I have a '68 327 with 13.5:1 compression, 58 cc camel hump heads (extensively modified to flow like mad), 3000 - 7500 cam, full roller valve train, stall converter.
It has a Holley 700 cfm mech. secondaries on a weiand single quad tunnel ram.
Question...how in the dickens do you tune the carb for this setup? I understand it is going to be difficult, but jeez. The engine only carries about 6.5 in. of vacuum at idle, so I put a 2.5 in. power valve in. I drilled small holes in each butterfly which allowed me to close the plates far enough to properly calibrate the transition slots. It still run excessively rich. I don't care about idle speed as this is just a toy, not a dayly driver, and I don't want to change intakes as now it is a challenge.
Any thoughts would be appreciated.
Thanks
It has a Holley 700 cfm mech. secondaries on a weiand single quad tunnel ram.
Question...how in the dickens do you tune the carb for this setup? I understand it is going to be difficult, but jeez. The engine only carries about 6.5 in. of vacuum at idle, so I put a 2.5 in. power valve in. I drilled small holes in each butterfly which allowed me to close the plates far enough to properly calibrate the transition slots. It still run excessively rich. I don't care about idle speed as this is just a toy, not a dayly driver, and I don't want to change intakes as now it is a challenge.
Any thoughts would be appreciated.
Thanks
I apologize, I intended to put this question in the carburetor forum. Go ahead and move it there if you please.
Supreme Member
F-BIRD'88
Supreme Member
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May 23, 2023
With that much cam you need to run a lot of initial advance at idle. Then the carb will run right.
The over lap of the big cam causes the fuel to burn slower at idle (EGR effect) so the extra advance at idle corrects the burn time.
Lock out the mechanical advance travel and set the timing at 36degrees BTC at a idle. (it will be the same at high rpm.)
You now can probabily plug the holes you made in the throttle plates. (probabily not nessessary)
Just remove the weights and springs and tie wrap the mechanism in a locked out position and reinstall the rotor.
Once you do this timing mod, things will fall into place.
If the carb has a PCV port on it hook it up to the PCV valve. This will allow some extra "idle air". Once you combine the timing mod with a functioning PCV, your carb blades will be in the right spot at idle. Don't throw away the distributor advance parts cause once you do this and get the carb working you can go back and recurve the distributor for a radical race timing curve. like 20-25deg at idle advanceing to 32-36 at 3200rpm.
You can still use vacuum advance too. But it may need to be adjusted for amount (travel).
Let the car warm up to full operating temp (so the tunnel ram manifolds' runners are warm) and then adjust the idle mixture screws for best idle.
let me know know this works for ya.
The over lap of the big cam causes the fuel to burn slower at idle (EGR effect) so the extra advance at idle corrects the burn time.
Lock out the mechanical advance travel and set the timing at 36degrees BTC at a idle. (it will be the same at high rpm.)
You now can probabily plug the holes you made in the throttle plates. (probabily not nessessary)
Just remove the weights and springs and tie wrap the mechanism in a locked out position and reinstall the rotor.
Once you do this timing mod, things will fall into place.
If the carb has a PCV port on it hook it up to the PCV valve. This will allow some extra "idle air". Once you combine the timing mod with a functioning PCV, your carb blades will be in the right spot at idle. Don't throw away the distributor advance parts cause once you do this and get the carb working you can go back and recurve the distributor for a radical race timing curve. like 20-25deg at idle advanceing to 32-36 at 3200rpm.
You can still use vacuum advance too. But it may need to be adjusted for amount (travel).
Let the car warm up to full operating temp (so the tunnel ram manifolds' runners are warm) and then adjust the idle mixture screws for best idle.
let me know know this works for ya.
Thanks for the advice. When I get the swap completed, I will try this. I broke the in the engine in a S10, now I am shoving it in my '85 Firebird. Still working on the swap. I will let you know.
