Cliff's Quadrajet Parts and Rebuild Kits
General Category => Quadrajet Carb Talk and Tips => Topic started by: novadude on September 18, 2025, 05:31:44 AM
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Let's talk about secondary air bleeds. I've noticed that most Chevy carbs are 0.026-0.028". Cliff's book recommends drilling all to 0.036", and I believe many BOP carbs came with .033-.036" tubes.
What is the effect of increasing the air bleed size? I am thinking that this might make the secondaries richer on initial opening - bigger bubbles lead to better initial flow when fuel starts moving thru the nozzle. I am also thinking that larger bleeds may trend a little leaner at higher flow due to more air mixed with the fuel. Basically, the slope of fuel curve relative to air flow would change. Am I correct in my thinking?
How much impact does the secondary air bleed have? Has anyone done back-to-back tests changing nothing but the air bleed size?
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Somebody did do a test on this but I can't remember where. Might have been Class Racer, but just not sure. I don't think it was here.
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Somebody did do a test on this but I can't remember where. Might have been Class Racer, but just not sure. I don't think it was here.
If you remember where, please share. Do you recall the conclusions?
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If I remember right a couple HP up to .043. Might have been on the other carb forum that kinda died out. Seems like I asked Tuner & I got salt to taste, so I take it whatever works.
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You are correct in your thinking.
The 0.036" did not work for me. I had a post earlier where it kept going lean at high rpm. One thing to note is that I'm near 600 HP with my setup so in order to get there the air door has to be open as far as possible (1.240"), yet I'm still 2" of manifold vac at WOT at the top of the pull. So there is less sec venturi effect vac to pull fuel.
For my 7043240, I reduced them to 0.025" and got really close with CE rods before the engine swallowed a screw. Next I will try 0.022" (the Ebrock RPM comes with 0.020"). I also closed the lower end of the tube and added extra emulsion holes like Tuner suggests although I don't agree with all his recommendations..
There is really good info here: https://www.tapatalk.com/groups/racingfuelsystems/viewtopic.php?p=16572#p16572
See the pic in the link above
So I don't think the 0.036" will work with higher HP applications. Unless you can close the air door a lot and run really small tip sec rods ( which I don't have) .
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The inverted bleed tubes and emulsion holes deal did not work well for me when I tried it. Seemed like it would start rich and trend lean. I didn't enlarge the well like he recommends, so maybe that had something to do with it? This was with a 0.026" bleed in the tube (moved to the top of the airhorn - other end soldered shut, 0.024" emulsion holes)
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You are correct in your thinking.
The 0.036" did not work for me. I had a post earlier where it kept going lean at high rpm. One thing to note is that I'm near 600 HP with my setup so in order to get there the air door has to be open as far as possible (1.240"), yet I'm still 2" of manifold vac at WOT at the top of the pull. So there is less sec venturi effect vac to pull fuel.
For my 7043240, I reduced them to 0.025" and got really close with CE rods before the engine swallowed a screw. Next I will try 0.022" (the Ebrock RPM comes with 0.020"). I also closed the lower end of the tube and added extra emulsion holes like Tuner suggests although I don't agree with all his recommendations..
There is really good info here: https://www.tapatalk.com/groups/racingfuelsystems/viewtopic.php?p=16572#p16572
See the pic in the link above
So I don't think the 0.036" will work with higher HP applications. Unless you can close the air door a lot and run really small tip sec rods ( which I don't have) .
Must have been you that I saw you had tested diff. size secondary bleeds.
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The inverted bleed tubes and emulsion holes deal did not work well for me when I tried it. Seemed like it would start rich and trend lean. I didn't enlarge the well like he recommends, so maybe that had something to do with it? This was with a 0.026" bleed in the tube (moved to the top of the airhorn - other end soldered shut, 0.024" emulsion holes)
That was the approach that worked for me. I do not agree with enlarging the well. It seems that will reduce the effects of emulsion air, which I need at lower rpms to enrichen it. I had a 0.025" bleed at the top, lower end soldered shut, and only 2 of 0.026" emulsion holes at the bottom. I will note that the orifice that feeds the secondary poe well was left at 0.027". If you have seen the videos of fuel being pulled out of the POE holes you'll understand the relevance of that so maybe my extra fueling from there is different.
