TyCor

Rebuild Q-Jet -- but secondary linkage doesn't seems right -- very flimsy and the linkage never really makes contact with anything when the vacuum is either open or closed. any ideas??? What am I missing. Does this linkage actuate the secondaries under vacuum? or do the secondaries open under internal vacuum pressure?



****Here I am holding the vacuum closed replicating what it looks like when I start the car.

'75

The secondaries are mechanical and are connected to the other side of the carb. The air valve which is at the top of the secondaries is operated by air velocity when the mechanical secondaries are opened. The vacuum pot in the pic functions to limit the function of the air valve.

...Roger...

First , I don't believe that is a Corvette carb.
2nd , what do you think is going on , secondarys not opening while driving ?

TyCor

I am concerned with two things:

1) the linkage in the pic appears to have no purpose (no matter where the vac diaphragm is, it would never contact or move the linkage as shown in the pic)
2) I am not sure the secondaries are operating. I do not believe they are mech as they do not move when I manipulate the accelerator. I'll get some pics of the other side of the carb -- that may help

I decoded the carb from a 1984 truck - I think.

lars

1. Your carb is a truck carb.
2. When vacuum is applied to the pulloff, it will keep the airvalve closed. That's what it's supposed to do.
3. Your Q-Jet is a mechanical secondary carb. Full description of this system is in my Q-Jet Tuning Paper, which has been available to Forum members for about 7 years. All you have to do is request the paper for a complete description of the operating system.

Lars
V8FastCars@msn.com

TyCor

Lars,

just sent a request via e-mail - thanks! Is it normal for the secondaries to NOT move when I pull the throttle all the way back. I was looking to see if there was a mechanical linkage tied directly to the secondaries, but I could not find one readily visible.

sturgeongeneral

This is very common when working on q-jets. You can have 4 carbs and linkage will differ from carb to carb. I tell everyone to take pics of the linkage before disasembly.

TyCor

I am on page 8 of Lars' tuning paper -- it was mentioned that my carb is a truck carb -- what difference does this make? what is the purpose of distinguishing as a truck or car version? thanks

mar

I believe mine is a truck carb too...it will have gobs of lo-end Tq

lars

If you give me your carb number I can tell you for sure, but the truck carbs built after 1975 used different booster venturies than the passenger car carbs. With the stronger low-velocity metering signal, intended for low-rpm torque and throttle response, the primary metering rods in the truck carbs have different power tips than the passenger car carbs - this is outlined in detail in the paper you have. Truck carbs 1974 and older do not have this unique primary rod configuration, and can be interchanged with passenger car carbs.

Lars

TyCor

Here is my carb number -- I think I have decoded it correctly?

170: >1976
8: '80's
4: 1984
2: Chevrolet
2: Auto
6: 49 state emissions
228: 228th day of the year
3: ??

lars

I wouldn't use that carb if I were you. There are no published jetting specs for that carb, and, if it's not an ECM-controlled carb, it's a truck carb. This means that it will be very difficult to determine any jetting baseline for it, making a good level of tune almost impossible without use of a dyno or a good wideband.

Lars

TyCor

Lars, do you recommend any particular carb -- I looking at ordering a summit intake manifold and carb combination kit they offer in either a 600 cfm or 750 cfm Edelbrock or Holley. I have not baseline for the best choice. The engine in the car is a 1968 327/HEI.

lars

Any Chevy Q-Jet passenger car carb from 1969 through 1979 will run very well when correctly set up. I like to avoid the '75 carbs, however. My Q-Jet Tuning Paper has a good listing of carbs and carb numbers.

If you want to go with something other than a Q-Jet, use any carb in the 650-750 cfm range other than the Edelbrock "Performer" carbs: The so-called "Performer" is a re-pop of the old AFB, and they are consistently the worst-performing carbs we see on the engine dyno. There are people here who will argue that, but Cliff R and I have documented this performance (i.e., lack thereof) consistently. The EDL Performer will run OK out of the box, but any other carb, properly set up, will outperfrom it by a significant margin. But, it's an "easy way out" for a replacement carb if you have no other option.

Also - don't buy cheap: There are cheap Holleys that are not worth the money. The new "Ultra" series from Holley is a good carb, and they have fixed many of the tuning issues from earlier street carbs with this carb. BG makes several very nice models. QuickFuel has an outstanding product series in the "Q" series carbs. It's all a matter of what you're comfortable with tuning.

Keep in mind that if you go from your stock Q-Jet setup to a square-bore aftermarket carb that you will have a significant amount of fabrication and custom work to do to get everything to work right - send me a request for my "BG Carb Installation" paper for an idea of the things you'll be needing to do for a swap.

Lars

TyCor

I sent Lars a request for the BG info. I wanted to know if anyone could recommend a QJET retailer or rebuilder for a passenger car. I would hate to by something off the shelf with out having a recommendation. Even after reading the carb papers, I am not sure I am skilled enought to to a basic tune on carb out of the box.