12a streetport holley tune questions
12a streetport holley tune questions
so I've been running a racing beat intake and carb setup (carb from racing beat) and the stock setup had 50 main jets and a #8 secondary plate. I've done a lot of looking around about General setup but it seems limited on info. I installed a wide band but am looking for knowledge on best path of getting it better dialed in..... set the float bowls, bumped the primary jets to 52 since the car stuttered on accelerating and got much better but the idle seemed to get different but I was surprised since I thought the jets shouldn't effect the idle speed or afr at idle which got richer just at idle? Do I need to adjust the idle screws every time I bump or drop the primary jets? Thanks!
Last edited by risingsunroof82; Apr 26, 2021 at 04:48 PM.
Old [Sch|F]ool
Joined: May 2001
Posts: 12,872
Likes: 574
From: Cleveland, Ohio, USA
Tuning Holleys is a lot of "fun" because a lot of the things that you need to tune, are not adjustable. You can swap main jets, but that is it, things like air bleeds and the whole of the idle circuits (secondary has idle circuits too, ensures the fuel in that bowl doesn't go stale) can only be adjusted by drilling and hoping you don't go too far.
That is why, say, Holley made about a dozen or two different 600CFM carbs, all of those things are calibrated for different applications.
This is also where Racing Beat carbs are worth it.
Adjusting the idle screws will adjust the IDLE mixture but it will not adjust the LOW LOAD mixture. You're running on the idle circuit under a lot of light load conditions up to some cruise conditions, this is where you get to drillin' and fillin' to adjust things to make a smooth transition to the main circuit. I am 90% certain that the idle circuit does get its fuel feed from the main fuel well, which is fed from the bowl by the main jet.... IN THEORY the idle shouldn't be affected by the mains, BUT a #50 jet is about as small as you can get for a Holley, so it may provide enough of a restriction to affect idle. To directly answer your question.
Do you have a street ported engine? Most of when someone has to muck around with the idle restrictors (can't call them jets, because they aren't replaceable unless you buy a $1200 carb) is when the idle and low load engine vacuum are different than what the carb was calibrated for.
That is why, say, Holley made about a dozen or two different 600CFM carbs, all of those things are calibrated for different applications.
This is also where Racing Beat carbs are worth it.
Adjusting the idle screws will adjust the IDLE mixture but it will not adjust the LOW LOAD mixture. You're running on the idle circuit under a lot of light load conditions up to some cruise conditions, this is where you get to drillin' and fillin' to adjust things to make a smooth transition to the main circuit. I am 90% certain that the idle circuit does get its fuel feed from the main fuel well, which is fed from the bowl by the main jet.... IN THEORY the idle shouldn't be affected by the mains, BUT a #50 jet is about as small as you can get for a Holley, so it may provide enough of a restriction to affect idle. To directly answer your question.
Do you have a street ported engine? Most of when someone has to muck around with the idle restrictors (can't call them jets, because they aren't replaceable unless you buy a $1200 carb) is when the idle and low load engine vacuum are different than what the carb was calibrated for.
Last edited by peejay; Apr 26, 2021 at 07:49 PM.
Tuning Holleys is a lot of "fun" because a lot of the things that you need to tune, are not adjustable. You can swap main jets, but that is it, things like air bleeds and the whole of the idle circuits (secondary has idle circuits too, ensures the fuel in that bowl doesn't go stale) can only be adjusted by drilling and hoping you don't go too far.
That is why, say, Holley made about a dozen or two different 600CFM carbs, all of those things are calibrated for different applications.
This is also where Racing Beat carbs are worth it.
Adjusting the idle screws will adjust the IDLE mixture but it will not adjust the LOW LOAD mixture. You're running on the idle circuit under a lot of light load conditions up to some cruise conditions, this is where you get to drillin' and fillin' to adjust things to make a smooth transition to the main circuit. I am 90% certain that the idle circuit does get its fuel feed from the main fuel well, which is fed from the bowl by the main jet.... IN THEORY the idle shouldn't be affected by the mains, BUT a #50 jet is about as small as you can get for a Holley, so it may provide enough of a restriction to affect idle. To directly answer your question.
