Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Fine tuning Q-jet
old cars:
I’ll share some results: 454BBC /approx 500hp turbo 400/2.73 axle 227@.050 camshaft
First a Quadrajet
Idle AFR 13:1 Cruise 15.5:1 50mph 2100rpm Light Acceleration: 16.5:1 WOT: 12.5:1
Holley 780cfm (originally)
Idle AFR 12.6:1 Cruise: 13.5:1 Light Acceleration: 13.5:1 WOT: 12:1
After
Idle AFR 13:1 Cruise 15.5:1
Results will vary but these AFR ratios produced no hesitations or loss of power you could feel on the street.
The Holley carb stopped blubbering at lower speeds. WOT was not finish tuned at the time.
Idle fuel reflects camshaft being used. Idle vacuum was 12” approximately in gear.
Cliff Ruggles:
You will find the Holley carb (no mention as to part number, model, mods made to it or type of boosters used) more difficult to tune for part throttle A/F than the Q-jet.
You will NOT find your results changing jets and PV's. A large portion of the fuel at part throttle comes from the transfer slots as the large primary bores and inefficient booster design aren't in the mix like they are on the Q-jet.
I remember my early tuning efforts with Holley carburetors dating clear back to the 1970's. I remember locking the emergency brake and blocking the tires, putting the car in gear, then climbing up and bracing myself over the carb, looking down in it and watching for fuel to start streaming from the boosters as I applied load to the engine against the converter.
NOTHING....WTF? ALL the fuel was coming from the idle discharge holes and transfer slots, not the first drop from the boosters. The Q-jets I was tuning at the time, or attempting to tune as it was all a new game for me had TONS of fuel coming from the boosters in a nice cone shaped well atomized pattern with the slightest movement of the throttle off idle.
Hum? An early lesson in Holley tuning and by that time I had worn the bowl thread clear out of the main casting changing jet sizes in vain to try to get some decent fuel economy out of them. Not having any luck sent me into the air bleeds and then into the metering blocks. I got good at them early on then really good at them after a decade or so of tuning them to the brink of extinction.
By design I was never able to equal the Q-jet or the TQ's in fuel economy and seamless transition right off idle and for normal driving, but any heavy throttle movements without the secondaries was very impressive in comparison. This simply happens due to the much greater CFM available w/o having to get into the secondaries like I did on the spread bore units.
In later years I played around more with the larger annular booster Holley carbs and found them to be better in that area, but it comes at a cost of CFM lost stuffing a much larger booster into the larger bores.
I finally decided to pick a direction and run with it, so in 2003 when I went full time into the carburetor rebuilding/restoration business I dropped just about everything from the list and stuck almost 100 percent with Q-jets, but would take in a factory Tri-Power set-up (2-JETS) on occasion. That didn't last long and very quickly my backlog went out past two years. So I dropped everything but Q-jets, hired more help and we ran full time till a couple of years ago. Most of my employees moved on, I did not replace any of them as it became a perfect opportunity to retire and not have to put anyone on the street.
There's the short version of a long story but at this time I'm only selling parts, rebuild kits, and custom rebuild kits with tuning parts. I have the best accl pump seals currently available and NOTHING will make them swell up like the cheap soft light blue seals showing up from all the other sources.........Cliff
https://cliffshighperformance.com/simplemachinesforum/index.php/topic,4546.0.html
bob69:
I've been adjusting and testing for the past few months and finally worked out the issues. So here is a recap.
The original carb was defective. Tearing it down I found some sizable voids in the base plate casting. The voids were located inside the idle needle screw holes. Probably why the carb was showing a lean off idle afr spike. Also, this lean off idle afr did not show up in any of the three other q-jets I tested.
I tested jets from 67 to 71 with various rods. Except for one test, none of the tunes gave the performance I felt the combination is capable of. The one test, with 71 jets, smoke the tires for about 30 feet and then nothing. Like turning off a light switch, performance disappeared. The car ran OK at low engine speeds but had no performance.
It became obvious there was a problem with fuel starvation. The fuel pump was replaced about 5 years ago, so it was probably a restriction in the tank or fuel lines. On these older chevys, the fuel pick up has a strainer or "sock" to filter out debris. I pulled the tank and it looked new inside. Very clean. Pulled the sock and found it was almost completely plugged up. Don't know why. It was a replacement sock that came with the reproduction fuel pick/gauge line. Perhaps the stock's material was reacting badly with today's fuels.
Replaced the sock, blew out the lines with compressed air and replaced all rubber fuel lines. Did another test and still no performance. Pulled the fuel pump and found that the pumps internal plunger was partially stuck open. Limiting the pumps stroke and limiting fuel flow. Replaced the fuel pump. The fuel starvation problem was solved.
The 71 jets were now way too rich everywhere. Put in 69 jets which cleaned up the afr's and still has performance. May try 68 in the future.
quadrajam:
Good. So how are you liking those SUMMIT heads?
Cliff Ruggles:
At least you found the root problem. IMPOSSIBLE to effectively tune when fuel delivery issues are present. So basically you are back to square one.
The good news is that you can make predictable changes at this point........
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