorced idle jet. Less obvious is the effects of those details on pressure difference across the bleed.
The smaller SIAB results in much stronger signal (lower pressure) in the secondary idle circuit between the IJ and T-slot discharge than a primary idle circuit with a much larger PIAB. (.026” vs. .076” = 9 X larger area)
The idle jet divorced from the main circuit sees float bowl pressure which is always higher than the T-slot discharge signal so fuel will always flow through the idle jet toward the low pressure of the T-slot.
Because the divorced SIJ is not downstream of the main jet, the pressure difference across the SIJ is not affected by the main nozzle depression acting on the restriction of the SMJ as it does with the normal location of SIJ downstream of SMJ.
At WOT the main nozzle depression acting in the main well downstream of the main jet is the lowest pressure in the carb. In the common Holley arrangement, like that of the 1273 primary (and nearly all other carbs) with the idle jet located downstream of the main jet, the idle circuit reverses at WOT when venturi booster nozzle vacuum becomes stronger than the vacuum acting on the T-slot and air from the large primary IAB flows through the IJ into the main well and then the IJ becomes a main well air bleed which has significant correction effect on WOT high airflow A/F.
In the 1273 primary with the bleed between the main nozzle and idle well, that bleed flows air in either direction depending which has strongest signal, toward idle well at small throttle opening and toward main nozzle at large throttle opening and WOT.
In your secondary plate divorced IJ and additional bleed arrangement, because the bowl pressure remains nearly constant and is always higher than T-slot signal, at WOT fuel flow through SIJ will continue toward the T-slot, instead of reverse through the idle jet at WOT, as in the case of common circuit configuration like the normal Holley secondary metering plate and primary metering blocks.
Because at WOT the main nozzle (main well) pressure is lowest in the carb the question then becomes what will flow through the additional .021” bleed in your jet plate, air or fuel?
The idle circuit signal, even though less than venturi signal, will be more than the 1/2” or so needed lift fuel from float level to the height of the top of the circuit. Because the SIAB is so near the size of the .021” restriction you drilled, I think the flow through the additional bleed between idle and main wells will be fuel or possibly a mix of fuel and air, depending on the ratio of the bleed areas, not just plain air from the bleed.
Without testing I don't see how to confirm that. I guess if it makes the carb do what your want it to do, so be it.
What is the reason you did this and what is the result? Have you looked at this with a WBO2? Divorcing the idle jet in the primary of a gasoline carb nearly always results in an extremely rich area in the throttle position where T-slot and main fuel flow overlap, but I suppose in the secondary at mid power level air flow that may not be necessarily so or cause drivability problems like it does in the primary.
P.S. I about made myself delirious proofreading this, so if anybody sees a need for correction be my guest.
Re: 1273 metering block questions
Thank you Tuner for taking the time to reply to my question with such a detailed response. I am glad to see that my thoughts on what is happening in that circuit are along the same lines as yours and your understanding as helped me greatly in consolidating my understanding.
I modified that metering plate while I was tuning a Holley List 3916 to my engine ( 302 sbf). My aims were to make fuel flow through the secondary more stable, continual and constant throughout its range and to increase responsiveness of the idle and main jet flows to changes in throttle position. So I wanted the idle jet to drink from the bowl directly like the main jet, but I didnt want it to just be always flowing like truly divorced idle circuits with no communication/transition between the main jet circuits. So thats what led me to the modifications. I dont have a WBO2 yet, it is high on my shopping list though so soon I will. All I can tell you for whatever its worth is that I was happy with my old girl when we went for a drive with the modified metering plate
I had 87 jets in the front metering block no PV and 88 jets in the rear . This was with an air gap dual plane, and solid lifter cam from the k code mustangs of 65 and 66.
My thinking about what was/is happening is:
At initial secondary opening fuel flows through the idle circuit to the T slot, and fuel is lifted to the top of the main jet circuit. When the secondary throttle is at the point of max flow from the T slot without the boosters engaged, the idle circuit is probably seeing enough pressure differential to pull some fuel from the MJ circuit and some air from the main jet bleed flow through the .021 hole connecting the two circuits.
As the throttle is further opened to the point whereby the T slot and main nozzle are seeing the same pressure differential both circuits will be flowing with no crossover of air or fuel from either circuit (boosters and Tslot giving fuel)
When the pressure differential seen by the main nozzle is greater than that seen by the T-slot, fuel stops flowing from the T-slot, and as the pressure differential at the main nozzle becomes greater and greater as the throttle approaches WOT, the main jet circuit will eventually begin to pull fuel from the idle jet and air from the T slot and idle bleeds through the 0.021 hole
I would very much like to confirm if this is what is happening, or if my understanding of these principles is correct, thats why I asked about it when you mentioned the 1273 extra bleed that I didnt know about. it sounded similar to what i was trying to achieve on that metering plate.
