Edelbrock Carb conversion
02-02-04, 11:24 PM
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Edelbrock Carb conversion
Have been reading the posts on the pros/cons of alternative carbs for rotary's with much interest. It seems that Webers or Holleys along with modified Nikki's are the most talked about, with Edelbrocks a question mark. I have been sucessfully running an Edelbrock 600cfm (manual choke #1405) on my street ported 85 12A on the street and autocross for over a year now and am highly impressed with this combination. At the moment I'm getting 20mpg in town and about 25 on the highway with excellent response and drivibility. The carb is mounted on a RacingBeat manifold with a Carter fuel pump.
There HAVE been a few quirks with this comb, however...
I have been experiencing the top-end over-rich problem that peejay talked about. My (eventual) cure was to drop the secondary jets to .077 -the smallest Edelbrock makes. I HAD thought about connecting the secondaries as he suggested but at the moment I'm using a four-hole 2 inch spacer to bring the powerband down a bit. I'm going to try that on my old 1 incher and see if it helps. Good suggestion, peejay, thanks!
I'm just running the stock needles and seats instead of the "off-road" spring loaded versions at the moment; the only cut-out it makes is in sharp, slow corners under light thottle. Also it tends to stall under heavy braking if you don't double clutch down the gears; sudden clutch-in stops can be heartstoppers if you need power suddenly! It also tends to spit if you back off the thottle partially at high rpm; air-valves fluttering? Going to try the seats soon...
I was originally looking for a 500cfm Edel. to mount on my stock-with-exhaust 12A but I blew up the motor and decided to go with a 600 and street-porting. Besides, 600's are much cheaper and easily found and Edelbrock says that the only difference between the two is the size of the secondary airflow. 48mm Webers have about 600cfm (I've heard) and I figured the individual runner RB manifold would allow the on-demand secondaries of the Edel. to work well in this application. I'm glad to say that it works just fine.
The secondary air-valves are counter-weighted for the air flow of a 5 liter engine so they don't open on the 2.3 liter 12A until about 6000rpm. Not only flat midrange but sudden power from 6 to 8000. Not conductive to smoothness OR blowing off that pesky Honda in the next lane... When inquired Edelbrock sent me instructions on how to drill the counterweights to allow quicker opening. I haven't gotten around to that yet so I came up with a VoTech solution; I simply tied the air-valves off so that they could only move from about 3/4 open to full! Works great, now has massive midrange at 4000 and SCREAMS from 5000 to 8000. The engine is now taking the full 600cfm from about 2500rpm (in the lower gears). Only at WOT in the upper gears and below 2500 does it protest (o.k.; it KILLS!). Is it a rig? Yes. Does it work? YES! I'll get around to drilling those weights someday; maybe it'll fill in the 4-to-5 gap....
So if you can't stand Holleys or afford a Weber think about the Edelbrock; it's working just fine for me.
Sanspistons (and Sansmoney, usually........!)
There HAVE been a few quirks with this comb, however...
I have been experiencing the top-end over-rich problem that peejay talked about. My (eventual) cure was to drop the secondary jets to .077 -the smallest Edelbrock makes. I HAD thought about connecting the secondaries as he suggested but at the moment I'm using a four-hole 2 inch spacer to bring the powerband down a bit. I'm going to try that on my old 1 incher and see if it helps. Good suggestion, peejay, thanks!
I'm just running the stock needles and seats instead of the "off-road" spring loaded versions at the moment; the only cut-out it makes is in sharp, slow corners under light thottle. Also it tends to stall under heavy braking if you don't double clutch down the gears; sudden clutch-in stops can be heartstoppers if you need power suddenly! It also tends to spit if you back off the thottle partially at high rpm; air-valves fluttering? Going to try the seats soon...
I was originally looking for a 500cfm Edel. to mount on my stock-with-exhaust 12A but I blew up the motor and decided to go with a 600 and street-porting. Besides, 600's are much cheaper and easily found and Edelbrock says that the only difference between the two is the size of the secondary airflow. 48mm Webers have about 600cfm (I've heard) and I figured the individual runner RB manifold would allow the on-demand secondaries of the Edel. to work well in this application. I'm glad to say that it works just fine.
The secondary air-valves are counter-weighted for the air flow of a 5 liter engine so they don't open on the 2.3 liter 12A until about 6000rpm. Not only flat midrange but sudden power from 6 to 8000. Not conductive to smoothness OR blowing off that pesky Honda in the next lane... When inquired Edelbrock sent me instructions on how to drill the counterweights to allow quicker opening. I haven't gotten around to that yet so I came up with a VoTech solution; I simply tied the air-valves off so that they could only move from about 3/4 open to full! Works great, now has massive midrange at 4000 and SCREAMS from 5000 to 8000. The engine is now taking the full 600cfm from about 2500rpm (in the lower gears). Only at WOT in the upper gears and below 2500 does it protest (o.k.; it KILLS!). Is it a rig? Yes. Does it work? YES! I'll get around to drilling those weights someday; maybe it'll fill in the 4-to-5 gap....
