Dbl Pumper/Tuning - Step In
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
He has a 383 that 750 should be very close right out of the box
OP I had something similar happen, wound up being too much timing but all we can do from here is guess while you pull your hair out
Does this seem to happen around 2800 or so?
If youre tired of dicking with it send it to Jeff at AED with every spec of your motor, car gear etc. It will be perfect when you get it back you may or may not have to adjust the idle speed. Service is top notch.
OP I had something similar happen, wound up being too much timing but all we can do from here is guess while you pull your hair out
Does this seem to happen around 2800 or so?
If youre tired of dicking with it send it to Jeff at AED with every spec of your motor, car gear etc. It will be perfect when you get it back you may or may not have to adjust the idle speed. Service is top notch.
Safety Car
Joined: Jan 2009
Posts: 3,736
Likes: 56
From: anchorage ak
This is exactly where I am at and the objective of the thread is to get some idea's on which way to go with these components. I do think the too much too fast thing is relevant, ( but I still think there could be a timing curve factor here.) There is no stumble or bog. I am going to pull the carb tonight, inventory the component sizes and re-adjust. I like to idea to block off the secondaries and run it on primaries so that will be first step after the carb is re-installed.
The motor spec:
350 block 4.040
Scat 9000 3.75
Scat I beam 5.7 rods
Diamond forged flat top piston -5cc
Howards Retrofit Roller .510/234 .530/238
RHS 72/200 Aluminum Heads
Dart dual plane intake
Dragon Fire HEI
Static Compression 10:1
DCR 8:1
Thanks for the input to date.
The motor spec:
350 block 4.040
Scat 9000 3.75
Scat I beam 5.7 rods
Diamond forged flat top piston -5cc
Howards Retrofit Roller .510/234 .530/238
RHS 72/200 Aluminum Heads
Dart dual plane intake
Dragon Fire HEI
Static Compression 10:1
DCR 8:1
Thanks for the input to date.
Le Mans Master
Joined: Mar 2001
Posts: 5,739
Likes: 644
From: Lincoln, CA
Doc, When I had my 496 build I ordered a 830 DP.
Sent it to Lars to be gone though and setup on his test motor.
Everything ran great but used a lot of fuel and smelled rich on my setup.
Played around with it trying different things but not the main jets.
Finally broke down and ordered a wide band. What I found was during cruse I was really rich. Like 10.5 rich. Idle was lean and the transition was too rich.
Following the article I liked to earlier I had to go down 12 sizes to the get target A/F correct. Idle to 2K is now a nice flat 13.5.
Yes it's taken a lot of time changing bleeds and restrictors but it's running perfect now.
Everything is a guess without having the tools to see what is happening.
Sent it to Lars to be gone though and setup on his test motor.
Everything ran great but used a lot of fuel and smelled rich on my setup.
Played around with it trying different things but not the main jets.
Finally broke down and ordered a wide band. What I found was during cruse I was really rich. Like 10.5 rich. Idle was lean and the transition was too rich.
Following the article I liked to earlier I had to go down 12 sizes to the get target A/F correct. Idle to 2K is now a nice flat 13.5.
Yes it's taken a lot of time changing bleeds and restrictors but it's running perfect now.
Everything is a guess without having the tools to see what is happening.
Last edited by ddawson; Apr 27, 2016 at 01:22 PM.
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
This is exactly where I am at and the objective of the thread is to get some idea's on which way to go with these components. I do think the too much too fast thing is relevant, ( but I still think there could be a timing curve factor here.) There is no stumble or bog. I am going to pull the carb tonight, inventory the component sizes and re-adjust. I like to idea to block off the secondaries and run it on primaries so that will be first step after the carb is re-installed.
The motor spec:
350 block 4.040
Scat 9000 3.75
Scat I beam 5.7 rods
Diamond forged flat top piston -5cc
Howards Retrofit Roller .510/234 .530/238
RHS 72/200 Aluminum Heads
Dart dual plane intake
Dragon Fire HEI
Static Compression 10:1
DCR 8:1
Thanks for the input to date.
The motor spec:
350 block 4.040
Scat 9000 3.75
Scat I beam 5.7 rods
Diamond forged flat top piston -5cc
Howards Retrofit Roller .510/234 .530/238
RHS 72/200 Aluminum Heads
Dart dual plane intake
Dragon Fire HEI
Static Compression 10:1
DCR 8:1
Thanks for the input to date.
Last edited by fishslayer143; Apr 27, 2016 at 12:19 PM.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
1961 Corvette - 283ci/270 HP 'Dual-Quad'
WCFB {#2613S; Front} and {#2614S; Rear}
Rated @ 385 CFM's
"With the Dual WCFB Carbs, this combination put out 770 CFM's for the
deadly 1961 283ci/270 HP {Engine Code #468} in the Corvette.
We ain't doing nothing here that has not already been done.
