Last post on Jan 26, 2011 at 12:39 AM
You are in the Classic Cars - Archived Discussions
This discussion is ARCHIVED. To reactivate the discussion, post a request in the Lost In The Town Hall... discussion.
What is this discussion about?
Fuel System, Performance Mods, Engine, Fuel System, Coupe, Convertible, Hatchback, Truck, Sedan, Wagon
#390 of 399 Re: Carb still bogging [Mr_Shiftright]
Jan 18, 2011 (9:58 pm)
In the "old days", a ton of problems were blamed on the carb that in reality were often something else. Among the most common were vacuum leaks.
The vacuum leak was probably the one most mis-diagnosed problem on the older carb engines. Second to vacuum leaks was timing. Especially on the old points engines.
But I agree that most carbs were susceptible to be extremely finicky about adjustments, especially on those that the people weren't certain what they were doing. Some of the older FoMoCo and Holley carbs were especially tempermental, as most guys I knew at the track that ran them, ended up bringing boxes of power valves for them.
Some guys used to cuss Rochester "Quadra-junks", saying they were the most complicated, yet I found them to be the most forgiving.
#391 of 399 Re: Carb still bogging [0patience]
Jan 24, 2011 (11:13 am)
Those Thermo Quads were a pain too along with Ford's Varible Venturi carbs.
I think our Honda shop had maybe one tech left that could overhaul a carb.
A lot of today's techs have never seen one!
Jan 25, 2011 (3:43 pm)
I never liked those carbs myself. But Rochester Quadra Jets are a great carb when working correct. They do have mutiple problems when aging. Nitrofill float soaking fuel--passage leaks-vacum motor failure, choke lock keeping secondarys from opening. When all is corrected they work great again.
I modified many for drag racing and they run very strong.
I guess I was lucky. In 1964 -the car dealer I worked for sent me to an advanced tune up one week class. The first 3 days were on the upcoming Quadrajet. So I had a heads up. At 69, I am prob. an minority now who knows how to rebuild and improve on one.
First thing is to remove the choke secondary lock. I found countless cars even new back then that the secondaries never worked.
The worst case I ever saw was a new 70s 454 El Camino. The choke lock out was hanging up-the factory had mounted the engine lift ring on the rear of the intake on top the vacum line that activated the Turbo' modulator, vacum adavnce and the choke motor on the carb.
The dealer had replaced the smoked convertor once. No way they could have test drove that car.
When I got done my customer almost craped his pants when we took it for ride and it all worked like it should. I also had turned the AC lid upside down so the engine could breathe. They moan real nice then! I also took 1/2 a turn off the rocker adjustments that gave it even more response and better mileage.
I had couple of my own cars with Q-Jets. My 70 429 CJ Cyclone could run 120 on primarys. When you floored it -it was like 2 soft N2O hits when the secondaries opened and the Ram Air hit. If you drove it decent it got 17 MPG with a 350 gear. It would top at 150. The 3.50 gear was perfect to sqeeze most the HP out. The mileage was not 17mpg doing that! LOL!
My other Q-Jet car was a 77 Olds. It ran real nice. I had to rebuild the car once in 160,000 miles.
#393 of 399 Re: Quadra Jet [animaljim]
Jan 26, 2011 (12:39 am)
As another 69 year old with roots in 1960's drag racing; I came to appreciate the many different ways the fuel metering on Quadrajets could be dialed in. There was (and probably still is) a southern California specialty shop called The Carb Shop. They were really into quadrajet tuning; and sold a kit to modify the quadrajet float chamber cover to install an adjustable stop for the primary metering rods. They also sold different taper rods which worked better with the stop. The adjustable stop was used to set the mixture at heavy throttle; and the faster taper of the rods would bring the power in sooner under load. There were many things that could be done with air bleeds and the main discharge nozzles to alter their response to changes in venturi vacuum. But it was frustrating to tune the late 1970s quadrajets that came with design features which prevented richening up the secondary metering rods.