jets sizes?
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
jets sizes?
Holley double pumper 750cfm
do the secondarys jets need to be bigger than the primaries by so many sizes??
do the jets need to stay with in a size range to each other?
is it ok to have the primaries jets bigger than the secondarys?
thanks
dave
do the secondarys jets need to be bigger than the primaries by so many sizes??
do the jets need to stay with in a size range to each other?
is it ok to have the primaries jets bigger than the secondarys?
thanks
dave
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,443
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
to the poster. the only way to really setup a carb is on an engine dyno with all the sniffers and inputs. On the primary you want to run as lean as possible and the secondary right at the max rich A/F for the most power. My Demon achived this with a spread of 10 jet sizes from front to back.
without a dyno. lean the primary jets down 2 numbers at a time till it runs ruff and kind of surges at a steady state cruise down a level highway at 60 -65 mph. Then back up 1-2 jet sizes to where it runs nice again. Check the plugs for a white- light tan color. Secondary is best done with a chase car. be talking on the cell phone and floor it to WOT in top gear. the chase car should see an initial blast of black smoke from the squirters and then nothing. Black should go away and no smoke at all
Last edited by gkull; Dec 29, 2008 at 11:31 AM.
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
Please explain to me how this could possibly work?
to the poster. the only way to really setup a carb is on an engine dyno with all the sniffers and inputs. On the primary you want to run as lean as possible and the secondary right at the max rich A/F for the most power. My Demon achived this with a spread of 10 jet sizes from front to back.
without a dyno. lean the primary jets down 2 numbers at a time till it runs ruff and kind of surges at a steady state cruise down a level highway at 60 -65 mph. Then back up 1-2 jet sizes to where it runs nice again. Check the plugs for a white- light tan color. Secondary is best done with a chase car. be talking on the cell phone and floor it to WOT in top gear. the chase car should see an initial blast of black smoke from the squirters and then nothing. Black should go away and no smoke at all
to the poster. the only way to really setup a carb is on an engine dyno with all the sniffers and inputs. On the primary you want to run as lean as possible and the secondary right at the max rich A/F for the most power. My Demon achived this with a spread of 10 jet sizes from front to back.
without a dyno. lean the primary jets down 2 numbers at a time till it runs ruff and kind of surges at a steady state cruise down a level highway at 60 -65 mph. Then back up 1-2 jet sizes to where it runs nice again. Check the plugs for a white- light tan color. Secondary is best done with a chase car. be talking on the cell phone and floor it to WOT in top gear. the chase car should see an initial blast of black smoke from the squirters and then nothing. Black should go away and no smoke at all
what do you mean by as lean as possible on the primary jets?, what exact level 12.5 or leaner??
when accerating Iam running at 14 to 15 on the primarys, then when I get into the secondary's it goes down to 11's
Iam at 70 front and 82 back at present, ive got a 383, big blower cam, tight convertor 2200 stall and high diff 308, I did have a off idle stumble, solved with bigger primary shooters and 50cc accerator kit
let me what you think?
cheers
dave
Last edited by corvettedave383; Dec 29, 2008 at 12:36 PM.
Pro
Joined: Dec 2006
Posts: 665
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From: wilmington nc
I doubt i can explane how it works, I"d be more than happy to talk about it though. So far everyone i have told this to is puzzled how it works. Most people say: how can you be riding around on large jets in the front all the time, makes no sence at all!!! Here"s a question for you to think about, lets say we have 70 jets in front and rear. When you make a dyno pull the engine goes lean first half of the pull, then on the second half of the pull it goes rich. What you going to do? What we did was increase the jet size in the front (to stop it from going lean) then we decreased the jet size in the rear (to stop it from going rich)
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Melting Slicks
Joined: Oct 2002
Posts: 2,314
Likes: 111
From: St Louis MO
Before you attempt to tune your carb, put it into the stock configuration and try that first...
http://www.holley.com/data/TechServi...%20Listing.pdf
If you have any way to measure your air-to-fuel ratios, cruise should be about 14 or 15:1, and full throttle should be 12.5 or 13.0:1.
http://www.holley.com/data/TechServi...%20Listing.pdf
If you have any way to measure your air-to-fuel ratios, cruise should be about 14 or 15:1, and full throttle should be 12.5 or 13.0:1.
Melting Slicks
Joined: Oct 2002
Posts: 2,314
Likes: 111
From: St Louis MO
I have a o2 sensor installed and the inovate box plugged into a laptop, I can record live date driving along the road, which I would have thought would be better than a rolling road, your getting real airflow, etc
what do you mean by as lean as possible on the primary jets?, what exact level 12.5 or leaner??
when accerating Iam running at 14 to 15 on the primarys, then when I get into the secondary's it goes down to 11's
Iam at 70 front and 82 back at present, ive got a 383, big blower cam, tight convertor 2200 stall and high diff 308, I did have a off idle stumble, solved with bigger primary shooters and 50cc accerator kit
let me what you think?
cheers
dave
what do you mean by as lean as possible on the primary jets?, what exact level 12.5 or leaner??
when accerating Iam running at 14 to 15 on the primarys, then when I get into the secondary's it goes down to 11's
Iam at 70 front and 82 back at present, ive got a 383, big blower cam, tight convertor 2200 stall and high diff 308, I did have a off idle stumble, solved with bigger primary shooters and 50cc accerator kit
let me what you think?
cheers
dave
Melting Slicks
Joined: Oct 2002
Posts: 2,314
Likes: 111
From: St Louis MO
If you're not running a blower, I'd recommend 35 shooters. If that doesn't help, go to 72 primary jets, and further to 74 primary jets. On the secondary side, I'd cut back to 78 right away.