Do you have a street ported engine? Most of when someone has to muck around with the idle restrictors (can't call them jets, because they aren't replaceable unless you buy a $1200 carb) is when the idle and low load engine vacuum are different than what the carb was calibrated for.
That is why, say, Holley made about a dozen or two different 600CFM carbs, all of those things are calibrated for different applications.
This is also where Racing Beat carbs are worth it.
Adjusting the idle screws will adjust the IDLE mixture but it will not adjust the LOW LOAD mixture. You're running on the idle circuit under a lot of light load conditions up to some cruise conditions, this is where you get to drillin' and fillin' to adjust things to make a smooth transition to the main circuit. I am 90% certain that the idle circuit does get its fuel feed from the main fuel well, which is fed from the bowl by the main jet.... IN THEORY the idle shouldn't be affected by the mains, BUT a #50 jet is about as small as you can get for a Holley, so it may provide enough of a restriction to affect idle. To directly answer your question.
Do you have a street ported engine? Most of when someone has to muck around with the idle restrictors (can't call them jets, because they aren't replaceable unless you buy a $1200 carb) is when the idle and low load engine vacuum are different than what the carb was calibrated for.
Tuning Holleys is a lot of "fun" because a lot of the things that you need to tune, are not adjustable. You can swap main jets, but that is it, things like air bleeds and the whole of the idle circuits (secondary has idle circuits too, ensures the fuel in that bowl doesn't go stale) can only be adjusted by drilling and hoping you don't go too far.
That is why, say, Holley made about a dozen or two different 600CFM carbs, all of those things are calibrated for different applications.
This is also where Racing Beat carbs are worth it.
Adjusting the idle screws will adjust the IDLE mixture but it will not adjust the LOW LOAD mixture. You're running on the idle circuit under a lot of light load conditions up to some cruise conditions, this is where you get to drillin' and fillin' to adjust things to make a smooth transition to the main circuit. I am 90% certain that the idle circuit does get its fuel feed from the main fuel well, which is fed from the bowl by the main jet.... IN THEORY the idle shouldn't be affected by the mains, BUT a #50 jet is about as small as you can get for a Holley, so it may provide enough of a restriction to affect idle. To directly answer your question.
Do you have a street ported engine? Most of when someone has to muck around with the idle restrictors (can't call them jets, because they aren't replaceable unless you buy a $1200 carb) is when the idle and low load engine vacuum are different than what the carb was calibrated for.
That is why, say, Holley made about a dozen or two different 600CFM carbs, all of those things are calibrated for different applications.
This is also where Racing Beat carbs are worth it.
Adjusting the idle screws will adjust the IDLE mixture but it will not adjust the LOW LOAD mixture. You're running on the idle circuit under a lot of light load conditions up to some cruise conditions, this is where you get to drillin' and fillin' to adjust things to make a smooth transition to the main circuit. I am 90% certain that the idle circuit does get its fuel feed from the main fuel well, which is fed from the bowl by the main jet.... IN THEORY the idle shouldn't be affected by the mains, BUT a #50 jet is about as small as you can get for a Holley, so it may provide enough of a restriction to affect idle. To directly answer your question.
Do you have a street ported engine? Most of when someone has to muck around with the idle restrictors (can't call them jets, because they aren't replaceable unless you buy a $1200 carb) is when the idle and low load engine vacuum are different than what the carb was calibrated for.
After some tinkering and adjusting heres where its sitting...... after driving the idle didn't want to stabilize for a few seconds and is probably due to the holley mighty mite fuel pump i just bought and installed..... may not be keeping up.
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I'm right behind you. I tore my suspension apart and then couldn't find the time to rebuild it, for the past year. But work is easing up and the days are long, so, it's coming together quick.
Do you have a writeup on that exhaust?
Do you have a writeup on that exhaust?
It is a racingbeat header, into a racingbeat presilencer, 2.5 stainless all welded with a high flow cat (which is even more high flow since I've found hunks of cat comb in the driveway) lol and the difference for mine is more than likely the aero turbine muffler on the back side. Haha hope that helps!
I've been still tuning in and it had bad breaking up in the 5k+ range so I grabbed my holley carb for a bridgeport and made a Frankenstein holley carb between it and a stock port racing beat holley carb..... and here's the outcome so far.