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The inverted bleed tubes and emulsion holes deal did not work well for me when I tried it. Seemed like it would start rich and trend lean. I didn't enlarge the well like he recommends, so maybe that had something to do with it? This was with a 0.026" bleed in the tube (moved to the top of the airhorn - other end soldered shut, 0.024" emulsion holes)
I kinda quit testing, I was trying to get serious about by logging AFR & RPM but it seemed to be a PITA with the Innovate stuff it seemed so finicky. I guess since I quit going to the track, & as long as it runs good & can still do a 300 ft. smokey burnout it's all good.
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That was the approach that worked for me. I do not agree with enlarging the well. It seems that will reduce the effects of emulsion air, which I need at lower rpms to enrichen it. I had a 0.025" bleed at the top, lower end soldered shut, and only 2 of 0.026" emulsion holes at the bottom. I will note that the orifice that feeds the secondary poe well was left at 0.027". If you have seen the videos of fuel being pulled out of the POE holes you'll understand the relevance of that so maybe my extra fueling from there is different.
I may need to revisit this. I had the four (4) .024" holes in the tube (and maybe a little too high) and it seemed like it was doing wierd things to the fuel curve.
0.027" is pretty small for the well re-fill holes (even in many stock model carbs I've seen). I would guess that the POE fuel probably isn't much of a player after intial WOT?
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I may need to revisit this. I had the four (4) .024" holes in the tube (and maybe a little too high) and it seemed like it was doing wierd things to the fuel curve.
0.027" is pretty small for the well re-fill holes (even in many stock model carbs I've seen). I would guess that the POE fuel probably isn't much of a player after intial WOT?
I think all the well holes I've seen were .04.
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Only 1974 and earlier Buick units uses the .029" secondary well fill holes.
All Pontiac 1966-74 units uses .034" holes.
Most, if not all other makes/years uses .038"-.039" secondary well fill holes.
And, i have found tuning the Qjet secondaries in a boat, me hanging above the carb, at extended WOT, i noticed fuel pulled out of the secondary "accelerator" ports constantly.
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Only 1974 and earlier Buick units uses the .029" secondary well fill holes.
All Pontiac 1966-74 units uses .034" holes.
Most, if not all other makes/years uses .038"-.039" secondary well fill holes.
And, i have found tuning the Qjet secondaries in a boat, me hanging above the carb, at extended WOT, i noticed fuel pulled out of the secondary "accelerator" ports constantly.
Like this.
http://www.ryanbrownracing.com/video/Qjet_dyno_vid.wmv
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Only 1974 and earlier Buick units uses the .029" secondary well fill holes.
All Pontiac 1966-74 units uses .034" holes.
Most, if not all other makes/years uses .038"-.039" secondary well fill holes.
And, i have found tuning the Qjet secondaries in a boat, me hanging above the carb, at extended WOT, i noticed fuel pulled out of the secondary "accelerator" ports constantly.
Does that change with small secondary well fill holes? Seems like a really small fill hole would allow the fuel level to drop in the well to where the fuel coming out the discharge would dramatically decrease.
Perhaps a different topic for a different time, but how do the drilled "cobra jet" POE tubes change the situation?
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I kinda quit testing, I was trying to get serious about by logging AFR & RPM but it seemed to be a PITA with the Innovate stuff it seemed so finicky. I guess since I quit going to the track, & as long as it runs good & can still do a 300 ft. smokey burnout it's all good.
I tried datalogging to, but I couldn't get a clean rpm trace with my Daytona Sensors WEGO IV. I gave up before trying a potentiometer or resistor to try and smooth it. Now I just catch glimpses of the meter while trying to keep the car on the road. LOL
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I may need to revisit this. I had the four (4) .024" holes in the tube (and maybe a little too high) and it seemed like it was doing wierd things to the fuel curve.
0.027" is pretty small for the well re-fill holes (even in many stock model carbs I've seen). I would guess that the POE fuel probably isn't much of a player after intial WOT?
The way I heard that explained was that it's fuel is fed by gravity once the fuel level drops below the re fill holes. So it's pretty much a constant flow rate after that ( limited by the 2 orifices). And with increasing air with increasing rpm and no correlating increase in fuel flow, it's contribution gets leaner at higher rpm. So yeah when first at WOT and lower rpms, it's not much of a player after that
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Now I just catch glimpses of the meter while trying to keep the car on the road. LOL
That's the same problem I've got, but I'm trying to push the record button on my little voice recorder to make notes with, while looking at the AFR gage and staying between the ditches and not bouncing off the guardrails.
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I think I was just doing some testing here looks to be about 9 years ago. The AFR & RPM numbers aren't right in the sidebar.