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Pick a head, any head.

General engine tech -- Drag Racing to Circle Track

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1980RS
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Re: Pick a head, any head.

Post by 1980RS »

c1500sbc wrote: Thu Jun 10, 2021 12:01 am I’m looking to change the heads on my 383. Currently running some Promaxx maxx series 200 heads. I really think they are holding the motor back.

Mostly a street driven truck. 95 Silverado short box, 2wd with a 5 speed. 10.68 static compression. Running 93 pump gas. Street avenger carb. Crower 238/240 at .050 mid .500 lift roller cam.

Any suggestions?
You are wrong on the heads holding you back, it's the Avenger carb that 's the problem.
c1500sbc
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Re: Pick a head, any head.

Post by c1500sbc »

1980RS wrote: Thu Jun 10, 2021 1:57 pm
c1500sbc wrote: Thu Jun 10, 2021 12:01 am I’m looking to change the heads on my 383. Currently running some Promaxx maxx series 200 heads. I really think they are holding the motor back.

Mostly a street driven truck. 95 Silverado short box, 2wd with a 5 speed. 10.68 static compression. Running 93 pump gas. Street avenger carb. Crower 238/240 at .050 mid .500 lift roller cam.

Any suggestions?
You are wrong on the heads holding you back, it's the Avenger carb that 's the problem.
Would you mind explaining why the carb, from an Avenger to a a double pumper, would make such a difference?
1980RS
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Re: Pick a head, any head.

Post by 1980RS »

c1500sbc wrote: Thu Jun 10, 2021 2:39 pm
1980RS wrote: Thu Jun 10, 2021 1:57 pm
c1500sbc wrote: Thu Jun 10, 2021 12:01 am I’m looking to change the heads on my 383. Currently running some Promaxx maxx series 200 heads. I really think they are holding the motor back.

Mostly a street driven truck. 95 Silverado short box, 2wd with a 5 speed. 10.68 static compression. Running 93 pump gas. Street avenger carb. Crower 238/240 at .050 mid .500 lift roller cam.

Any suggestions?
You are wrong on the heads holding you back, it's the Avenger carb that 's the problem.
Would you mind explaining why the carb, from an Avenger to a a double pumper, would make such a difference?
If it's a 750 Avenger there is no reference tube in the pri side just a hole and those carbs are notorious for sec side not opening all the way or at all. Like I said in my other post I have fixed a bunch of them last winter and just tested the 770 at the track and it now preforms as well as my 750 HP vs, which I also added the tube to, both carbs were within .002 of each other. That HP carb picked up over .5 and 4 mph with just that one mod compared to when I tested it stock. I am only talking on the 750 Avenger, if you are running the 670 I have not tried one of those yet.
c1500sbc
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Re: Pick a head, any head.

Post by c1500sbc »

1980RS wrote: Thu Jun 10, 2021 4:55 pm
c1500sbc wrote: Thu Jun 10, 2021 2:39 pm
1980RS wrote: Thu Jun 10, 2021 1:57 pm

You are wrong on the heads holding you back, it's the Avenger carb that 's the problem.
Would you mind explaining why the carb, from an Avenger to a a double pumper, would make such a difference?
If it's a 750 Avenger there is no reference tube in the pri side just a hole and those carbs are notorious for sec side not opening all the way or at all. Like I said in my other post I have fixed a bunch of them last winter and just tested the 770 at the track and it now preforms as well as my 750 HP vs, which I also added the tube to, both carbs were within .002 of each other. That HP carb picked up over .5 and 4 mph with just that one mod compared to when I tested it stock. I am only talking on the 750 Avenger, if you are running the 670 I have not tried one of those yet.
Sorry, maybe I missed where you said you had fixed a bunch of them. I feel like an idiot here, but can you tell me how you added the reference tube?
1980RS
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Re: Pick a head, any head.

Post by 1980RS »

c1500sbc wrote: Thu Jun 10, 2021 5:22 pm
1980RS wrote: Thu Jun 10, 2021 4:55 pm
c1500sbc wrote: Thu Jun 10, 2021 2:39 pm

Would you mind explaining why the carb, from an Avenger to a a double pumper, would make such a difference?
If it's a 750 Avenger there is no reference tube in the pri side just a hole and those carbs are notorious for sec side not opening all the way or at all. Like I said in my other post I have fixed a bunch of them last winter and just tested the 770 at the track and it now preforms as well as my 750 HP vs, which I also added the tube to, both carbs were within .002 of each other. That HP carb picked up over .5 and 4 mph with just that one mod compared to when I tested it stock. I am only talking on the 750 Avenger, if you are running the 670 I have not tried one of those yet.
Sorry, maybe I missed where you said you had fixed a bunch of them. I feel like an idiot here, but can you tell me how you added the reference tube?
I think there is a site that has all the info on how to do tube mod, it's not very hard but the results are worth the work.

https://www.tapatalk.com/groups/racingf ... t2166.html
F-BIRD'88
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Re: Pick a head, any head.

