Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
1903 Won't Respond to Tip-In Procedure
bry593:
This truck is my daily driver and I have put some miles on it since the beginning of this thread. Noticed this morning my sticky throttle seems to be wearing in. One less thing, eh?
New vac advance should arrive in a couple of days.
Cliff Ruggles:
"K" series rods are next to useless for tuning with the APT system.
We offer a full tapered 45C instead, which tapers from .045" all the way to .026".
"K" rods only have .005" taper on the entire supper section of the rods then "step" clear down to .026". If you look at where they start at in the actual orifice of the jets you'll see that you have very little actual room for adjustments for the entire travel of the rod with the APT screw.
When tuning the primary side determine the jet size FIRST. Do this by climbing a hill or steep grade in a higher gear w/o the secondaries. Once you nail down the correct jet size then use the APT for light load/part throttle tuning.
For the later SMAB APT carbs with very few exceptions .036" tipped primary rods will be the best choice. I have tuned a few with .026" rods and dropped the main jet size down about 4-5 numbers to sort of split the difference between using .036" and .026" tipped rods, with good results.
Even with that said I only took that approach before we had the 50C rods made and I didn't have a decent set of "M" series rods available for that particular build......
bry593:
Thanks Cliff. I will take that into consideration if my vacuum advance mods are insufficient to cure the ping. I know that with the pedal to the metal, the vac should be all out, but I'm hoping for a miracle :)
Tonight I checked my initial timing and checked your vacuum advance curve. Turns out, I must have done a parking lot adjustment because my initial was 12 degrees and was supposed to be 10.
Here is your curve as tested using a Mighty Vac and dial-back timing light:
4 in Hg = 0
6 in Hg = 5
8 in Hg = 10
10 in Hg = 18
12 in Hg = 18
14 in Hg = 18
So basically, you have a 9 degree that starts at 6 in Hg and is all in by 10. I need the advance to be all in closer to 14 since my engine produces 14-22 at typical cruise loads. Actually, a better way of looking at it is that I need the vac advance to start falling out at 12-14. Don't want that advance under load. This cam and compression is prone to detonation. My new advance should arrive in a couple days and I'll run the same test on it.
Just for posterity, my current total cruise (2450ish rpm @ 75 mph) is 12 + 18 + 16 = 46 crank degrees.
bry593:
Edit: The Mighty Vac might read low by 1 in Hg. I calibrated against my vacuum gauge and there was 1 unit difference. Even so, that would mean your pod starts at 7 in Hg and is all in by 11.
--- Quote ---If you didn't notice the vast majority of the time your engine is making enough vacuum to both apply the vacuum advance and the power piston would be DOWN no matter what power piston spring you'd have in it.
"K" rods only have .005" taper on the entire supper section of the rods then "step" clear down to .026". If you look at where they start at in the actual orifice of the jets you'll see that you have very little actual room for adjustments for the entire travel of the rod with the APT screw.[\quote]
--- End quote ---
bry593:
Edit: The Mighty Vac might read low by 1 in Hg. I calibrated against my vacuum gauge and there was 1 unit difference. Even so, that would mean your pod starts at 7 in Hg and is all in by 11.
--- Quote ---If you didn't notice the vast majority of the time your engine is making enough vacuum to both apply the vacuum advance and the power piston would be DOWN no matter what power piston spring you'd have in it.
--- End quote ---
Yes, this engine makes great vacuum. But, it is a '71 3/4 ton truck with 76lb wheel assemblies and a 4.10 differential (locker, 10-1/8 ring gear). It likes a bit of primary taking off. Not unusual to drop to 10 in Hg. I have the stiffest power piston spring + a brass shim.
--- Quote ---"K" rods only have .005" taper on the entire supper section of the rods then "step" clear down to .026". If you look at where they start at in the actual orifice of the jets you'll see that you have very little actual room for adjustments for the entire travel of the rod with the APT screw.
--- End quote ---
Yes, similar to the 50M rods that come stock in the Edelbrock are 2-step. If you hit the 6-10 in Hg range, my truck would suddenly come alive well before the 2ndys. It is obvious when it reaches the step down and continuous length of .036". Problem is that it is max primary flow range, and pre-WOT vacuum signal. If you are towing and reduce vacuum enough to lift the needles to the step down, gas mileage is over...
Please remember my application is not a hot rod. It is a daily driver, 1971 truck that I can tow a tractor with when needed.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version