Drilled throtle blades?
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From: Hoosier
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Drilled throtle blades?
I've seen in some magazines that drilling a 1/32 or a 1/16 hole in each throtle blade has a similar affect as leaning out the idle circuit. Has anyone done this?
Reason I'm asking is that up to about 1800-1900 rpm cruise speed my carb is running rich, mainly on the idle circuit. I'm barely on the gas pedal at 40 mph. At about 2000 rpm and over cruise speed, the primary circuit is coming on and running at stoich.
The idle screws at set to highest vacuum right now (21), at 700 rpm and 12 degrees advance timing. The transistion slot is barely showing under the closed blades at idle. The air/fuel gauge is showing an equivalent of 12.5 to 1. I have a new base model crate engine with a 600cfm Edelbrock carb, manifold, and headers.
BTW. my state sniffer test verified the accuracy of the gauge.
Reason I'm asking is that up to about 1800-1900 rpm cruise speed my carb is running rich, mainly on the idle circuit. I'm barely on the gas pedal at 40 mph. At about 2000 rpm and over cruise speed, the primary circuit is coming on and running at stoich.
The idle screws at set to highest vacuum right now (21), at 700 rpm and 12 degrees advance timing. The transistion slot is barely showing under the closed blades at idle. The air/fuel gauge is showing an equivalent of 12.5 to 1. I have a new base model crate engine with a 600cfm Edelbrock carb, manifold, and headers.
BTW. my state sniffer test verified the accuracy of the gauge.
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Re: Drilled throtle blades? (AlwaysWave)
seems like drilling the throttle plates would for sure lean it out some, but I have a gut feeling you wouldn't get enough air through a 1/16" hole to make much of a difference. Now I don't claim to be an expert, but wouldn't you be better off changing the primary needles instead of drilling? Drilling would be awful permanent as well, but with a needle or jet change you could always try something else. I heard GM purposely ran a rich mixture as they wanted the rich mix to continue burning in the catalytic converter (to make it hotter) to reduce emissions. :cheers:
[Modified by Big Fish, 9:20 PM 10/20/2002]
[Modified by Big Fish, 9:20 PM 10/20/2002]
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From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Drilled throtle blades? (AlwaysWave)
Before drilling your throttle plates, try sticking in a set of slightly fatter (leaner) rods to see if this will correct your mixtures. Then, if you're still convinced of the need to drill the plates due to a very large cam (drilling the plates is only needed if your cam is so big that you have to crack the idle speed open beyond the transition slot in the throttle bore), try drilling holes in your PCV hose first: bleeding more air in through the PCV hose will have the same effect as drilling the plates, except the hose is easily replaceable. If bleeding more air through the PCV doesn't cure your problem, drilling the plates will not do anything differently.
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From: Orange Park Florida
Re: Drilled throtle blades? (lars)
Lars, it's allways been my experience that funny air/fuel problems in a Q jet are more from worn out throttle plates and such with the lean settings in the off idle xfer circuit...that slot....that compensating with jets/rods and or differance idle screws/settings...that so many combinations of screw-ups are possible, that in reality, it takes a super careful rebuild by someone who really knows the carbs well enough to make engineering type adjustments to the basics.....most of these carbs have been played with, worn out after so many miles, came from other cars...etc....hard to say what in hell is the matter with them....
wonder if you agree or not...
GENE
wonder if you agree or not...
GENE
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From: Hoosier
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Re: Drilled throtle blades? (mrvette)
Big Fish, I have a 72..no catalytic converter
Lars, the jets and rods don’t control the idle circuit, unless you mean the idle jet in the bowl, which is not interchangeable. Bleeding more air through the PCV hose is a great thought. At about 2000 rpm the fuel mix is a-ok. But driving around town at speeds less than 45-50 mph it’s rich and using too much fuel. It’s a miles per gallon thing.
Mrvette, I’m using an Edelbrock carb, not a Q-Jet. The carb is relatively new and in great shape.
If a small hole in the PCV line doesn't do well....Perhaps I could try inserting a wire into the idle jet to restrict the fuel flow? Or increase the air bleed size ?
BTW…if the throttle blades are drilled, the holes can be sealed with JB Weld.
