Holley HP idle mixture adjustment
Thread Starter
Pro
Joined: Sep 2004
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From: Philadelphia PA
Holley HP idle mixture adjustment
I have a new Holley HP 950, squared 77 jets and 2 6.5 PVs.
It idles fine, but adjusting the idle mixture screws shows no change. I cant lean them out to stall the engine.
Can anyone diagnose? Suggestions. Thanks
It idles fine, but adjusting the idle mixture screws shows no change. I cant lean them out to stall the engine.
Can anyone diagnose? Suggestions. Thanks
Melting Slicks
Joined: Oct 2005
Posts: 2,056
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From: Redruth Cornwall
may be a dumb question but which way are you turning the screws?
Surley if you gently bottom the screws and the back them off 1 1/2 turns she should run....turn them in again and it will stall, backing them out will richen mix.
Surley if you gently bottom the screws and the back them off 1 1/2 turns she should run....turn them in again and it will stall, backing them out will richen mix.
Le Mans Master
Joined: Apr 2004
Posts: 7,106
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From: Newark DE
WOW!! a 950cfm carb? what size motor? that is a huge carb!
Unless you are running a huge BB motor you are probably way over carbed.
Even the VERY radical L88 motors "only" ran an 850CFM carb.
You may have to jet that carb down a lot
best way to adjust mixture is to set it with a vacuum gauge and adjust your mixture screws to achieve the highest vacuum reading.
Unless you are running a huge BB motor you are probably way over carbed.
Even the VERY radical L88 motors "only" ran an 850CFM carb.
You may have to jet that carb down a lot
best way to adjust mixture is to set it with a vacuum gauge and adjust your mixture screws to achieve the highest vacuum reading.
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Pro
Joined: Sep 2004
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From: Philadelphia PA
Yes. It runs. I started as 1 1/2 out. Turning them in didnt stall the car or make any idle change.
Originally Posted by jotto
may be a dumb question but which way are you turning the screws?
Surley if you gently bottom the screws and the back them off 1 1/2 turns she should run....turn them in again and it will stall, backing them out will richen mix.
Surley if you gently bottom the screws and the back them off 1 1/2 turns she should run....turn them in again and it will stall, backing them out will richen mix.
Le Mans Master
Joined: Apr 2004
Posts: 7,106
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From: Newark DE
Originally Posted by Highhat
Yes. It runs. I started as 1 1/2 out. Turning them in didnt stall the car or make any idle change.
I can turn the mixture screws in all the way on my Holley 650cfm on my '65 and it doesn't stall
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Originally Posted by BarryK
WOW!! a 950cfm carb? what size motor? that is a huge carb!
Unless you are running a huge BB motor you are probably way over carbed.
Even the VERY radical L88 motors "only" ran an 850CFM carb.
You may have to jet that carb down a lot
best way to adjust mixture is to set it with a vacuum gauge and adjust your mixture screws to achieve the highest vacuum reading.
Unless you are running a huge BB motor you are probably way over carbed.
Even the VERY radical L88 motors "only" ran an 850CFM carb.
You may have to jet that carb down a lot
best way to adjust mixture is to set it with a vacuum gauge and adjust your mixture screws to achieve the highest vacuum reading.
Bigger Is Better Part II!
The buddy of a racer saw my carburetor shop work shirt while we were watching cars launch on test day. He asked, "Hey, my buddy just got a new, bigger Holley and we can't get it jetted right. It's slower now than it was with the smaller carb. What's the deal?" As it turned out, just the reverse was true. He bought a Holley "HP 950" and took off a Holley "850."
It's understandable why he made the mistake of thinking he'd gotten a "bigger carb." After all, 950 is more than 850 right? Wrong! In this case, Holley decided to rate their new line of "HP" carbs like their competition does, with inflated CFM figures. They had to. Otherwise no one would buy them because they are more expensive. An HP 950 Holley is a 750 main body with an 850 base. That makes it an 800. You can do the math. The "950," in this case, refers to the CFM potential, not the size of the venturi and throttle blade diameter.
Do not be misled or confused by the difference between CFM and size--all custom carb companies sell their carbs by the CFM potential instead of the size because they know that it's harder to get a large sized carb to feel responsive at low rpm with wide throttle angles. You could say that the smaller is easier and bigger is stronger. The smaller sized carbs don't need as much professional help to make them work their best, but the bigger sized carbs will make more horsepower at top end when they are working at their best.
You can't expect to bolt on a carb fresh out of the box, or any race part for that matter, and it be just right. It will need on-the-car tuning to correctly work your combination. The fact is, the only Holley that's bigger than the 850 is the Dominator (the HP 1,000 is an 850 with air-horn treatment.) I explained the above to my new racer buddy and he got a refund and bought a 1050 Dominator. After I "tuned it on the car," it ran better and stronger than ever. I even made it more streetable.
Le Mans Master
Joined: Apr 2004
Posts: 7,106
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From: Newark DE
thanks gkull
I didn't know that about the new HP carbs. so his HP950 is really a 750cfm. That size is fine than.
disregard my comment on it being a "HUGE" carb. my mistake.
Hey, what do you expect? if there call it a 950 i expected it to be a 950.........
d*mn marketing people
I didn't know that about the new HP carbs. so his HP950 is really a 750cfm. That size is fine than.
disregard my comment on it being a "HUGE" carb. my mistake.
Hey, what do you expect? if there call it a 950 i expected it to be a 950.........
d*mn marketing people
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
An HP 950 Holley is a 750 main body with an 850 base. That makes it an 800.
