Carburetor Tuning

   The best way to tune a carburetor is to start with the right one. Get a carb that fits your application. All you will need to adjust is the idle mixture and possibly the jets. If you need to adjust much else you probably don't have the right carb for your application.

    Those $900 Race Demon carbs with the replaceable venturis, boosters, air bleeds and such look pretty cool but are made for experienced racers who need every micro-horse and hundredth of a second to be competitive.  They have the equipment and know how to set them up. I've seen guys put them on their daily drivers expecting to see a huge power gain only to have a car that stutters, spits back through the carb, and gets horrible gas mileage. I've also heard of guys feeling a big power increase but were replacing an old, worn out, stock carb that had been screwed up by some carb "expert". They would have felt the same increase in power (and probably better gas mileage) using a new Holley 600 which you can find for under $200. 

    Tuning a carburetor is always the last step I take in power tuning my car. A lot of "carburetor" problems are actually electrical problems. Once your sure you have carb problems, check your ignition system. I suggest you setup your distributor before your carb. Lucky for you I also have an ignition section.

    If your engine isn't running right and you have eliminated all other variables, (i.e. vacuum leaks, valve timing, and ignition timing), then you should work on the carb. It helps to have a good understanding of air pressure, velocity, and vacuum before doing too much with carburetors. There are plenty of books available about modifying carburetors. You should get one dealing with your specific type of carb to use as a guide.

    If you're like me you don't have the cash for a new carb so you will have to tweak with what's available. I was planning to make a page that described all the ins and outs of carb tuning then I found . It is the best guide I've seen.

    What he doesn't mention is track tuning. Once you get your accelerator pump setup so it doesn't bog or hesitate you can fine tune it based on 60ft times. Try different shooters and cams until you reach the fastest 60ft time. I put together a chart to show the profiles of the different Holley pump cams. I've seen a couple charts floating around but they all used the same data which was flawed. I hooked up a dial indicator and degree wheel to a throttle body and measured all the cams myself.

    The jets can be adjusted by the mile per hour at the end of the track. The mph is the best indicator of power output. Change jets up and down to reach the highest mph. 

    If you are running a double pumper, disable the secondaries by removing the link between the throttle shafts. Adjust the primary accelerator pump and jets for the fastest 60ft and mph, then hook up the secondaries and adjust the accelerator pump and jets on them to reach the best 60ft and mph.

    With vacuum secondaries do not remove the throttle link but find some way to keep the secondaries shut. I usually tie the secondaries shut with a piece of wire from the secondary shaft to the throttle return spring bracket. With the secondaries disabled adjust the primaries for best 60ft and mph. Vacuum secondary carbs don't have a second accelerator pump so to fine tune the 60ft times you will change the secondary spring which determines when the secondaries open. Again, adjust the secondary jets for best mph. If you have a nonadjustable metering plate you should ditch it and use one with replaceable jets or convert to a metering block.

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