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Re: How to add vac. advance port to Holley carb?
[Re: JimG]
#571760
01/07/10 07:14 AM
01/07/10 07:14 AM
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Joined: Feb 2007
Posts: 316
Coolville, Ohio
Lew
enthusiast
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enthusiast
Joined: Feb 2007
Posts: 316
Coolville, Ohio
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Quote:
If your front metering block has no vacuum port, it's almost certain that the main body and baseplate also lack the required orfices.
You'll need to drill the 90* intersecting passages in the baseplate. You can use a gasket to get the location of the vertical passage and eyeball the horizontal one, copying another carb (if you have one) that has ported vacuum.
You'll also need to drill 90* intersecting passages in the main body, again using gaskets as the template.
I've always opted to replace the metering block with one already equipped with the port, although some people report successfully drilling them themselves. I'm no machinist, so I just take the easy way out there.
It's not a big deal at all as long as you take your time.
Jim
at this point i guess it goes without saying that all these carbs are not created equal,all i did was drill, tap, and nipple my metering block and i had spark adv. vacuum, got lucky i guess.
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Re: How to add vac. advance port to Holley carb?
[Re: DaveRS23]
#571761
01/07/10 09:54 AM
01/07/10 09:54 AM
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Joined: Jul 2007
Posts: 531
Virginia
JimG
mopar
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mopar
Joined: Jul 2007
Posts: 531
Virginia
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Quote:
Okay, if the base plate is easier to plumb, then that is the route I would prefer to go. Besides, if I end up buying parts, wouldn't it be a lot easier to buy a base that I wouldn't have to re-tune like a metering block?
I'm afraid I might have been unclear: your carb must have the necessary passages in the baseplate AND the main body AND the metering block for this to work. Sorry for any confusion. Still, it's not a big deal, so don't throw in the towel!
Maybe you'll get lucky like Lew and some of the work will already be done for you.
Attached is a pic of a 950 HP baseplate I drilled to add ported vacuum.
Use a baseplate gasket to get the location of the vertical passage. The depth isn't critical as long as you don't go all the way through. I never documented the depth; I just measure another baseplate and duplicate it. I can measure a few I've got laying around if someone needs the number.
For the horizontal passage, I pick my spot (again using measurements taken from another baseplate), tap it with a punch, and drill until the two passages intersect.
For both passages, I always start with a small bit and work my way up.
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Re: How to add vac. advance port to Holley carb?
[Re: DaveRS23]
#571763
01/07/10 10:20 AM
01/07/10 10:20 AM
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Joined: Jul 2007
Posts: 531
Virginia
JimG
mopar
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mopar
Joined: Jul 2007
Posts: 531
Virginia
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In my experience (your results may vary), on 750 and smaller carbs, the horizontal passage will intersect the vertical passage and align perfectly with ported vacuum passage on the metering block. On 850 and larger carbs, you must use a dremel or something to cut a short channel similar to the one shown in the photo. Otherwise, when you bolt on the metering block, the vacuum passage will not align with the one in the main body, and there will be no continuous path. An alternative is to cut this slot in the metering block gasket, but you'll have to remember to do it every time you change the gasket.
So, here's the proceedure: use a baseplate gasket as a template and drill the vertical passage.
CHECK THE FOLLOWING CAREFULLY FOR YOURSELF BEFORE PROCEEDING: On 750 and smaller carbs, use a metering block gasket as a template and drill the horizontal pasage so it intersects the vertical one you just drilled. On 850 and larger, drill the horizontal passage in the correct location so it intersects, irrespecitive of gasket hole location. Then cut out a channel as shown with a dremel, or modify the metering block gasket with a similar channel. AGAIN, DOUBLE CHECK THE HORIZONTAL PASSAGE LOCATION FOR YOURSELF CAREFULLY FOR YOURSELF BEFORE PROCEEDING. This paragraph represents only my personal experience. You may find an exception. The moral is - check carefully and have a plan before you start and it's really not a big deal.
The attached photo is an 850 main body with the described horizontal channel. It also shows the location of the hole that needs to be drilled.
ETA: I'm not sure if 850 is the smallest size that needs the little horizontal channel. At some point above 750, the passages in the main body are moved forward and toward the center of the carb to accomodate the larger bore size. I've never examined an 800 to see where these passages are. Every 750/down main body I've seen have the passages in the "standard" location. Every 850 I've seen has them moved to the location shown in this photo.
If someone knows where they are on a 800, please chime in.
Last edited by JimG; 01/07/10 10:46 AM.
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Re: How to add vac. advance port to Holley carb?
[Re: JimG]
#571765
01/07/10 08:49 PM
01/07/10 08:49 PM
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Joined: Jan 2003
Posts: 12,078
Benton, IL.