Lars, the jets and rods don’t control the idle circuit, unless you mean the idle jet in the bowl, which is not interchangeable. Bleeding more air through the PCV hose is a great thought. At about 2000 rpm the fuel mix is a-ok. But driving around town at speeds less than 45-50 mph it’s rich and using too much fuel. It’s a miles per gallon thing.
Mrvette, I’m using an Edelbrock carb, not a Q-Jet. The carb is relatively new and in great shape.
If a small hole in the PCV line doesn't do well....Perhaps I could try inserting a wire into the idle jet to restrict the fuel flow? Or increase the air bleed size ?
BTW…if the throttle blades are drilled, the holes can be sealed with JB Weld.
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Re: Drilled throtle blades? (AlwaysWave)
I have .125 or 1/8th inch holes in my primary throttle blades. but that is more of a double pumper idea for carbs like my speed Demon and race Demon.
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right out of my edelbrock book. The Idle Screw Port is a variable discharge restriction that is adjusted by the engine tuner to achieve the desired A/F Ratio at engine idle.
I ran a 750 Edel carb for over 10 years. You can do your own custom jetting outside the very few choices that the book gives you.
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right out of my edelbrock book. The Idle Screw Port is a variable discharge restriction that is adjusted by the engine tuner to achieve the desired A/F Ratio at engine idle.
I ran a 750 Edel carb for over 10 years. You can do your own custom jetting outside the very few choices that the book gives you.
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From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Drilled throtle blades? (AlwaysWave)
Your jet and rod combo has a significant impact on the idle circuit. The idle circuit draws its fuel through the main jets, and changes in oriface size of the main jetting will impact your idle circuit even though the idle circuit has down-stream orifaces of its own. If you want to verify it, simply drop in a nice, fat primary metering rod and see what it does to your idle. I assure you it will change it radically...
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Re: Drilled throtle blades? (MikeC)
My 750 was way rich out of the box and I did lots of rod /jet changes to tweak it in on a sniffer and rear wheel dyno. I never achieved vacuum readings over 15 or so inches at idle and driving with a vacuum gauge I don't think I ever saw over 20
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From: Hoosier
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Re: Drilled throtle blades? (gkull)
OK....I'll buy into the main jet/rod affects the gas volume in the idle bowl. But the current combo is working well for engine speeds over 1700 rpm.
The other thing about the idle mixure is that it is actually already an emulsion by way of the air bleeds and gas draw from the bowl. The idle screws restrict the flow of that emulsion right? So if the emulsion itself is rich or lean, doesn't it play a large part in the final idle mixture?
gkull, are you saying maybe I should adjust to the air/fuel gauge and let the vacuum be where it may?
I'm confused.
The other thing about the idle mixure is that it is actually already an emulsion by way of the air bleeds and gas draw from the bowl. The idle screws restrict the flow of that emulsion right? So if the emulsion itself is rich or lean, doesn't it play a large part in the final idle mixture?
gkull, are you saying maybe I should adjust to the air/fuel gauge and let the vacuum be where it may?
I'm confused.
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Re: Drilled throtle blades? (AlwaysWave)
gkull, are you saying maybe I should adjust to the air/fuel gauge and let the vacuum be where it may?
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I was just saying that I have never seen that much vacuum. You should always ajust the idle mixture screws for the highest vacuum. So your doing that right. Which is also the smoothest idle. You might try a rod one or two numbers bigger on the larger diameter part of the rod and leave the power diameter the same.
I always tell people that the easiest way to tell if you carb is rich at idle. all you have to do is uncap the full vacuum port on the side of the carb. It causes a leaner condition. If your idle rpm jumps up your rich on the A/F mixture. Drilling the throttle blades is the same idea. Your inducing a straight shot of air. I've tuned 600 and 750 Edel's and i never had to resort to drilling.
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I was just saying that I have never seen that much vacuum. You should always ajust the idle mixture screws for the highest vacuum. So your doing that right. Which is also the smoothest idle. You might try a rod one or two numbers bigger on the larger diameter part of the rod and leave the power diameter the same.
I always tell people that the easiest way to tell if you carb is rich at idle. all you have to do is uncap the full vacuum port on the side of the carb. It causes a leaner condition. If your idle rpm jumps up your rich on the A/F mixture. Drilling the throttle blades is the same idea. Your inducing a straight shot of air. I've tuned 600 and 750 Edel's and i never had to resort to drilling.