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Author
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Topic: Jet Sizing between primary & secondary
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GregK
Gearhead Posts: 165
From: Clear Spring, Md
Registered: Mar 2004
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posted 09-23-2004 07:30 AM
   
I just aquired a Barry Grant Speed Demon carb that is a 750 in size with vacum secondaries. My question is how far should the sizes in jetting be between the primary and the secondary? I was using a holley street avenger 670 with vacum secondaries and the jetting was 70's in the front and 72's in the rear. There is no power valve in the rear so should the jetting be 6-8 sizes larger than in the front ? There was no power valve in the rear of the street avenger also. I've got the jetting set up on the Speed Demon with 72's in the front and 74's in the rear. Would this be a good starting point with my set up?------------------
1968 coupe 351W, Bored.030,Stock Crank, Stock Rods,Decked,KB Hyper. flat top pistons. Bullet Cam 231/[email protected]/.541 lift,Stock heads,RPM Air Gap,Holley 670 Street Avenger,MSD Ignition,C4 Trans.,9" rear with 4.11 posi. best time 12.381 @ 105.04 mph in the 1/4
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bluestreek
Gearhead Posts: 1724
From: Athens,GA
Registered: Jul 2001
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posted 09-23-2004 10:19 AM
The last BG 750 I played with was 77/84 and some needed larger, so don't be afraid to go bigger if it feels boggy. More agressive squirter cams are usually needed too. The high flow venturis and downleg boosters need more jet than a standard straightleg Holley. The 4 corner idle circuits are set up richer for this reason. Make sure to set the front and back butterflies equally just below the transition slots and set all 4 idle screws about 1 full round out. Then after you fire it up be sure to keep all settings as close to the same as possible when making your final idle adjustments and check that the float levels are about 1/2 way up the sight glass. After that you're on your on.------------------
1966 Mustang Coupe: Custom glass hood and BIG scoop sits atop a 289 stroked to 331 c.i., Steel crank, rods and girdle, TFS alum. heads, Stealth 8020 intake, Xtreme 268 Solid Roller, Holley 750 HP, long tubes, 4speed, 9" 3.50 posi.
11.86 @ 116 mph (7.62 @ 93 mph)daily driver!
DanH
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GregK
Gearhead Posts: 165
From: Clear Spring, Md
Registered: Mar 2004
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posted 09-23-2004 10:32 AM
Bluestreak, when you say to set the butterflies should any of the transition slots be exposed or not at all? Also are the holley pump cams compatiable with the BG? Right now with the set up I have on this carb (72front/74 rear, silver vac sec. spring) The four corner idle screws are only 1/2 turn out for best idle with vacum guage.------------------
1968 coupe 351W, Bored.030,Stock Crank, Stock Rods,Decked,KB Hyper. flat top pistons. Bullet Cam 231/[email protected]/.541 lift,Stock heads,RPM Air Gap,Holley 670 Street Avenger,MSD Ignition,C4 Trans.,9" rear with 4.11 posi. best time 12.381 @ 105.04 mph in the 1/4
IP: Logged |
bluestreek
Gearhead Posts: 1724
From: Athens,GA
Registered: Jul 2001
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posted 09-23-2004 11:03 AM
1/2 turn out is not unusual on a 4 corner system. The larger the cam the more air and fuel you'll need for good idle. As long as the transition slots are positioned equally between front and back and are not exposed more than .020" (when viewing from the top) you're OK. Ideally I like mine to be dead even with the top edge, but .020" higher or lower is a good range to be in for good off idle response. Holley pump cams are compatible and a quicker/stronger shot is usually needed for a engine that has to pull at lower rpms. Looks like you have the VE series with the vacuum secs.. The silver spring may be OK but try the yellow one anyway after you get the carb dialed in. BTW That carb came with 76/83 jets. That's the same thing I run in my 750 HP (downleg boosters) in the summer. Since it got cool I had to go up to 77/86 and picked up about 2 mph! ------------------
1966 Mustang Coupe: Custom glass hood and BIG scoop sits atop a 289 stroked to 331 c.i., Steel crank, rods and girdle, TFS alum. heads, Stealth 8020 intake, Xtreme 268 Solid Roller, Holley 750 HP, long tubes, 4speed, 9" 3.50 posi.
11.86 @ 116 mph (7.62 @ 93 mph)daily driver!
DanH [This message has been edited by bluestreek (edited 09-24-2004).]
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GregK
Gearhead Posts: 165
From: Clear Spring, Md
Registered: Mar 2004
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posted 09-23-2004 01:19 PM
Thanks for all the help Bluestreak. One more question, what size squirter came with this carb. Yes it is a VE series 750------------------
1968 coupe 351W, Bored.030,Stock Crank, Stock Rods,Decked,KB Hyper. flat top pistons. Bullet Cam 231/[email protected]/.541 lift,Stock heads,RPM Air Gap,Holley 670 Street Avenger,MSD Ignition,C4 Trans.,9" rear with 4.11 posi. best time 12.381 @ 105.04 mph in the 1/4
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grego37
Gearhead Posts: 132
From: los angeles,CA,USA
Registered: May 2004
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posted 09-23-2004 02:39 PM
I was allways told when you dont use a rear power valve, bump up the rear jet size 6 sizes.
example:
if your carb has 72 pri and 74 sec. and you decide to remove the rear pwr. valve, you'd need to change the sec. jets size to 80.
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bluestreek
Gearhead Posts: 1724
From: Athens,GA
Registered: Jul 2001
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posted 09-23-2004 11:33 PM
Here you go. Use this page to pull up the specs on any new Demon carb.http://www.barrygrant.com/demon/default.aspx?page=29&specs=29cc ------------------
1966 Mustang Coupe: Custom glass hood and BIG scoop sits atop a 289 stroked to 331 c.i., Steel crank, rods and girdle, TFS alum. heads, Stealth 8020 intake, Xtreme 268 Solid Roller, Holley 750 HP, long tubes, 4speed, 9" 3.50 posi.
11.86 @ 116 mph (7.62 @ 93 mph)daily driver!
DanH
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GregK
Gearhead Posts: 165
From: Clear Spring, Md
Registered: Mar 2004
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posted 09-24-2004 03:47 PM
Blue streak thanks for all the help. I found the BG web site before you sent it to me. Its a good thing because I was a little lean on the jetting. Stock set up called for 76/83 on the jets, 31 squirter, and silver Vac. Sec. spring. I set mine up with 78/84 jets, 35 squirter, and silver spring in secondary. Before I had the jetting set at 72/74 and noticed the exhause pipes looked a little white after a few short blast down our country road. Now they have a nice grayish color and much darker.It pulls a lot harder now with no bog from the secondaries.  I have to wait a week before I can got to the track and test. I have to fly to Tucson in the morning.  for work will be gone a whole week,I don't know if I can wait that long. Thanks againGreg
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