WCFB {#2613S; Front} and {#2614S; Rear}
Rated @ 385 CFM's
"With the Dual WCFB Carbs, this combination put out 770 CFM's for the
deadly 1961 283ci/270 HP {Engine Code #468} in the Corvette.
We ain't doing nothing here that has not already been done.
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
1961 Corvette - 283ci/270 HP 'Dual-Quad'
WCFB {#2613S; Front} and {#2614S; Rear}
Rated @ 385 CFM's
"With the Dual WCFB Carbs, this combination put out 770 CFM's for the
deadly 1961 283ci/270 HP {Engine Code #468} in the Corvette.
We ain't doing nothing here that has not already been done.
WCFB {#2613S; Front} and {#2614S; Rear}
Rated @ 385 CFM's
"With the Dual WCFB Carbs, this combination put out 770 CFM's for the
deadly 1961 283ci/270 HP {Engine Code #468} in the Corvette.
We ain't doing nothing here that has not already been done.
Instructor
Joined: Apr 2016
Posts: 104
Likes: 7
From: Denver Colorado
1961 Corvette - 283ci/270 HP 'Dual-Quad'
WCFB {#2613S; Front} and {#2614S; Rear}
Rated @ 385 CFM's
"With the Dual WCFB Carbs, this combination put out 770 CFM's for the
deadly 1961 283ci/270 HP {Engine Code #468} in the Corvette.
We ain't doing nothing here that has not already been done.
WCFB {#2613S; Front} and {#2614S; Rear}
Rated @ 385 CFM's
"With the Dual WCFB Carbs, this combination put out 770 CFM's for the
deadly 1961 283ci/270 HP {Engine Code #468} in the Corvette.
We ain't doing nothing here that has not already been done.
You are absolutely right BUT those early 4-barrel WCFB's used a type of vacuum secondaries to prevent too much too soon throttle opening. The secondary butterflies had counter-weighted air valves above them that slowly opened as the engine gained rpm to prevent too much too soon throttle opening. The Carter 4-barrels used the same vacuum secondary idea until the much more accurate AVS style replaced them.
Mechanical secondary carburetors do NOT belong on street engines because they allow too much too soon which is why not a single engine in the world has ever come equipped with them.
And when it comes to accurate carburetor sizing Summit Racing will recommend NO MORE than 1-1/2 cfm per cubic inch of displacement which means a 350" only requires 525 cfm, a 383" only requires 575 cfm, a 427" only requires 640 cfm, and a 454" only requires 681 cfm.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
At WOT the 283ci in 61Vette is using all 770cfm @ 6000RPM that all that is needed to know, not when they kick in or a bunch of other BS.
So much for that ridiculous formula for figuring out CFM. No one uses that anymore, except for those that don't know how to size a carb for any particular engine
So much for that ridiculous formula for figuring out CFM. No one uses that anymore, except for those that don't know how to size a carb for any particular engine
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
At WOT the 283ci in 61Vette is using all 770cfm @ 6000RPM that all that is needed to know, not when they kick in or a bunch of other BS.
So much for that ridiculous formula for figuring out CFM. No one uses that anymore, except for those that don't know how to size a carb for any particular engine
So much for that ridiculous formula for figuring out CFM. No one uses that anymore, except for those that don't know how to size a carb for any particular engine
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,456
Likes: 102
From: Piedmont Va
St. Jude Donor '11-'12-'13,'19-'20
I said it could be too much too fast because you did not elaborate on your engine specs.. and contrary to popular belief, bigger IS NOT always better and dyno testing proves it.. A 750 DP mech sec. on a stock L48 is wasted effort.. The engine will run far better with a 650 cfm or a 780 vac sec like GM put on its Corvette performance sbc engines..then the secondaries open only as far and as soon as the engine can consume the A/F mix.. This was why the Q-Jet was so widely used on GM engines.. BTW, none came with double pumpers . Not even the LT-1... You can get any engine to run even with a 1050 carb, doesn t mean its the best option. That said, your engine is modified enough that it should handle the 750 without much issue. You should concentrate on the Accelerator pump and nozzle..
First, my regrets for re-kindling of religious debates. I have been at this in my garage for many years as most of you have, and have indeed myself acquired sure fire opinions about stuff that I considered bible ...until proven wrong.
I am aware that some folks think that Dp's are not good street carbs. I have also seen some DP's tuned to the point where it was hard to tell there was no FI. Nice smooth linear acceleration through all stages. In my experience you cannot get that from a Vac Sec carb. I used the 780 as an example purely from from the context of CFM.
The 750 is not in any way too much CFM for the 383 so we can end that part of the discussion.
I created this thread at the same place Fishslayer is here; The issue most likely associated with the pump shot, with the secondary jetting and power valve being possible factors. What I was hoping to gain here was some experience based on my symptom? Based on the conversation around "too much too soon", I am going to approach it from that angle and start with the lash adjustment and cam size, then move to the squirter. Stay tuned, and thanks again for all the input!
