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Carb Tips: Tune In, Tune Up, Turn On...Carb tips... To begin with, has your carb been sitting for a while? It's pretty common for the gaskets to dry out. This can cause idling problems. If gas has been sitting in it, your passages may be gummed up... If you need to, do a quick rebuild with new gaskets and clean it out. It's cheap, and will save you major headaches trying to figure out why you can't get it to run right... Ok, now that we have a clean carb to start with lets get into it... First tip: Don't overdo it! Don't try to run too big of a carb. Many think a bigger carb is better, but it can be as bad for performance as too small a carb. A stock or slightly modified engine for the street doesn't need more than a 600 or 750 cfm carb. If you're unsure what size you need, use this formula: Multiply the engine size (cubic inch) by the maximum rpm it will see. Now divide that by 3,456. Example: 350 cubic inch engine multiplied by 5500 rpm = 1,925,000. 1,925,000 divided by 3,456 = 557cfm. So, you want to run a minumum of slightly over a 550 cfm. So a 600 cfm carb would work great. (You can go a little bigger than your minimum...) Second tip: Vacuum or mechanical secondaries? This is a good guideline... -Use a vacuum secondary carb on midsize or large cars especially if you have an automatic trans. These cars don't respond well to the super quick reaction of the mechanical secondaries and can cause them to bog. The engines can't build rpm's fast enough... Vacuum secondaries will lead to better performance and better gas mileage in these cars. -Use a mechanical secondary carb on smaller cars or with a manual transmission, especially if it will see lots of time at the dragstrip. This type of setup can get the rpms up quick and can use the quick action of the mechanical secondaries. Third tip: How do I hook up my electric choke? A wire from "ignition on" from the fuse block goes to the positive (+) terminal on the choke cap. Another wire from the negative (-) terminal on the choke cap gets attached to the carb base stud to ground it. Fourth tip: How do I set my mixture screws? Make sure the engine is up to temperature and in park. Now turn the mixture screws in lightly till they just seat. Now back them both out 1 1/2 to 2 turns. Make sure they are even. Now turn one 1/4 of a turn in and then the other. Repeat until the engine rpm drops. Now back them back out 1/4 of a turn. A better way is to have a vacuum gauge. Put the hose on manifold vacuum at the base of the carb. Now adjust the mixture screws until you get the highest vacuum. Readjust your idle speed, and make sure the mixture screws are where they make the most vacuum again. Fifth tip: Make sure your timing is right! Timing can cause a hard starting issue. It can also cause your engine to idle rough or not at all... Also, make sure all your ignition components are good. There's an old saying: "If it acts like a fuel problem, it's ignition. If it acts like ignition, it's a fuel problem..." So, before you start wondering about your carb, make sure your timing is correct, and your ignition is good shape. Sixth tip: Make sure you don't have any vacuum leaks! A vacuum leak at the carb base or intake will give you fits and act like carb problems. So, with the engine running, take some carb cleaner and spray around the base of the carb, around the edge of the intake, and all vacuum ports and hoses. If the rpm jumps up at a certain spot, you have found a vacuum leak. Seventh tip: Fuel bowl level. Make sure your fuel level in your fuel bowl is adjusted correctly. This varies on different carbs. Just adjust it as listed for your specific carb. A fuel level that is too low can cause a bog or stumbling if you floor it. Eighth tip: Adjust your WOT Wide Open Throttle... As silly as it seems, lots of people forget this. And they don't get all the horsepower that's available. The carb isn't opening all the way. Have someone sit in the car with the engine off. While you are looking down the throat of the carb, have them push the gas pedal to the floor and hold it. Have the plates opened all the way? (Straight up and down) If not, adjust the throttle linkage... We hope these carb tips help you with your carb... Return from Carb Tips to Hot Rod Engines
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