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
I doubt i can explane how it works, I"d be more than happy to talk about it though. So far everyone i have told this to is puzzled how it works. Most people say: how can you be riding around on large jets in the front all the time, makes no sence at all!!! Here"s a question for you to think about, lets say we have 70 jets in front and rear. When you make a dyno pull the engine goes lean first half of the pull, then on the second half of the pull it goes rich. What you going to do? What we did was increase the jet size in the front (to stop it from going lean) then we decreased the jet size in the rear (to stop it from going rich)
dave
Thread Starter
Burning Brakes
Joined: Apr 2008
Posts: 825
Likes: 40
From: Glasgow, Scotland
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,443
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
I doubt i can explane how it works, I"d be more than happy to talk about it though. So far everyone i have told this to is puzzled how it works. Most people say: how can you be riding around on large jets in the front all the time, makes no sence at all!!! Here"s a question for you to think about, lets say we have 70 jets in front and rear. When you make a dyno pull the engine goes lean first half of the pull, then on the second half of the pull it goes rich. What you going to do? What we did was increase the jet size in the front (to stop it from going lean) then we decreased the jet size in the rear (to stop it from going rich)
The primary cuircut gets fuel from the idle cuircut/power valve, and primary jets. that is why the primary used much small jets than the secondary. It has three fuel sources to make the 14-15 A/F ratio.
Your secondary has an idle curcuit if you have the 4 corner idle screws, but the butterflies don't open until you start to floor the car and you want a richer 12 ish fuel ratio to make max power.
When you make a dyno pull the engine goes lean first half of the pull, then on the second half of the pull it goes rich. What you going to do? what you do is find a competant carburator shop and have them set up the air bleeds for the correct flow to the boosters. Your boosters are not flowing enough air at lower rpm to pull the fuel out. I would increase the primary air bleed diameter and go down on the jetting. another fix is the booster type/size. In an overcarbed setup annular boosters work. I kind of like down leg style
Team Owner
Joined: Apr 1999
Posts: 21,953
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From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Pro
Joined: Dec 2006
Posts: 665
Likes: 0
From: wilmington nc
The problem you would have to have to make a carb do this is the Wrong dia air bleeds and or plugged power valve
My carb does not have adjustable air bleeds, it does only have 1 power valve so the rear is plugged.
Your boosters are not flowing enough air at lower rpm to pull the fuel out
Does this have anything to do with low engine vacume? or is it strictly caused by to small of air bleeds......
My carb does not have adjustable air bleeds, it does only have 1 power valve so the rear is plugged.
Your boosters are not flowing enough air at lower rpm to pull the fuel out
Does this have anything to do with low engine vacume? or is it strictly caused by to small of air bleeds......
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,443
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
The problem you would have to have to make a carb do this is the Wrong dia air bleeds and or plugged power valve
My carb does not have adjustable air bleeds, it does only have 1 power valve so the rear is plugged.
Your boosters are not flowing enough air at lower rpm to pull the fuel out
Does this have anything to do with low engine vacume? or is it strictly caused by to small of air bleeds......
My carb does not have adjustable air bleeds, it does only have 1 power valve so the rear is plugged.
Your boosters are not flowing enough air at lower rpm to pull the fuel out
Does this have anything to do with low engine vacume? or is it strictly caused by to small of air bleeds......
the booster flows fuel depending on how much air is rushing by it. You can look in the carb and see the booster. Just open the throttle to 4000 rpm and the booster will look like it is just spraying gas into the air stream. The air bleed diameter and the jet fuel flow determine the amount of fuel that the booster can feed into the fuel stream. which sets your A/F ratio
The type of booster it's self determines at what air flow it workd the best. Like I said annular for over carbed and big cammed engine that have lots of low rpm reversion.
second is the down leg booster. Closer to the butterflies to get the faster air speed.
Straight leg. I would only use them on a higher air flow dual plane equiped motor. Like 400+ ci
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,443
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
Dave, I would try to keep the primaries 8-10 jet sizes smaller than the secondaries. I increased the diameter of my high speed air bleeds and was able to run smaller jets without getting lean popping.
Out of the box double pumpers tend to have too rich at idle. I have drilled my throttle plates with a .110 drill bit. Lots of articles on idle feed restriction methods
http://www.chevyhiperformance.com/te...ics/index.html
Out of the box double pumpers tend to have too rich at idle. I have drilled my throttle plates with a .110 drill bit. Lots of articles on idle feed restriction methods
http://www.chevyhiperformance.com/te...ics/index.html