Tuner why did Holley not put that bleed arrangement(from the 1273) in other carbs?
I modified that metering plate while I was tuning a Holley List 3916 to my engine ( 302 sbf). My aims were to make fuel flow through the secondary more stable, continual and constant throughout its range and to increase responsiveness of the idle and main jet flows to changes in throttle position. So I wanted the idle jet to drink from the bowl directly like the main jet, but I didnt want it to just be always flowing like truly divorced idle circuits with no communication/transition between the main jet circuits. So thats what led me to the modifications. I dont have a WBO2 yet, it is high on my shopping list though so soon I will. All I can tell you for whatever its worth is that I was happy with my old girl when we went for a drive with the modified metering plate
I had 87 jets in the front metering block no PV and 88 jets in the rear . This was with an air gap dual plane, and solid lifter cam from the k code mustangs of 65 and 66.My thinking about what was/is happening is:
At initial secondary opening fuel flows through the idle circuit to the T slot, and fuel is lifted to the top of the main jet circuit. When the secondary throttle is at the point of max flow from the T slot without the boosters engaged, the idle circuit is probably seeing enough pressure differential to pull some fuel from the MJ circuit and some air from the main jet bleed flow through the .021 hole connecting the two circuits.
As the throttle is further opened to the point whereby the T slot and main nozzle are seeing the same pressure differential both circuits will be flowing with no crossover of air or fuel from either circuit (boosters and Tslot giving fuel)
When the pressure differential seen by the main nozzle is greater than that seen by the T-slot, fuel stops flowing from the T-slot, and as the pressure differential at the main nozzle becomes greater and greater as the throttle approaches WOT, the main jet circuit will eventually begin to pull fuel from the idle jet and air from the T slot and idle bleeds through the 0.021 hole
I would very much like to confirm if this is what is happening, or if my understanding of these principles is correct, thats why I asked about it when you mentioned the 1273 extra bleed that I didnt know about. it sounded similar to what i was trying to achieve on that metering plate.
Tuner why did Holley not put that bleed arrangement(from the 1273) in other carbs?
Re: 1273 metering block questions
I confess this made me chuckle and think this is an application where it might make sense to divorce the SIJ. Well played.I modified that metering plate while I was tuning a Holley List 3916 to my engine ( 302 sbf).
If you have a primary metering block with a main well emulsion tube, such as from an early 1850, you might give that a try. Ideal would be one from a 3418 because the PVCR is close to correct for the venturi size so you could use a (high flow) PV and economy MJ, allthough the IJ may be somewhat generous for the 302. With an 1850 carb block you could walk up on the PVCR size. The 3418 PVCR is .093" like the original 3085 Mopar carb. The block in the 3916 (stamped 5615) has (or had, if it hasn't been modified) .040" PVCR.
Because they had the example of the 3085 to crib from, I don't understand how the 3916 could have been "engineered on Edelbrock's dyno" <--(was what the magazines said) and come up with such a small PVCR. Lucky for me, having previous experience with the 3085 gave me a big head start on curing the huge lean problem that came no extra charge in the 3916 which was initially marketed only through Edelbrock.
Several people put their money down and preordered them and when they finally showed up after several "next week" delays from Edelbrock they either had a massive bog or some wouldn't run at WOT at all without using a huge plug fouling PMJ. 1968 was a fun summer when a .093" drill bit was a miracle maker.
It was used in some other carbs too, mostly 2bbls for Y-Block 292s. I have seen it in other late 50s applications but I don't recall specifically which, Ramblers, IHC, other Ford engines and Ford big truck engines likely. Only 50-55 years ago, seems like just yesterday.Tuner why did Holley not put that bleed arrangement(from the 1273) in other carbs?
Re: 1273 metering block questions
I dig, 50 to 55 years ago and still a day doesnt go by you that you aint thinking about it. I should add thank you for helping me to tune the 3916, because It was through reading all the posts you made on here about them that got me headed in the right direction. Yes my metering block had the unmolested 0.039 hole to begin with.Only 50-55 years ago, seems like just yesterday.
That carb actually turned out to be highly street-able even with no front PV I was able to get groceries and sit at lights for hours on end with no plug fouling , and still get the response when i put my foot down. Combustion had a different sound too with that carb on, a slower, real deep boomy rumble note. The layout of the carb and booster type/arrangement/position really changes how air and fuel flow through the engine and the manner in which they interact, even at idle when the plates are closed and you would think it wouldn't have much impact . Ive been making some radical alterations to boosters and this phenomenon has become even more pronounced.
I dunno what is with PVs, Ive wanted to love them and repeatedly given a relationship with them another shot, but it always ends with disagreement in some form or another. No PV in the front of any of my carbs on the other-hand always has worked out. Never fouled a set of plugs with no front PV ever, dunno why my experience is contrary to the norm?.