DaveRS23
OP
Special needs idiot
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OP
Special needs idiot
Joined: Jan 2003
Posts: 12,078
Benton, IL.
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YEAH!!!!! That's the info I need. I figured it wasn't that hard, I just wanted some experience to go by. Thanks a lot for sticking with this thread, I really appreciate it. Dave
Master, again and still
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Re: How to add vac. advance port to Holley carb?
[Re: Dcuda69]
#571766
01/07/10 09:20 PM
01/07/10 09:20 PM
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Joined: Jan 2003
Posts: 12,078
Benton, IL.
DaveRS23
OP
Special needs idiot
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OP
Special needs idiot
Joined: Jan 2003
Posts: 12,078
Benton, IL.
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Quote:
This is a good thread!!! I'm interested in your combo...engine,ignition,trans,gear etc.I've tried a couple times to make my vac. advance work(MP dist)but usually end up just leaving the hose off. I know I have to shorten the throw on the arm(too much advance)just have not done it yet. Keep us posted on how it works out for you!! 
Always glad to talk about my car. 
I run a local true street tire King of Street class with the car and have worked very hard to make the car as fast as I can while keeping a relatively stock look AND be able to drive it as much as I want on the street on 93 pump.
While tuning with Drag Radials I ran a best this summer of 1.5 60', 6.89 @ 105. On street tires I can run low 1.6s 60', 7.10s @ 104, if I don't screw up the launch.
The car is a 70 Barracuda cloned to look like a Cuda. 3,450 lbs with me and fuel.
RB Ohio Crank kit stroked to 499. Eddies with MCH porting. Scott Brown solid cam. Indy -3 intake with 9375 Dom 2 circuited with the 850 metering blocks. All under a Shaker hood. Mopar electronic dist. with Summit ignition box.
PTC 4,100 stall converter, full auto home-built 727 done with info from here on Moparts. With the A&A governor and Rick Allison's help. I race the car in drive - no manual shifting
3.91s in 8 3/4 with Dr. Diff axles and clutch Sure-Grip with extra springs. Cal-Tracs and offset springs. 325 tires on 10" Cragars.
With vacuum advance, this ride will just about be perfect-------for me!
Master, again and still
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Re: How to add vac. advance port to Holley carb?
[Re: DaveRS23]
#571767
01/07/10 09:30 PM
01/07/10 09:30 PM
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Joined: Jul 2007
Posts: 531
Virginia
JimG
mopar
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mopar
Joined: Jul 2007
Posts: 531
Virginia
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Quote:
YEAH!!!!! That's the info I need. I figured it wasn't that hard, I just wanted some experience to go by. Thanks a lot for sticking with this thread, I really appreciate it. Dave
Great, let me know how it works out for you.
If you take apart a carb that already has ported vacuum, you can reverse engineer it, especially with the info I gave you.
Your car sounds cool - this should make it a better cruiser!
Jim
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Re: How to add vac. advance port to Holley carb?
[Re: screamindriver]
#571770
01/08/10 09:15 AM
01/08/10 09:15 AM
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Joined: Jul 2007
Posts: 531
Virginia
JimG
mopar
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mopar
Joined: Jul 2007
Posts: 531
Virginia
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Quote:
For a while now I've been contemplating the addition of the vacuum channel on the Promax 6-bbl metering block...It's typically a race piece but I'm wondering about the advantages for street cruizing by doing the mod...Looks like some real work would need to be done...Here's a comparison of the factory block compared to the Promax billet piece....Note the small hole and vacuum channel on the original piece 
Yes, your block looks like every other billet block I've seen...the dogleg channel for the ported vacuum isn't there.
It looks like surgery by a machinist the put it there, probably not worth the effort.
Come to think of it, has anyone ever seen a billet block equipped with ported vacuum?
Jim
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Re: How to add vac. advance port to Holley carb?
[Re: JimG]
#571771
01/09/10 04:10 AM
01/09/10 04:10 AM
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Joined: Jan 2003
Posts: 5,791
Hamilton, Ontario Canada
Magnum
master
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master
Joined: Jan 2003
Posts: 5,791
Hamilton, Ontario Canada
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Before you do any drilling or purchasing a metering block.
Try manifold vacuum advance. I've tried both and find ZERO difference in mileage.
I know you will not see this in an slippery automatic but I have slightly more power in the first 1/4" of throttle with my 5 speed manual.
It idles higher with advance during idle which makes me wonder if that is better for mileage.
When I did the comparison I was just moving the vacuum line from ported to manifold with no other changes to the carb.
69 Super Bee, 93 Mustang LX, 04 Allure Super
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