Post by F-BIRD'88 »

1980RS wrote: Thu Jun 10, 2021 6:45 pm
c1500sbc wrote: Thu Jun 10, 2021 5:22 pm
1980RS wrote: Thu Jun 10, 2021 4:55 pm

If it's a 750 Avenger there is no reference tube in the pri side just a hole and those carbs are notorious for sec side not opening all the way or at all. Like I said in my other post I have fixed a bunch of them last winter and just tested the 770 at the track and it now preforms as well as my 750 HP vs, which I also added the tube to, both carbs were within .002 of each other. That HP carb picked up over .5 and 4 mph with just that one mod compared to when I tested it stock. I am only talking on the 750 Avenger, if you are running the 670 I have not tried one of those yet.
Sorry, maybe I missed where you said you had fixed a bunch of them. I feel like an idiot here, but can you tell me how you added the reference tube?
I think there is a site that has all the info on how to do tube mod, it's not very hard but the results are worth the work.

https://www.tapatalk.com/groups/racingf ... t2166.html
I think Tuner posted about this before.
c1500sbc
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Re: Pick a head, any head.

Post by c1500sbc »

Someone should rename this thread "why not to jump to conclusion!" :D

Here is a copy of the instructions I pulled on adding the secondary tube. All of it makes sense. Pretty straight forward, except for the very last part. At the bottom it says to "Plug the passage where you removed the lead plug, either with another lead plug or tap the hole with 8-32 thread and use a setscrew" Maybe it will make more sense when I start to do it, but aren't I adding the tube where the lead plug used to be? It sounds like i'm removing the plug and then drilling the hole where the plug is/was?



The ”715”, “735”, “750”, “780”, “800” and “850” (1&5/16” and larger primary venturi) vacuum secondary carbs will not function correctly without the little slash-cut brass tube in the primary which was in all the old carbs in the vacuum passage to the secondary diaphragm, to amplify the secondary opening signal. Holley quit putting that tube in the carbs more than 30 years ago. Dumbassery at it’s finest for 99¢. Without it, the carbs open the secondary late and not all the way, even with the lightest spring.

The secondary butterfly is offset with the larger area on the upstream side of the throttle shaft and the impact force of the air flow is strong enough to keep the throttle from going all the way open. The more the secondary opens, the less air flows through the primary and the less the opening signal becomes until the force of the diaphragm and the air impact on the offset butterfly balance.

The sharp edged venturi don't have the problem and the carbs smaller than 600 CFM don't have the problem, though I have added the tubes to some on occasion when the secondary is desired to function as one-to-one at low engine speed. When there is two carbs the air flow per each is halved and the secondary can be lazy sometimes, the little tube quickens things up and stronger springs can be used.

A lead plug seals the passage from the vacuum diaphragm mounting boss to the primary venturi. Center punch the lead plug and drill it out with a 1/8” drill. Drill only deep enough to remove the plug, about 3/16” or so. The drill bit will catch it and pull it out.

Drill through the passage into the venturi with .120” (#31), take care to not hit the bottom of the booster.

Use a smaller bit or toothpick and Sharpie mark to make a gauge of the length of the hole to the venturi wall, it will probably be a little over an inch. Use this gauge to Sharpie mark the succeeding drill bits so you can drill to the desired depth.

Drill with .125” to a depth which leaves about 3/16” of the .120”. Drill with .128” (#30) to a depth that leaves about 3/16” of the .125”.

From the venturi wall back to the diaphragm mounting boss the hole is stepped with 3/16” of .120”, 3/16” of .125” and the remainder .128”.

Brass tubing is available at most hardware stores. A foot of 1/8” ID x 3/32” ID is about $2.50. Cut a piece about ¾” long. File one end at a 45° angle and file a screwdriver slot in the other end. Align the slot with the angled tip so you can clock it in position before driving the tube in place. You need a 1/8” diameter screwdriver.

Put the tube in the passage, properly align the tip angle, and use a 1/8” punch and a small hammer to drive it in place with about 1/8” protruding into the venturi bore.

Plug the passage where you removed the lead plug, either with another lead plug or tap the hole with 8-32 thread and use a setscrew.

Do not plug the bleed hole in the secondary venturi.
F-BIRD'88
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Re: Pick a head, any head.

Post by F-BIRD'88 »

You keep this up, we going to name you "The Carb Whisperer".. When you done fixing this Holley you can get back to finishing the job on that Edelbrock carb.
c1500sbc
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Re: Pick a head, any head.

Post by c1500sbc »

F-BIRD'88 wrote: Fri Jun 11, 2021 5:41 pm You keep this up, we going to name you "The Carb Whisperer".. When you done fixing this Holley you can get back to finishing the job on that Edelbrock carb.
Haha! I gave up on the Edelbrock. Sure did learn a lot from guys like yourself though. No matter what I did I couldn't get the motor and edelbrock to get along. I bought a few carb tuning/modifying books and i'm really trying to slow down, learn/perfect the basics and get this one ironed out.
1980RS
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Re: Pick a head, any head.

Post by 1980RS »

All my Eddy carbs run great, they are just slow as all hell at the track. :lol:
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