Technical SBC and quadrajet issues

actually they are very crude , kind of like a abacus vs a pocket calculator , for the time they were made they filled the need till they go the technology we have in todays cars , but I prefer either mechanical fuel injection or a good old carby .and a carb thats kept in tune will be as clean as one of them old beasts . and one of the main reasons we had to do this is people got lazy on the maintance of vehicles , today people just get in and drive and some even change the oil once in the cars lifetime ..

 

I have a small theory I'll try tonite, if I adjust timing to 8-12 degrees base timing and use manifold vacuum for advance, it will run faster and smoother at idle thus I could lower the idle speed screw and maybe regain idle mixture control. Then attack the stumble......

 

There is a spec for the secondary air valve , however it is for the stock engine. Loosen the set screw (allen head) and rotate the arm until it has more tension. It sounds like your distributor does not allow full advance. To adjust this you will have to elongate the slots in the advance plate with a file. Start with 8-12 initial timing (with out vacuum advance) and end with 36 (approx.) at 2500 rpm. Rev the engine past that to make sure it does not advance further. using light springs on the advance plate with allow it to advance faster. 90% of all carb problems are ignition related.
After it is running properly, THEN you can mess with the metering rods, arms and secondary accelerator circuit to fine tune it.

 

Great carb when done right,,,Agree not for someone with little skill or knowledge of them..They take some patience when tuning but when you get it right they are hard to beat for performance and mileage.
Last one I did for my sons chev stepside. 355 with 272 crane 441 heads . Light the tires at wot and cruise 17.5-17.8 mpg. reach thru the window and fire up...they are just too sophisticated for some ...

 

Well, I did some playing last nite and I set it to 10 initial and used full manifold vacuum and it worked pretty good. I also lightened the air baffle spring and got most of the stumble. Still going to change out the whole dist. The left idle mixture screw is not as sensitive as the right, the right one I can stall it by closing it off.
May try and find another carb for kicks and try it too.

 

Check out Cliff Ruggles, good to talk to,,get your carb number and on his forum enter your number ,He'll tell you what it is and what is needed for your application,,,You won't be sorry

 

In my experience, carburetors work best on a manifold designed for them. Spreadbores on a squarebore manifold or squarebores on spreadbore manifolds require too many compromises. I first found this out when running stock cars in a class requiring a 2 barrel carburetor. Everybody ran 4 barrel intakes with a 2 barrel adapter. We got access to a dyno and found out that we got a few more horsepower with a 2 barrel manifold on a Ford 289. The stock Ford manifold fit the 500cfm Holleys. GM engines required an adapter but they also gained a little power with a 2 barrel intake. On the circle track, it also helped in transitioning between WOT and closed throttle.

We also found that using manifold vacuum on the vacuum advance worked best. When you open the throttle, the vacuum drops backing off the timing until the engine revs up enough to build the vacuum back up.

10 inches of vacuum at idle sounds a little low. You also say that one idle mixture screw doesn't affect the idle as much. The vacuum leak is likely on that circuit.

 

I checked for leaks using a good spray intake/carb cleaner and got nothing.

 

For what I know it’s not unusual for the idle jets (maybe there’s another word for them?) in those carbs to be more or less clogged. They were totally clogged on my q-jet.
Found videos on youtube how to pull the idle jets out and clean them and the small passages etc. After this I could close my butterflies and the carb/engine responded to adjusting the mixture screws. (1967 327 with summit 1103 cam).



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Do you have a link to the video you mentioned or are you going to make me look for it the hard way?

 

Carburetors HATE adapter plates from either square->spread or spread->square.

IF you have the proper manifold/carb combination THEN typically the cause of hesitation on a relatively mild engine when bringing in the secondaries in order or frequency:
(1) Improper spring adjustment (GENERALLY, on a mild engine 1/2 turn past just touching closed)
(2) Fatigued air valve spring (these come in the better rebuilding kits)
(3) Worn secondary plastic cam (again, these come in the better rebuilding kits)
(4) CPO not working, not correctly adjusted, wrong vacuum port (someone modified incorrectly)

The only better STREET carb than the Q-Jet is the TQ! But asking it to function on a spread->square adapter is like taking a switch-blade to a gun-fight!

Do the math:

Spread-bore 750 CFM (150 primary, 600 secondary)
Square-bore 750 CFM (375 primary, 375 secondary)

BEST POSSIBLE RESULT:

Primary 150 (limited by the spread-bore)
Secondary 375 (limited by the square-bore)
Total 525 CFM

The best will not be realized at higher venturi velocities because of forcing the air/fuel mixture to make extra corners. When making a corner, the fuel will NOT fill the entire port, rather it will be forced against the outside wall (same as river current going around a bend).

Math and physics are your friends, if you let them be so.

Jon.

 

Check the secondary throttle blades to see if they are completely closing. The low vacuum is troubling.

 

LOL! I love it that you keep everybody on their toes with a statement like this. A fair number of people here get a little twitchy when you mention Q-Jets. But throwing a T-Quad into the same sentence must give them a full-blown case of the heebie-jeebies.

 

Clay - the Q-Jet came out in 1965, and the TQ in 1969. When did the e-clones often mentioned in these forums arrive?

Jon.

 

The biggest deal with the Thermo quad is the large amount of plastic now than they are a least 34 years old.

 

Plastic, yes, but the plastic is not effected by ethanol. In fact, one can run E-85 in these carbs, if one so desires.
With approximately 150k carbs, I run a TQ on my own hot rod (er, "sleeper" production vehicle).

EDIT: And just for the record, I am more than 34 years old, and some of my O.E. parts have been replaced with plastic!

Jon.

 

Old threads never die. They just get resuscitated......

@#38/39

 

This is the description of this I have ever read. Thank you.

 

Check the float it might be water logged witch raises the fuel level and makes for unmetered fuel dripping into the air stream.

 

I have worked on countless QJets , they are very tunable and work well when properly tuned . Well plugs leaking is a given it will happen . If it has married choke , there is a seal that is a given it will be leaking vacuum and need replacing .This may require , power valve adjustment , idle tube tube change and metering rod and rod holder change . The bog is so normal at low speed , if at high speed there is not enough fuel for air entering at that moment .

 

this thread is old, I cant even remember what I did to fix it! she hauls pretty good now!

 

Now that's funny..hahaha
3 yr old post that all of a sudden is brought to the top with new answers to a problem that was fixed so long ago that you don't even remember what fixed it..hahaha

 

Actually it was earlier, my '79 was the last pre-computer carb. In '80 the rebuilds got more expensive according to the shop I use to do mine.

 

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My '84 Silverado was not computer controlled but my Neighbor's '85 was. Maybe trucks were different.

 

Depends on where it was sold perhaps. I had an '82 Chevy wagon I bought in 1991, needed a car desperately. Someone had swapped in an earlier SBC and unhooked the computer.

 

I have to agree the QJ is a good carb, but like most things it has its ways. Fixed a lot of them back in the day, but haven’t fooled with one in a while. Always liked the QJ.

We had some TQs in our fleet of police cars, back in the seventies and had a few warp the plastic center part and cause leaks. Other that that they ran good on the 440 Magnums they were on.

Bones