Carb Tech Tip's
Convert your 4-bbl Scout II from a Thermoquad to a Rochester Q-jet carburetor.
This conversion is worth the trouble!
The first thing to do is locate the correct carb from a 73-75 chevy truck for the "70" series carb, and a 80-86 for the "170" series carb. Ask the scrap yard if you can also go through the scrap motor pile. Find a few Q-jets and get the secondary hangers and rods from them making sure that you grab different rod sizes. The sizes are stamped on the lower half of the rod. Refer to my rod chart for the sizes that you might need. You can't have too many of them as you will never know what you will need.
The carb that you want is a "70" series, although a "170" series will work also. The fuel inlet needs to come in from the side and not the front. This is because the front inlet style carb will not clear the thermostat housing. Your stock air cleaner housing will fit without altering it at all. I recommend using a K&N air filter, PN# E-1090. Make sure to buy the "recharger" kit for it also. The carb number is located toward the rear of the carb, throttle linkage side. Write this number down as you will need it to buy the right carb kit. The float that is in the carb should be ok, as far as the size and shape for this application. You will also need a small bucket of carb cleaner or spray.
The next step is to disassemble the carb to clean it and then rebuild it. Also buy some good quality epoxy. You will want to epoxy the well plugs on the bottom of the carb bowl center section. The rebuild kits include a small piece of foam rubber to help seal these off. It does not work for long. Be sure to write down the main and secondary jet sizes for future reference. I always set the float about 2/32nds lower than the stock specs for the spec.'s on the carb that I am working on. This is because Q-jets will meter the fuel better with the float level a little under the specs and it keeps them from having a "hot soak" problem. Also note the letter stamped on the secondary rod hanger. Before putting the carb back together, remove the plugs hiding the mixture screws, then screw the mixture screws in until they lightly bottom, then back them both out two turns. This is the base setting that you will fine tune from when the carb is installed. Also, set up the choke linkage per instructions that came with the rebuild kit. I have found that a good running IH engine does not require much choke enrichment except when it is being started cold. I disable my choke in the summer.
Once you have the carb rebuilt and ready to install, you will need to make a linkage adapter so the throttle cable and the transmission TV linkage will line up correctly. This is because the Q-jet body is about an inch and a half narrower than the Thermoquad. I have not done this conversion on a Scout II with cruise control, but it should not be to hard to include this when doing this swap. The bracket needs to be in the shape of a "U". The "add on" bracket that the factory uses for cruise is perfect for this, but if you don't have a one, you will have to make one. The "U" bracket is about an inch long on two sides, and about a half of an inch wide at the bottom. You can also fashion a stud for this instead. Either way will work. Look at your current set up and it should explain it's self. I also moved the return spring anchor point from in front of the carb to the rear of the carb, anchoring it on the TV linkage/ throttle cable bracket. It has been brought to my attention that when going from a 2bbl to the 4bbl that the linkage mounted on the intake for the 2bbl is a direct bolt up with minor modifications. (Thanks,George!)
The fuel line is next to modify. The factory size line is 5/16, and the Q-jet uses 3/8. This different size does not hurt the fuel flow any. You can either use the factory size line with an adapter at the carb, or use a new 3/8 size fuel line from the fuel filter to the carb. It looks better to use the factory size with an adapter at the carb than to go the other route. The fitting from your old Thermoquad fuel inlet works perfect for an adapter at the carb when using the factory size line. Use this or go to the hardware and buy one like it. It must be a pipe thread fitting or it will leak!
You can either use a manual choke, manifold choke, or an electric choke setup. The manual setup is the easiest thing to do. But if you don't want to pull the knob out every morning, you can make the manifold choke or the electric choke work. You will have to use the late "170" series Q-jet for an electric choke. You can either use the factory coil by flipping it over, or buy a new one for an early chevy. The early chevy one that you will need is opposite of the standard chevy one. Then you will have to make a choke rod. You can either use the Thermoquad choke rod, or make one using any vacuum pull off rod from any GM carb that is long enough. You have to use one of those because it must be strong enough to with stand the choke coil's force. Study how the factory set up works, and you will get a general idea of how to bend it. It will take a little trial and error to get it to were it works right for you. Sounds harder than it really is!
Now all that needs to be done is a few adjustments. Also, before starting the engine, make sure that the throttle opens fully and closes fully by it's self. Once you have started the truck up, verify that the timing is correct. Then set the idle. When this is done then set the mixture screws for the best idle, starting with the settings that you set when you built the carb. Using a vacuum gauge is the best way to get it right. Now recheck the idle. Now go for a test drive. Note if the secondaires come in too early or too late. If they open too early, tighten up the tension on them and loosen the tension if they are too late. Don't worry if it is too lean or rich at this point. Get the "cut in" point set right first, then play with the secondary jetting after it is adjusted. Refer to my rod sizes page for tuning tips on what to use.
Now you are ready to go Scout'n with a better running Scout, plus better mpg!
Good luck!
© By Scott Hattery 98-05