Ask Away! with Jeff Smith: Swapping a Late 1980s Quadrajet with a Throttle Sensor for an Aftermarket Carb & TV Cable

I just read your article titled “The Missing Link: The Best way to Install a Carburetor TPS” in Street Muscle Magazine.  

I’m trying to install an Edelbrock 1906 Carburetor to replace a Rochester Quadrajet in a 1986 GMC van. The Quadrajet has a TPS with two wires, but there’s also a cable going from the throttle linkage down to the 700-R4 transmission.

Is it possible that the TPS is being used for something else other than controlling the transmission? Seems to me that back in 1986 computers were doing more monitoring than controlling. I guess the real question is do you think this would work if I installed it without a TPS?

Thank you in advance!

P.P.

We’ll get into the TV cable question in a moment. First and more importantly, have you tried to mount the Edelbrock on your intake manifold?

If the intake is a stock Quadrajet manifold then the Edelbrock carb will not fit.

This is because the manifold is designed only for a Q-jet that employs a spread bore bolt pattern. The Edelbrock carb is designed to be bolted to what is generally called a square bore or Holley style bolt pattern. Because the Q-jet uses very large secondaries, the Edelbrock bolt pattern and the throttle bores won’t match up with the Edelbrock carburetor.

There are carburetor adapters to mate the Edelbrock to a Quadrajet pattern. There’s a Mr. Gasket adapter (MRG-1932) but personally I really don’t like adapters since they add height, which reduces hood clearance and they are a shortcut I prefer to avoid. But that’s just my opinion. 

The best solution is a new intake manifold that will accommodate the Edelbrock square bolt pattern. Keep in mind that your engine is a later model, 1986 small block engine. Roughly in 1987 is when Chevy changed the intake manifold bolt pattern from the standard one that went all the way back to 1955. Chevy changed the angle of the center four intake manifold bolts to a more vertical orientation. If your engine uses this pattern, you have to look close to see the difference in the angle. But with 1986, it appears your engine may still retain the traditional bolt pattern. This means any traditional dual plane intake for a small block Chevy will work. This is really the best way to go although this does make for additional work and expense.  

Swapping a Quadrajet for an Edelbrock Carburetor

Assuming there are others who might want to convert a Q-jet to an Edelbrock carburetor, there are some items that will be needed to complete the swap. You will need a separate bracket designed for the Edelbrock carburetor to mount the TV cable to the rear of the carburetor since the Edelbrock is a different bolt pattern than the Q-jet. I found a bracket that will work from the TCI trans people (TCI-376700). Then you will also need a separate small bracket from Edelbrock to mount the end of the TV cable at the correct place on the Edelbrock throttle linkage (EDL-8026). 

Once that is complete then you will need to accurately adjust the TV cable to make sure that the cable end moves the TV cable in the trans with any movement of the throttle. This is critical. You also need to make sure the cable travel is sufficient to allow wide-open- throttle (WOT) of the carburetor but also that the TV cable is tight at WOT. One of the best ways to know that the TV cable is operating properly is to install a transmission line pressure gauge to the transmission. With the engine in gear, monitor the line pressure to ensure that once the throttle opens that line pressure immediately increases. If the line pressure does not increase with throttle opening, then the TV cable (or the actual throttle valve in the transmission) is not operating properly. If the cable appears worn, it should be replaced but they tend to last a long time

Rebuild the Quadrajet?

It would appear that the easiest solution might be to just rebuild the Q-jet. Many enthusiasts downplay the Q-jet because of its complexity, but that’s also why it works so well. Plus, a Q-jet is a 750 cfm carburetor—the one you are replacing it with is only 650 cfm. Granted the engine will not require 750 cfm for maximum power and may not even max out a 650 cfm carburetor since this is not a high output application.

I can recommend a couple of carb rebuilders who can make your Q-jet like new. Sean Murphy Induction in Huntington Beach, CA really knows his Q-jet, plus this will maintain the van’s feedback carburetor situation which is actually quite useful to improve fuel mileage. The other is Jet Performance also in Huntington Beach. Either of these companies can certainly bring your carburetor back to better-than-new performance.

I also just learned that Jet is now casting its own Quadrajet carburetor bodies out of the original zinc material. They have had to resort to this major effort because the pool of rebuildable cores has all but dried up. This effort will be aimed at making only complete carburetors at this time, but if you are a hard-core Quadrajet fan, this should be cause for celebration for all carburetor fanatics out there.

The advantage of your current Q-jet besides retaining the feedback application is that the later model versions like yours can be easily tuned for better fuel mileage. These versions usually include an externally accessible adjustable part throttle (APT). This can be identified by a pipe plug located in the top of the carb that when removed allows access to an adjustment pin in the top of the primary metering rods. By lowering the rods into the jets, this will lean out the part throttle operation and might improve fuel mileage. It’s just another tuning opportunity presented by the more sophisticated Q-jet carburetor. It’s something to seriously consider.