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Author Topic:   Carb Tuning
Oldsmobile13
Dirt Full Roller

Total posts: 66
posted July 07, 2001 02:03 PM
I have a good 2bl carb off a truck (chevy of course) Besides removing the choke, how can I get the most performance out of this? *Anything goes*

dryslickracer
Dirt Roller

Total posts: 14
posted July 08, 2001 02:20 AM
Me Again!!
Are you allowed to modify the existing carb body?
You need to eliminate the old float,needle& seat-Replace it with a brass float& good needle and seat.
Most of us in Iowa run our carbs with holley jets in them, I believe it takes a 28 or 30X1/4 I'll checklater today-tap out the jet holes.I'vebeen able to just runthe tap through the existing threads with no problem.

When you go to dial the carb in use a vacume gauge while your setting the fuel mixture *****s on thebase- watch for the sweetspot.

Run a good air cleaner like K&N at least 3" tall

sideways
Dirt Forum Racer

Total posts: 81
posted July 08, 2001 09:46 PM
hey dryslickracer please elaborate on"sweet spot"for a rookie I need the help.Thanks.

dryslickracer
Dirt Roller

Total posts: 14
posted July 09, 2001 01:26 AM
Proper idle settings, in terms of both butterfly position and idle mixture ***** position, are
absolutely critical for maximum acceleration from a racing engine.

Remove the carburetor from the engine and invert it. If you'll open the butterflies somewhat
you'll see a slot in each throttle bore- the transfer idle slot. Turn the idle speed ***** and
position the butterflies to where about .020" of the slot (eyeballing is good enough) is visible
below the bottom edge of the butterflies and exposed to manifold vacuum. Then lightly seat
each idle mixture needle and turn them out 1 1/2 turns, which is a good starting point.

Start the engine and let it warm up. You'll likely have to open the butterflies some in order to
get it to run. Make very sure that you remember exactly how much you had to turn the idle
speed ***** to get the engine to run- this is very critical. It wouldn't hurt to make some kind
of index mark.

Now, if you have a vacuum gauge, attach it to the port in the base that gives the highest
vacuum reading (if you don't have a gauge just listen to the rpm or use a good tach). Pick
one idle mixture needle, doesn't matter which one, turn it in 1/4 turn and let the rpm stabilize.
If vacuum/idle speed increases, turn it in another 1/4 turn, let the rpm stabilize and seed if
vacuum/idle speed increases again. If it does, keep going in that direction until it begins to
decrease. If turning it in hurts vacuum/idle speed right off the bat, then turn it out and go
through the same procedure. The idea is to find the "sweet spot", in terms of vacuum/idle
speed, for that mixture needle.

Now that idle speed has increased, you can close the butterflies somewhat. Repeat the
procedure with the other mixture needle. The idea is to adjust the mixture needles for the
correct idle mixture, which should raise the rpm enough to where you can return the
butterflies back to the original, correct position, i.e., .020" of the transfer idle slot exposed to
manifold vacuum.

If your butterflies were open too far when you started this procedure, you'll notice a definite
difference in throttle response and acceleration on the track when you set the idle system up
correctly.

This info was from another forumthat Iposted on concerning the mixture *****adjustments.
It helped alot- so I'mhappy to pass it on

Dryslickracer

Oldsmobile13
Dirt Full Roller

Total posts: 66
posted July 09, 2001 11:55 AM
I have seen guys polish the inside of their carbs, Is this something worth doing?

KPLugnut
Dirt Full Roller

Total posts: 57
posted July 09, 2001 12:26 PM
The proper size for tapping the holes for Holley jets is 1/4" x 32NCF.
Dryslickracer's "EarlParker-esque" explanation of idle mixture settings is right on, tho.
And yes, enlarging and reshaping venturis and baseplates is benefitial to airflow, BUT....it is very easy to turn a good carb into junk by doing so. Carburetor customization of this sort quite often will create a carb that will not idle and will not flow/pull fuel properly and nothing the user can do will help eliminate the carb from stumbling all over itself.
If there is anything I can help you with, feel free to contact me. Kinetic Performance sells these sorts of carbs quite often and we believe we have found that "sweet spot". I am also always available for advice (read as "free").

Just trying to help...

KPLugnut

dryslickracer
Dirt Roller

Total posts: 14
posted July 10, 2001 01:03 AM
Hey KP.
What about the powervalve?
Is there much that can be done in this area without detection? I have never seen much concerning this area,can they be changed out like the Holleys or is it a whole different ballgame with the Rochester?

Thanks

KPLugnut
Dirt Full Roller

Total posts: 57
posted July 10, 2001 07:28 AM
The actual power valve itself is not nearly as easily tunable as with a Holley (tho it can be done if necessary. The power piston (and it's spring tension) are however, VERY tunable and therefore, you can get the same power valve "rating" or point at which the circuit opens or closes (depending how ya look at it) as you can with a Holley.
Probably the first (and most commonly done) thing to do is to cut 1.5 coils off the power piston spring to bring the tension more into the ballpark of what the typical race motor would need with one of these carbs.

Hope that helps,
KPLugnut

avila
Dirt Roller

Total posts: 12
posted July 13, 2001 12:03 PM
quote:
Originally posted by KPLugnut:
The actual power valve itself is not nearly as easily tunable as with a Holley (tho it can be done if necessary. The power piston (and it's spring tension) are however, VERY tunable and therefore, you can get the same power valve "rating" or point at which the circuit opens or closes (depending how ya look at it) as you can with a Holley.
Probably the first (and most commonly done) thing to do is to cut 1.5 coils off the power piston spring to bring the tension more into the ballpark of what the typical race motor would need with one of these carbs.

Hope that helps,
KPLugnut



I can attest to this single change. I rebuilt my 4bbl Rochester 750 this season. After the rebuild it ran much better than before. I then started playing with primary jets, needles, secondary meter robs/hangers, 2ndary trottle butterfly spring rate and some others. Moving back and fourth in each direction. I was right on the money for all of these after the rebuild. I then bought a
power pistion spring set, that included 3 springs. One of the springs was the same size as the spring I had currently in the carb; one was shorter in lenght and the other was longer in length. I threw in the smallest length spring and my bog off the turn disappered! Of course it turns out this was the correct tension spring for the cam i"m running.

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