75gmck25
Full Access Member
- Joined
- Oct 13, 2016
- Posts
- 2,266
- Reaction score
- 2,210
- Location
- Northern Virginia
- First Name
- Bruce
- Truck Year
- 1975
- Truck Model
- K25 Camper Special TH350 NP203
- Engine Size
- 5.7
Edelbrock makes a dual plane intake (Performer) that has a mounting pad designed to work with either squarebore or spreadbore carbs. They also sell the Performer EPS that is squarebore only. They are available new from many sources, and also appear frequently on eBay. I don't have any experience with using a squarebore to spreadbore adapter, but it might be worth trying on your stock intake if you switch to squarebore.
Replacing the intake manifold is not hard, but does require some time and mechanical skill. Its a very common mod to improve performance of a Chevy SBC, so there are lots of how-to articles out there. I assume your engine is stock 1955-1986 design, but you need to check. The heads and intake manifold bolt pattern changed for 87 and later.
I have used the Edelbrock 1406, and its a reliable carburetor with relatively easy tuning adjustments. I also think the QuickFuel Slayer carburetor may be a good budget choice, but I've never used one myself. I like Quadrajets and I have one on my truck, but repairing and tuning a Quadrajet with unknown problems is not a simple task. You never know what has been changed on your carburetor.
A few simple checks for the Quadrajet
- Start out with a new fuel filter in the fuel inlet, setting correct float level, and a good cleanup of the junk inside the float bowl. Measure fuel pressure and verify good fuel flow. Also check the float to make sure it does still float (some develop holes).
- Verify that the accelerator pump works (you can look down the venturi and see it squirt fuel when you move the throttle linkage.
- Check all the vacuum lines and make sure they are in good condition. I recommend connecting the distributor to unported vacuum (always has vacuum) to give more advance at idle.
- Also check the larger hose to the PCV valve and the hose to the brake booster to make sure they are in good shape and properly connected. You may also have a hose running to an evaporative fuel canister mounted up near the firewall. If these hoses deteriorate they can cause a very significant vacuum leak, which will really screw up your idle mixture.
- Turn the mixture screws about 2 turns out, and then gradually turn them out further (about 1/4 turn at a time) until you get best idle vacuum. These screws are a PIA to reach, so be patient. As you make it richer and the idle goes up you may need to adjust the throttle stop screw to set the idle back down. You want to check everything at about 700-800 RPM in Park, but ensure you still get 650-700 rpm when you put it in Drive.
- With the engine warm, make sure the choke plate goes fully 90 degrees after warm-up.
- Now disconnect and plug the vacuum advance hose to the distributor, and check base timing. Usually about 8 degrees BTDC at idle is a good starting point if you use 87 octane fuel, but up to 12 should still work fine. Then keep the timing light pointed at the balancer and rev up the engine to make sure you see it advance as engine RPM goes up. Then reconnect the vacuum advance line to the distributor and verify that at idle it advances when vacuum is applied.
It just occured to me that if you have an original '73 distributor you may have a points ignition system that requires maintenance and setting the points gap. Or maybe someone has already installed an HEI distributor (GM started using them in '75).
Bruce
Replacing the intake manifold is not hard, but does require some time and mechanical skill. Its a very common mod to improve performance of a Chevy SBC, so there are lots of how-to articles out there. I assume your engine is stock 1955-1986 design, but you need to check. The heads and intake manifold bolt pattern changed for 87 and later.
I have used the Edelbrock 1406, and its a reliable carburetor with relatively easy tuning adjustments. I also think the QuickFuel Slayer carburetor may be a good budget choice, but I've never used one myself. I like Quadrajets and I have one on my truck, but repairing and tuning a Quadrajet with unknown problems is not a simple task. You never know what has been changed on your carburetor.
A few simple checks for the Quadrajet
- Start out with a new fuel filter in the fuel inlet, setting correct float level, and a good cleanup of the junk inside the float bowl. Measure fuel pressure and verify good fuel flow. Also check the float to make sure it does still float (some develop holes).
- Verify that the accelerator pump works (you can look down the venturi and see it squirt fuel when you move the throttle linkage.
- Check all the vacuum lines and make sure they are in good condition. I recommend connecting the distributor to unported vacuum (always has vacuum) to give more advance at idle.
- Also check the larger hose to the PCV valve and the hose to the brake booster to make sure they are in good shape and properly connected. You may also have a hose running to an evaporative fuel canister mounted up near the firewall. If these hoses deteriorate they can cause a very significant vacuum leak, which will really screw up your idle mixture.
- Turn the mixture screws about 2 turns out, and then gradually turn them out further (about 1/4 turn at a time) until you get best idle vacuum. These screws are a PIA to reach, so be patient. As you make it richer and the idle goes up you may need to adjust the throttle stop screw to set the idle back down. You want to check everything at about 700-800 RPM in Park, but ensure you still get 650-700 rpm when you put it in Drive.
- With the engine warm, make sure the choke plate goes fully 90 degrees after warm-up.
- Now disconnect and plug the vacuum advance hose to the distributor, and check base timing. Usually about 8 degrees BTDC at idle is a good starting point if you use 87 octane fuel, but up to 12 should still work fine. Then keep the timing light pointed at the balancer and rev up the engine to make sure you see it advance as engine RPM goes up. Then reconnect the vacuum advance line to the distributor and verify that at idle it advances when vacuum is applied.
It just occured to me that if you have an original '73 distributor you may have a points ignition system that requires maintenance and setting the points gap. Or maybe someone has already installed an HEI distributor (GM started using them in '75).
Bruce
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it will effectively put a ceiling on your truck's potential but that may not be a bad thing FOR YOUR application..Edelbrock carbs are more street friendly and tolerant of dirt. And they make less power.