I ran this setup on my 83 about 10 years ago, it had a freshly rebuilt streetported 12a at the time. Unsure of porting size/timing.
I had two different 465 holley (1848) one definitely from Racing Beat. Both had a rather large hole between the primary and secondary circuit. I tried, like you, to fatten up the primary circuit (49 to 51/52) and swap out the accelerator pump spring and cam to try and bridge that hole and get it to smooth out.Make sure yours does not have a power valve; our vacuum signal is not strong enough to get that to work correctly. If you can get that to work, good on you. The one from RB had the power valve deleted for this reason. The thing I considered doing was spacing out the accelerator diaphragm to increase the fluid volume available to aid the transition as the primary circuit leaned out before the vacuum secondaries were able to engage. I had a 3 wire, narrow band, 7 light O2 sensor, it was simple, but it worked. Also considered swapping over to mechanical secondaries to control the timing of the transition directly. A combination of both of those things is probably the way to get better control of the primary to secondary hole.
On the dyno at KDR, I was able to get power pulls done and ended up swapping back to 49 on the main jet. I can dig up the dyno graphs, peak power was 141 rwhp at 6500 or 7000 rpm. After that, the hole was still there, I just learned to live with it and started to change my accelerator pedal behavior. 0-1/4 throttle was all primary, quickly moving to 1/2-3/4 would get the secondaries to kick in without much of a stumble. Anything beyond 3/4 it didn't feel like anything changed power wise. Should have checked the pedal position to butterfly position at the time.
The other thing I wanted to address was the sustained left turn fuel starvation issue. In normal driving around me, there are enough tight switch backs (and turning onto my road) that would induce starvation at 15-20 mph. I think the end goal was to convert over to cathedral float bowls with center hung floats and even that would not have fixed the issue 100%. Since our setup is rotated 90 degrees from how these things usually sit on v8's almost all of these aftermarket 4 barrel setups are going to have some trade offs running on a rotary. I think the other thing that turned me off of this was the cost of the float and float bowls.
Take this with a grain of salt, this is 10 year old memory working here and I usually can't remember what I had for breakfast come lunchtime. I can dig up any info that I may have saved from that time. I decided to learn more about the Nikki and start from a stock one and start working on modifying that. That's still in the future though.
Other random things:
-The holley blue pump is aggressively loud. The mallory pump is a slight improvement even with both of them on rubber isolators. I can't quite wrap my head around why a 7psi pump is this loud and a stock 58 psi external EFI pump is nearly silent by comparison.
-Don't use the fancy clear float bowl sight plugs. The heat and today's fuel make them brittle and crack off in place quickly.
-I had my secondary circuit completely shut off and stuck closed once when I tried to make a small adjustment to the bowl level. Had to pull the adjuster out completely to reattach the adjuster nut to the threaded post.
-Are you still using the RB style oil feed line that directs the OMP into the primary float bowl? I redirected mine directly into the intake primary runners. Though, there's issues with this too.
-What choke setup do you have? Electric or the mechanical pitot tube?
I had two different 465 holley (1848) one definitely from Racing Beat. Both had a rather large hole between the primary and secondary circuit. I tried, like you, to fatten up the primary circuit (49 to 51/52) and swap out the accelerator pump spring and cam to try and bridge that hole and get it to smooth out.Make sure yours does not have a power valve; our vacuum signal is not strong enough to get that to work correctly. If you can get that to work, good on you. The one from RB had the power valve deleted for this reason. The thing I considered doing was spacing out the accelerator diaphragm to increase the fluid volume available to aid the transition as the primary circuit leaned out before the vacuum secondaries were able to engage. I had a 3 wire, narrow band, 7 light O2 sensor, it was simple, but it worked. Also considered swapping over to mechanical secondaries to control the timing of the transition directly. A combination of both of those things is probably the way to get better control of the primary to secondary hole.
On the dyno at KDR, I was able to get power pulls done and ended up swapping back to 49 on the main jet. I can dig up the dyno graphs, peak power was 141 rwhp at 6500 or 7000 rpm. After that, the hole was still there, I just learned to live with it and started to change my accelerator pedal behavior. 0-1/4 throttle was all primary, quickly moving to 1/2-3/4 would get the secondaries to kick in without much of a stumble. Anything beyond 3/4 it didn't feel like anything changed power wise. Should have checked the pedal position to butterfly position at the time.