So if you can't stand Holleys or afford a Weber think about the Edelbrock; it's working just fine for me.
Sanspistons (and Sansmoney, usually........!)
02-03-04, 01:25 PM
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This thread ROCKS!!! Sanspistons, can you post those instructions on drilling the weights? I'm running a Carter (samething as Edelbrock) 650 and i'm having the same annoying drivability problems as you... except i'm getting 11 MPG.... looking for a cheap solution to hold me over untill i go EFI, any suggestions?
02-03-04, 02:25 PM
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I'm pretty sure the image works for everyone else as it's hosted right here on this server.
750 probably is a little larger than what most would have used, but I bought it for a 20B as a cheap solution until I can afford something more expensive. Well, that plan didn't work out so it's free to go on the 13B.
750 probably is a little larger than what most would have used, but I bought it for a 20B as a cheap solution until I can afford something more expensive. Well, that plan didn't work out so it's free to go on the 13B.
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02-06-04, 12:18 PM
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Jeff20B: Yes I have thought of connecting the primaries together also but see no real reason for it; the 12A works so beautifully on the primaries alone. I know: I disconnected the secondaries (easy to do on this carb) while I was diagnosing the top-end problems and it ran PERFECTLY smoothly from idle to 8000rpm; it just didn't have any real POWER! At one time I was running an insulator gasket (85-up Ford truck [mustang] holley piece) with BOTH primary and secondaries connected but with seemingly no effect. Not enough flow between, I guess; probably needs an inch or so to work right. Been watching your 20B carb/manifold thread with interest; don't have any real solutions to your dilemma; maybe six (!) Mikuni's on individual runners...??? Interesting to tune (to say the least!) but one of the better carbs ever made.
mwatson: I don't think the 750 is too large for the 13B; the primaries will be no problem and, like I said, it's the point were the secondaries kick in that determines the "bog" or lack of it on these carbs. You have to remember that on the air-valve carbs, unlike the vacuum controlled secondary Holleys and Nikki's, the air flow through the secondaries is controlled by the air-valves, NOT by manifold vacuum. THAT'S why the vacuum carbs are so sensitive to CFM size. Think of the Edelbrock/Carter as a CV carb (SU, Mukuni, etc.) on the secondaries at least and it'll make more sense.
Stoops417: I'm not sure that the Carter's counter- weights are the same size/configuration as the Edelbrock's; simple not sure if that's one of the (few) areas they updated the Carter. I'm also not sure if the Edelbrock instructions were copyrighted (!) or not so I'm reluctant to post them on the web. Third: I just don't have the facilities to do it!!! I'll have to see if my Dad can scan it for me! Seriously, why don't you mail Carter and see if they don't have something simular? If that don't work I'll see what I can do. 11mpg? At the moment I'm running 70x47 primary needles and 98 primary jets; this is the stock electric choke 600 Edel configuration. I run the yellow 'step up' springs on the street and either the orange or pink at the track; the orange "feel" better but hurt your mileage. Have you checked your float settings? After much experimenting I reset mine to stock; too high a float will run rich. Also check your fuel pressure; too much over 7psi will make the Edel run rich.
I confess I drive my car like a grandma most of the time; your mileage may vary.....!!
Sanspistons (and Sans-common sense!) -pining away for the new (old!) Edelbrock (Carter!) AVS spring controlled air-valve carbs!!!
mwatson: I don't think the 750 is too large for the 13B; the primaries will be no problem and, like I said, it's the point were the secondaries kick in that determines the "bog" or lack of it on these carbs. You have to remember that on the air-valve carbs, unlike the vacuum controlled secondary Holleys and Nikki's, the air flow through the secondaries is controlled by the air-valves, NOT by manifold vacuum. THAT'S why the vacuum carbs are so sensitive to CFM size. Think of the Edelbrock/Carter as a CV carb (SU, Mukuni, etc.) on the secondaries at least and it'll make more sense.
Stoops417: I'm not sure that the Carter's counter- weights are the same size/configuration as the Edelbrock's; simple not sure if that's one of the (few) areas they updated the Carter. I'm also not sure if the Edelbrock instructions were copyrighted (!) or not so I'm reluctant to post them on the web. Third: I just don't have the facilities to do it!!! I'll have to see if my Dad can scan it for me! Seriously, why don't you mail Carter and see if they don't have something simular? If that don't work I'll see what I can do. 11mpg? At the moment I'm running 70x47 primary needles and 98 primary jets; this is the stock electric choke 600 Edel configuration. I run the yellow 'step up' springs on the street and either the orange or pink at the track; the orange "feel" better but hurt your mileage. Have you checked your float settings? After much experimenting I reset mine to stock; too high a float will run rich. Also check your fuel pressure; too much over 7psi will make the Edel run rich.
I confess I drive my car like a grandma most of the time; your mileage may vary.....!!
Sanspistons (and Sans-common sense!) -pining away for the new (old!) Edelbrock (Carter!) AVS spring controlled air-valve carbs!!!
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