Race Director
Joined: May 2006
Posts: 18,755
Likes: 2,589
it's amazing how hostile some of these threads get! Back to your problem, I dont think you ever listed the number on the airhorn so I'm going with the assumption that you have 6.5 power valves front and back. Rather than going the extreme route of inserting a plug in the secondary metering block and increasing jet size by 10 numbers, I think that was intended for racing, have you considered a 8.5 power valve which would come in sooner? Still curious what you will find concerning the clearance at the pump arm.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Your response does not even qualify for an answer. I am sure those who have read the whole page know what I am talking about. And I am also glad you will never get near my Vette with logic like you just posted.
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
[/B]
First, my regrets for re-kindling of religious debates. I have been at this in my garage for many years as most of you have, and have indeed myself acquired sure fire opinions about stuff that I considered bible ...until proven wrong.
I am aware that some folks think that Dp's are not good street carbs. I have also seen some DP's tuned to the point where it was hard to tell there was no FI. Nice smooth linear acceleration through all stages. In my experience you cannot get that from a Vac Sec carb. I used the 780 as an example purely from from the context of CFM.
The 750 is not in any way too much CFM for the 383 so we can end that part of the discussion.
I created this thread at the same place Fishslayer is here; The issue most likely associated with the pump shot, with the secondary jetting and power valve being possible factors. What I was hoping to gain here was some experience based on my symptom? Based on the conversation around "too much too soon", I am going to approach it from that angle and start with the lash adjustment and cam size, then move to the squirter. Stay tuned, and thanks again for all the input!
First, my regrets for re-kindling of religious debates. I have been at this in my garage for many years as most of you have, and have indeed myself acquired sure fire opinions about stuff that I considered bible ...until proven wrong.
I am aware that some folks think that Dp's are not good street carbs. I have also seen some DP's tuned to the point where it was hard to tell there was no FI. Nice smooth linear acceleration through all stages. In my experience you cannot get that from a Vac Sec carb. I used the 780 as an example purely from from the context of CFM.
The 750 is not in any way too much CFM for the 383 so we can end that part of the discussion.
I created this thread at the same place Fishslayer is here; The issue most likely associated with the pump shot, with the secondary jetting and power valve being possible factors. What I was hoping to gain here was some experience based on my symptom? Based on the conversation around "too much too soon", I am going to approach it from that angle and start with the lash adjustment and cam size, then move to the squirter. Stay tuned, and thanks again for all the input!
Drifting
Joined: Aug 2009
Posts: 1,346
Likes: 160
From: new iberia la
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
At WOT the 283ci in 61Vette is using all 770cfm @ 6000RPM that all that is needed to know, not when they kick in or a bunch of other BS.
So much for that ridiculous formula for figuring out CFM. No one uses that anymore, except for those that don't know how to size a carb for any particular engine
So much for that ridiculous formula for figuring out CFM. No one uses that anymore, except for those that don't know how to size a carb for any particular engine
The CFM was there....but the heads nor the bore size would ever move that kind of air.......
You can put a big carb on an engine as long as it is a vacuum secondary.....the engine will use what it wants and no more than that...
Jebby
Instructor
Joined: Apr 2016
Posts: 104
Likes: 7
From: Denver Colorado
Carburetor Sizing Chart
Here's a carburetor sizing chart I have posted on other forums over the years that shows how much cfm your engine requires at 75%, 80%, 85%, and 100% volumetric efficiency. Note the 100% efficiency column only applies to off road track racing engines running open headers so that column does not apply to street engines running mufflers. A L-48 will be around 75% efficient, an L-82 will be around 80% efficient, and an LT-1 will be around 85% efficient. Summit Racing recommends 1-1/2 cfm per cubic inch for street engines and the 85% column almost exactly reflects that formula. NOTE. These figures are for 6000 rpm operation so 3000 rpm operation will only require half the cfm stated.
I suggest you print this carburetor sizing chart for future reference.
I suggest you print this carburetor sizing chart for future reference.
Last edited by Mountainoakie; Apr 27, 2016 at 11:11 PM.
Team Owner
Joined: Oct 2004
Posts: 81,241
Likes: 3,063
From: altered state
St. Jude Donor '05
Only reason GM didnt put DP on Im betting for warranty/complaints
VS carbs are pretty idiot proof, probably made bean counters happy too. Bet money if they knew every buyer would know how to drive a DP car there woulda been plenty of vettes with them on out of the factory
Could care less about the debates people like what they like
Like lots of you had both types both have + and -
VS carbs are pretty idiot proof, probably made bean counters happy too. Bet money if they knew every buyer would know how to drive a DP car there woulda been plenty of vettes with them on out of the factory
Could care less about the debates people like what they like
Like lots of you had both types both have + and -
Last edited by cv67; Apr 28, 2016 at 02:06 AM.