The other thing I wanted to address was the sustained left turn fuel starvation issue. In normal driving around me, there are enough tight switch backs (and turning onto my road) that would induce starvation at 15-20 mph. I think the end goal was to convert over to cathedral float bowls with center hung floats and even that would not have fixed the issue 100%. Since our setup is rotated 90 degrees from how these things usually sit on v8's almost all of these aftermarket 4 barrel setups are going to have some trade offs running on a rotary. I think the other thing that turned me off of this was the cost of the float and float bowls.
Take this with a grain of salt, this is 10 year old memory working here and I usually can't remember what I had for breakfast come lunchtime. I can dig up any info that I may have saved from that time. I decided to learn more about the Nikki and start from a stock one and start working on modifying that. That's still in the future though.
Other random things:
-The holley blue pump is aggressively loud. The mallory pump is a slight improvement even with both of them on rubber isolators. I can't quite wrap my head around why a 7psi pump is this loud and a stock 58 psi external EFI pump is nearly silent by comparison.
-Don't use the fancy clear float bowl sight plugs. The heat and today's fuel make them brittle and crack off in place quickly.
-I had my secondary circuit completely shut off and stuck closed once when I tried to make a small adjustment to the bowl level. Had to pull the adjuster out completely to reattach the adjuster nut to the threaded post.
-Are you still using the RB style oil feed line that directs the OMP into the primary float bowl? I redirected mine directly into the intake primary runners. Though, there's issues with this too.
-What choke setup do you have? Electric or the mechanical pitot tube?
Last edited by swbtm; Jun 10, 2021 at 07:59 PM.
I ran this setup on my 83 about 10 years ago, it had a freshly rebuilt streetported 12a at the time. Unsure of porting size/timing.
I had two different 465 holley (1848) one definitely from Racing Beat. Both had a rather large hole between the primary and secondary circuit. I tried, like you, to fatten up the primary circuit (49 to 51/52) and swap out the accelerator pump spring and cam to try and bridge that hole and get it to smooth out.Make sure yours does not have a power valve; our vacuum signal is not strong enough to get that to work correctly. If you can get that to work, good on you. The one from RB had the power valve deleted for this reason. The thing I considered doing was spacing out the accelerator diaphragm to increase the fluid volume available to aid the transition as the primary circuit leaned out before the vacuum secondaries were able to engage. I had a 3 wire, narrow band, 7 light O2 sensor, it was simple, but it worked. Also considered swapping over to mechanical secondaries to control the timing of the transition directly. A combination of both of those things is probably the way to get better control of the primary to secondary hole.
On the dyno at KDR, I was able to get power pulls done and ended up swapping back to 49 on the main jet. I can dig up the dyno graphs, peak power was 141 rwhp at 6500 or 7000 rpm. After that, the hole was still there, I just learned to live with it and started to change my accelerator pedal behavior. 0-1/4 throttle was all primary, quickly moving to 1/2-3/4 would get the secondaries to kick in without much of a stumble. Anything beyond 3/4 it didn't feel like anything changed power wise. Should have checked the pedal position to butterfly position at the time.
The other thing I wanted to address was the sustained left turn fuel starvation issue. In normal driving around me, there are enough tight switch backs (and turning onto my road) that would induce starvation at 15-20 mph. I think the end goal was to convert over to cathedral float bowls with center hung floats and even that would not have fixed the issue 100%. Since our setup is rotated 90 degrees from how these things usually sit on v8's almost all of these aftermarket 4 barrel setups are going to have some trade offs running on a rotary. I think the other thing that turned me off of this was the cost of the float and float bowls.
Take this with a grain of salt, this is 10 year old memory working here and I usually can't remember what I had for breakfast come lunchtime. I can dig up any info that I may have saved from that time. I decided to learn more about the Nikki and start from a stock one and start working on modifying that. That's still in the future though.
Other random things:
-The holley blue pump is aggressively loud. The mallory pump is a slight improvement even with both of them on rubber isolators. I can't quite wrap my head around why a 7psi pump is this loud and a stock 58 psi external EFI pump is nearly silent by comparison.
