Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Rich at WOT, lean at idle and cruise
modrummer:
--- Quote from: Cliff Ruggles on June 07, 2018, 03:54:19 AM ---Open up the idle tubes and DCR's and go back to ported vacuum for the advance. It's simply WAY too lean at idle if you need to run 18 initial plus 15-20 more degrees from the vacuum advance.
I see this a lot with aftermarket camshafts as they have more overlap and degrade vacuum at idle (signal) to the carburetor and the carb was originally calibrated for a "stock" engine making at least 20" vacuum at idle speed.
Everything will get better once you modify the idle system......Cliff
--- End quote ---
When you say "open up" I"m assuming you mean "replace" the idle tubes with bigger ones? What size would you reccommend? My MAB are ~.120", and my camshaft (Lunati 268) is currently pulling 12" vacuum at idle.
Cliff Ruggles:
No need to modify the MAB's, but remove and enlarge the idle tubes and DCR's to get enough idle fuel to the mixture screws for what you are doing.
That cam is mild enough that you shouldn't have any problem making plenty of vacuum at idle with 18 degrees initial timing, which is pretty high for what you are doing, IMHO.
We typically like to have the initial timing around 11-12 degrees for most of the engines we build and/or tune here. This will prevent difficult hot restarts when the engine is well heat soaked and is plenty of timing provided the CID, compression ratio and camshaft are well chosen......Cliff
modrummer:
--- Quote from: Cliff Ruggles on June 07, 2018, 06:14:26 AM ---No need to modify the MAB's, but remove and enlarge the idle tubes and DCR's to get enough idle fuel to the mixture screws for what you are doing.
That cam is mild enough that you shouldn't have any problem making plenty of vacuum at idle with 18 degrees initial timing, which is pretty high for what you are doing, IMHO.
We typically like to have the initial timing around 11-12 degrees for most of the engines we build and/or tune here. This will prevent difficult hot restarts when the engine is well heat soaked and is plenty of timing provided the CID, compression ratio and camshaft are well chosen......Cliff
--- End quote ---
Ok sounds good. I don't have your book in front of me (it's at home), but do I need to replace the idle tubes and restrictions with bigger ones? Or can I just drill them out to the size I need?
And what sizes would you recommend for them?
EDIT: Also just FYI if it helps, motor is a 350, 9.5:1 compression, Brodix IK200 aluminum heads, ported vic jr. intake, Lunati 268 cam, 4-speed with 3.55 rear gear.
77cruiser:
You can drill them out if you have a numbered drill set. It will only take .002-.003 more to get what you need I think. One other thing your engine might be happier with a RPM instead of the Vic.
modrummer:
--- Quote from: 77cruiser on June 07, 2018, 10:28:47 AM ---You can drill them out if you have a numbered drill set. It will only take .002-.003 more to get what you need I think. One other thing your engine might be happier with a RPM instead of the Vic.
--- End quote ---
Ok cool. I did a little googling and saw that I could drill out the Idle Restriction. Didn't know if I could drill out the idle tubes. Guessing I have to remove them carefully and then drill them.
So I have a performer RPM intake that I was running on the motor. The problem is that the intake port to head fitment on the Brodix IK200 heads isn't great. The performer RPM intake is meant to use a 1204 gasket, and I could MAYBE port it up to a 1205 gasket (Fel pro part #s). The Brodix heads call for a 1206 gasket. So with the Performer RPM, part of the intake runner ports gets blocked. Not much, but enough that it was bothering me knowing that.
The Vic had enough material to gasket match to a 1206 gasket and mates up well with those heads. I would definitely rather run the RPM intake for the street, but I'm not sure how to make it work without having like a shop add material to the runners or something.
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