The flip side of the coin is Demon carbs. The venturie/throttle blades are bigger than their holley double pumper counter parts. So the demon 750 is really more like an 800 cfm Holley. that's why Demons can make more HP on a dyno
The flip side of the coin is Demon carbs. The venturie/throttle blades are bigger than their holley double pumper counter parts. So the demon 750 is really more like an 800 cfm Holley. that's why Demons can make more HP on a dyno
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Thread Starter
Pro
Joined: Sep 2004
Posts: 500
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From: Philadelphia PA
I refer to stalling as tuning indicator. Usually (at least with my old 750) I knew I was were leaning out when the car began to studder towards stalling.
What I was getting at was that the idle mixture screws seem to make no change, even when all the way in
(I have a 388 with 215 Dart Pro ported heads. Both Dart and AFR (for the 210 heads) in fact recommended using the 950 HP.)
What I was getting at was that the idle mixture screws seem to make no change, even when all the way in
(I have a 388 with 215 Dart Pro ported heads. Both Dart and AFR (for the 210 heads) in fact recommended using the 950 HP.)
Originally Posted by BarryK
why would you want to stall the car out anyway?
I can turn the mixture screws in all the way on my Holley 650cfm on my '65 and it doesn't stall
I can turn the mixture screws in all the way on my Holley 650cfm on my '65 and it doesn't stall
Pro
Joined: Jun 2005
Posts: 660
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From: Fredericksburg VA
Gkull is correct. You will need to do some on cat test and tune. To start with it sounds like your primary blade to transition slot is not set correctly. This is causing you carb to run on the transition circuit instead of just the idle circuit. Pull the carb and turn it over. (Drain fuel first) Check to see if the primary and secondary blades are open the same amount. I find that if my secondaries are slightly opened more I have better tunability. From the underside use a small slotted screwdriver to adjust the secondaries. For a start adjust the primary side to show about 20 thousandths of the transition slot. This should get you carb back on the idle circuits and off the transition circuit. Now you should be able to use you idle mixture screws properly. (Way lean stall to way rich) See if this gets you back on the ball field.
Waterplay
Waterplay
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Not working idle screws bugs me. Something is not right. I've been trying to find an article on the 950 HP.
My original 383 heads were the Dart pro 1 215cc - I ordered them from Dart with the optional 2.08/1.625 valves setup with springs for a solid roller. They arrived with bowl work and i had them measured. They were right around 221 cc. Nice trouble free heads
My original 383 heads were the Dart pro 1 215cc - I ordered them from Dart with the optional 2.08/1.625 valves setup with springs for a solid roller. They arrived with bowl work and i had them measured. They were right around 221 cc. Nice trouble free heads
Safety Car
Joined: Jul 2002
Posts: 3,770
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From: St Louis MO
Originally Posted by waterplay
To start with it sounds like your primary blade to transition slot is not set correctly. This is causing you carb to run on the transition circuit instead of just the idle circuit.
Brett
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
That is correct the 950 HP carb is smaller than the 850 DP, and the 825 Race Demon flows more than both of them, go figure
That carb is probably running so rich the idle mixture screws do very little, probably have to decrease the size of the idle feed restrictors, they are screw in on the HP carbs, tiime to get some 6-32 blanks and do some experimenting. I went from .037" stock to .031" in my race demon and it made the idle mixture screws more responsive
That carb is probably running so rich the idle mixture screws do very little, probably have to decrease the size of the idle feed restrictors, they are screw in on the HP carbs, tiime to get some 6-32 blanks and do some experimenting. I went from .037" stock to .031" in my race demon and it made the idle mixture screws more responsive
Pro
Joined: Jun 2005
Posts: 660
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From: Fredericksburg VA
Highhat
If setting the primary and secondary throttle blades properly don’t work its time to get dirty and do what Motorherd suggests. Lets hope that's not the case. That’s just a tedious PITA to do. Good luck. If it works you can buy me a beer.
Waterplay
Brettmc
Who is waterman? Is that the guy who goes out and gets the bushel of crabs for us?
If setting the primary and secondary throttle blades properly don’t work its time to get dirty and do what Motorherd suggests. Lets hope that's not the case. That’s just a tedious PITA to do. Good luck. If it works you can buy me a beer.
Waterplay
Brettmc
Who is waterman? Is that the guy who goes out and gets the bushel of crabs for us?
Safety Car
Joined: Jul 2002
Posts: 3,770
Likes: 3
From: St Louis MO
Originally Posted by waterplay
Brettmc
Who is waterman? Is that the guy who goes out and gets the bushel of crabs for us?
Who is waterman? Is that the guy who goes out and gets the bushel of crabs for us?
Seriously, I don't know where I got waterman.
Pro
Joined: Jan 2004
Posts: 535
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From: Darwin, Australia NT
950 jetting
What jetting do you run on your 950. If it only has 750 venturis and two power valves the factory 78 jetting all round seems way too fat. Ive had to take lots of fuel out of mine (down to 70/74). Frustrated without a wide band, two places I tried wouldnt ship to Australia and suddenly my card isnt accepted by Summit. Arrrgh!!!
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Joined: Sep 2004
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From: Philadelphia PA
Idle Adjustment Follow-up
Follow-up: It seems that the slot on the secondaries starts deeper in the throttle bore than the primary, thus not enabling me to get an 0.02 transition slot.
So I would presume that to make the primaries and secondaries balance, you would set the primaries at 0.02, and then set the secondaries at the equivalent throttle blade depth or opening?
So I would presume that to make the primaries and secondaries balance, you would set the primaries at 0.02, and then set the secondaries at the equivalent throttle blade depth or opening?