-Don't use the fancy clear float bowl sight plugs. The heat and today's fuel make them brittle and crack off in place quickly.
-I had my secondary circuit completely shut off and stuck closed once when I tried to make a small adjustment to the bowl level. Had to pull the adjuster out completely to reattach the adjuster nut to the threaded post.
-Are you still using the RB style oil feed line that directs the OMP into the primary float bowl? I redirected mine directly into the intake primary runners. Though, there's issues with this too.
-What choke setup do you have? Electric or the mechanical pitot tube?
I had two different 465 holley (1848) one definitely from Racing Beat. Both had a rather large hole between the primary and secondary circuit. I tried, like you, to fatten up the primary circuit (49 to 51/52) and swap out the accelerator pump spring and cam to try and bridge that hole and get it to smooth out.Make sure yours does not have a power valve; our vacuum signal is not strong enough to get that to work correctly. If you can get that to work, good on you. The one from RB had the power valve deleted for this reason. The thing I considered doing was spacing out the accelerator diaphragm to increase the fluid volume available to aid the transition as the primary circuit leaned out before the vacuum secondaries were able to engage. I had a 3 wire, narrow band, 7 light O2 sensor, it was simple, but it worked. Also considered swapping over to mechanical secondaries to control the timing of the transition directly. A combination of both of those things is probably the way to get better control of the primary to secondary hole.
On the dyno at KDR, I was able to get power pulls done and ended up swapping back to 49 on the main jet. I can dig up the dyno graphs, peak power was 141 rwhp at 6500 or 7000 rpm. After that, the hole was still there, I just learned to live with it and started to change my accelerator pedal behavior. 0-1/4 throttle was all primary, quickly moving to 1/2-3/4 would get the secondaries to kick in without much of a stumble. Anything beyond 3/4 it didn't feel like anything changed power wise. Should have checked the pedal position to butterfly position at the time.
The other thing I wanted to address was the sustained left turn fuel starvation issue. In normal driving around me, there are enough tight switch backs (and turning onto my road) that would induce starvation at 15-20 mph. I think the end goal was to convert over to cathedral float bowls with center hung floats and even that would not have fixed the issue 100%. Since our setup is rotated 90 degrees from how these things usually sit on v8's almost all of these aftermarket 4 barrel setups are going to have some trade offs running on a rotary. I think the other thing that turned me off of this was the cost of the float and float bowls.
Take this with a grain of salt, this is 10 year old memory working here and I usually can't remember what I had for breakfast come lunchtime. I can dig up any info that I may have saved from that time. I decided to learn more about the Nikki and start from a stock one and start working on modifying that. That's still in the future though.
Other random things:
-The holley blue pump is aggressively loud. The mallory pump is a slight improvement even with both of them on rubber isolators. I can't quite wrap my head around why a 7psi pump is this loud and a stock 58 psi external EFI pump is nearly silent by comparison.
-Don't use the fancy clear float bowl sight plugs. The heat and today's fuel make them brittle and crack off in place quickly.
-I had my secondary circuit completely shut off and stuck closed once when I tried to make a small adjustment to the bowl level. Had to pull the adjuster out completely to reattach the adjuster nut to the threaded post.
-Are you still using the RB style oil feed line that directs the OMP into the primary float bowl? I redirected mine directly into the intake primary runners. Though, there's issues with this too.
-What choke setup do you have? Electric or the mechanical pitot tube?
I've had setups with racing beat and holley setups that never had cornering fuel starve and others i couldn't get away from but not sure why..... this one hasn't done it to me yet.... knock on wood. Lol I think the perfect float bowl height and proper fuel pressure and volume all together are key tho. And these pumps are really stupid loud. Haha and what secondary stuff and nut are you referring to? Thanks!
wow man! That's what I'm talking about for imput! So as of choke.... none. I removed the flap and electric choke assembly. It never did work that great and only messed with my tuning. At least t till it starts getting cooler again. Lol OMP is blocked off and running premix idemitsu oil for premix. Power valve is a blank plug. I've bumped up to 59 main jets and it pulls to 7k without breaking up and afr number is about 11.8 at 7k in 3rd. So secondaries must be operating.
I've had setups with racing beat and holley setups that never had cornering fuel starve and others i couldn't get away from but not sure why..... this one hasn't done it to me yet.... knock on wood. Lol I think the perfect float bowl height and proper fuel pressure and volume all together are key tho. And these pumps are really stupid loud. Haha and what secondary stuff and nut are you referring to? Thanks!
I've had setups with racing beat and holley setups that never had cornering fuel starve and others i couldn't get away from but not sure why..... this one hasn't done it to me yet.... knock on wood. Lol I think the perfect float bowl height and proper fuel pressure and volume all together are key tho. And these pumps are really stupid loud. Haha and what secondary stuff and nut are you referring to? Thanks!
Update.... still got a lean issue so I put in the 6.5 power valve to test and didn't seem to do anything different in afr but will really lay rubber so who knows. 😆 I may buy a smaller number valve to compensate for low vac in the rotaries.....???
Getting back to thinking after a hellish couple weeks of driving here and back again.
Power valve 6.5? I want to say the rotary does not have a consistent enough vacuum signal to trigger the valve reliably. Going off of memory and talking to RB years back, that's why their setups came with the power valve blocked off. If you can figure out a setup that would consistently add in the correct volume to the flow, then you could be good. Maybe an accumulator that gets drawn down over time and the accelerator trips that and uses it as a vacuum source
try looking into how the super aggressive cam v8 guys use these carbs. While tiny, the 465 should behave similarly to their larger setups (maybe?)
I was never able to dial in the accelerator pump to do anything. Maybe a custom 3d printed cam along with the diaphragm spaced out could help fill the fuel hole?
I was just curious about the choke because the carb I got from RB in 2009 had the mechanical copper pitot tube type.
Power valve 6.5? I want to say the rotary does not have a consistent enough vacuum signal to trigger the valve reliably. Going off of memory and talking to RB years back, that's why their setups came with the power valve blocked off. If you can figure out a setup that would consistently add in the correct volume to the flow, then you could be good. Maybe an accumulator that gets drawn down over time and the accelerator trips that and uses it as a vacuum source
try looking into how the super aggressive cam v8 guys use these carbs. While tiny, the 465 should behave similarly to their larger setups (maybe?)I was never able to dial in the accelerator pump to do anything. Maybe a custom 3d printed cam along with the diaphragm spaced out could help fill the fuel hole?
I was just curious about the choke because the carb I got from RB in 2009 had the mechanical copper pitot tube type.
Old [Sch|F]ool
Joined: May 2001
Posts: 12,872
Likes: 574
From: Cleveland, Ohio, USA
Accelerator pump contribution becomes very low once you get the transition tuned correctly.
This requires some fine drill bits and a decently in depth knowledge of how the metering circuits work, unless you have an aftermarket carb where all this stuff is tuneable.
This requires some fine drill bits and a decently in depth knowledge of how the metering circuits work, unless you have an aftermarket carb where all this stuff is tuneable.
Getting back to thinking after a hellish couple weeks of driving here and back again.
Power valve 6.5? I want to say the rotary does not have a consistent enough vacuum signal to trigger the valve reliably. Going off of memory and talking to RB years back, that's why their setups came with the power valve blocked off. If you can figure out a setup that would consistently add in the correct volume to the flow, then you could be good. Maybe an accumulator that gets drawn down over time and the accelerator trips that and uses it as a vacuum source try looking into how the super aggressive cam v8 guys use these carbs. While tiny, the 465 should behave similarly to their larger setups (maybe?)
I was never able to dial in the accelerator pump to do anything. Maybe a custom 3d printed cam along with the diaphragm spaced out could help fill the fuel hole?
I was just curious about the choke because the carb I got from RB in 2009 had the mechanical copper pitot tube type.
Power valve 6.5? I want to say the rotary does not have a consistent enough vacuum signal to trigger the valve reliably. Going off of memory and talking to RB years back, that's why their setups came with the power valve blocked off. If you can figure out a setup that would consistently add in the correct volume to the flow, then you could be good. Maybe an accumulator that gets drawn down over time and the accelerator trips that and uses it as a vacuum source
try looking into how the super aggressive cam v8 guys use these carbs. While tiny, the 465 should behave similarly to their larger setups (maybe?)I was never able to dial in the accelerator pump to do anything. Maybe a custom 3d printed cam along with the diaphragm spaced out could help fill the fuel hole?
I was just curious about the choke because the carb I got from RB in 2009 had the mechanical copper pitot tube type.
Last edited by risingsunroof82; Jul 11, 2021 at 02:28 AM.
Mine would back fire too, don't recall exact situation but I think i trained myself to idle/roll home the last little bit to allow it to settle down before shut down.
The draw back to the holley is the secondary metering block is not threaded for air bleeds and maybe even has fixed fuel jets (?? Memory fails me here). I think the primary metering block only has swappable fuel jets and fixed air bleeds. I want to say there was a way to retrofit some "air bleeds" by tapping those ports and using fuel jets instead, but tuning you were on your own getting the sizing dialed and they weren't cut for air flow. Mikuni jets come to mind when thinking about this too.
On the general subject of carburetors, this is a very well done video. Not mine, just something I found
https://m.youtube.com/watch?v=toVfvRhWbj8
The draw back to the holley is the secondary metering block is not threaded for air bleeds and maybe even has fixed fuel jets (?? Memory fails me here). I think the primary metering block only has swappable fuel jets and fixed air bleeds. I want to say there was a way to retrofit some "air bleeds" by tapping those ports and using fuel jets instead, but tuning you were on your own getting the sizing dialed and they weren't cut for air flow. Mikuni jets come to mind when thinking about this too.
On the general subject of carburetors, this is a very well done video. Not mine, just something I found
https://m.youtube.com/watch?v=toVfvRhWbj8
Old [Sch|F]ool
Joined: May 2001
Posts: 12,872
Likes: 574
From: Cleveland, Ohio, USA
The 4160 type carbs have a secondary metering "plate", not a metering block like the primary. Easy to see the difference because the secondary float bowl seals directly to the main body. These have 100% fixed jetting and can only be tuned with drilling or with replacing the plate.
Most (not all) 4150 type carbs have a secondary metering block, similar to the one on the primaries. I think you can only do it this way with centerhung float bowls, because of the fuel transfer tube on the sidehung units.
Air bleeds are all in the main body, not the bowls/metering blocks. This is one of the few things I appreciate about Holleys, after discovering that some/many/most/all? sidedrafts use unfiltered air for the air bleeds. (There is a workaround for Dell'Orto carbs: get bowl vent covers for a turbo Esprit, since the early ones had blowthrough dual DHLAs and they needed to boost reference the bowls)
Most (not all) 4150 type carbs have a secondary metering block, similar to the one on the primaries. I think you can only do it this way with centerhung float bowls, because of the fuel transfer tube on the sidehung units.
Air bleeds are all in the main body, not the bowls/metering blocks. This is one of the few things I appreciate about Holleys, after discovering that some/many/most/all? sidedrafts use unfiltered air for the air bleeds. (There is a workaround for Dell'Orto carbs: get bowl vent covers for a turbo Esprit, since the early ones had blowthrough dual DHLAs and they needed to boost reference the bowls)
Last edited by peejay; Jul 11, 2021 at 06:39 PM.
I just received a 2.5 and 3.5 power valves...... gonna install the 3.5 first...... I'm only getting lean numbers during cruising speed... as high as 18.5. When full throttling it stays around 11.5-13. Anyone know the vacuum a 12a pulls at any and or all ranges?
Old [Sch|F]ool
Joined: May 2001
Posts: 12,872
Likes: 574
From: Cleveland, Ohio, USA
Power valve may not really help that, it sounds more like an air bleed or idle fuel restriction channel size issue, depending on how deep into the throttle you are.
FWIW, I have a wideband on my stock Nikki now, and while cruising on the primaries a little before the secondaries open, I'm at about 15.5-16.5:1. It's happy there and gets great fuel economy.
FWIW, I have a wideband on my stock Nikki now, and while cruising on the primaries a little before the secondaries open, I'm at about 15.5-16.5:1. It's happy there and gets great fuel economy.
Ran the 3.5 PV today and it seemed to lower the numbers to the 16ish range! I do need to get to an exhaust shop cause I got some leaks which will throw numbers around and cause the back fires....
