Big engine, big carbs… right? Wrong! Most people I’ve dealt with feel that they should base the size of their carburetors on the size of their engine. However, in most cases that is not the best option. Here’s why: Think about this, all a carburetor knows is vacuum, period, end of story. It doesn’t care what size the engine is. So, less vacuum means a smaller carb and more vacuum means a bigger carb.
With that said, you can have a 2110cc motor that needs 40IDFs and a 1600cc that could need 48 IDAs. Really? Yes really. A carburetor needs a vacuum signal to pull down the gas. If you have no vacuum, it doesn’t mater what jets you have in it; it won’t pull the fuel down the throat. Make sense?
So, what creates vacuum in an engine? The main factors are: port size in the heads, cam lift and duration, and the biggest factor is the compression ratio. The trick here is to get all these things to work together. I have found the best compression ratio for a daily driver that runs off of regular gas tends to be 7.7 to 8.0 to one. Back in the good ol’ days, when gas was red and lovin’ was easy, we would run the ratio down to 7.0 to one. With the new gas however, running that low of a compression ratio, no longer burns the gas efficiently, so we’ve had to boost up the compressions a bit.
For a daily driver, port size should be mild; this will keep the vacuum signal strong in the head. Also, don’t get carried away with your cam. For example, in a 1776-2007cc engine, you should have around .430 lift and around .284 duration on a 40 by 35.5 head. These numbers should be smaller for a smaller size head or cc engine. This will give you good bottom end and great mid range. You won’t be gaining anything over 5500 RPM. Again this is a daily driver not a Friday/Saturday night car. That combo would take a set of 40IDFs.
Lets say we had a motor that had a 9 to one compression ratio, a cam with over .430 lift and .294 duration, and large heads. That is what we would label a Friday/Saturday night motor and it would take 44IDFs because the vacuum signal would be a lot stronger.
What about the motors that fall in between the daily driver category (8 to one compression) and the weekend warrior category (9 to one compression or more)? Let’s say you have something like 8.7 to one compression. Well, 40s are too small and 44s are too big. What now? You will need to up the size of the venturies in the 40s or neck down the venturies in the 44s. To do this, you will need to first pull the tops off the carbs, then remove the accelerator pump nozzles. Next, pull the auxiliary vent tubes out (the tubes in the middle of the throat). Then loosen the main venturi set screw and pull them up and out. While your carbs are apart double check your float levels.
The big question: “How do I know if my carbs are right? I don’t know all the things you just mentioned and I don’t want to take my motor apart.” Luckily, there are a couple of easy indicators you can use to help diagnose a carburetor issue. If your jetting is close and you’re sure you have no vacuum leaks, then roll on the throttle. If you have a major bog off the bottom when accelerating then more than likely your carbs are too big (which means no vacuum signal). On the other hand, if you rev it up then let off, and the idle stays high before it comes back to normal, then they’re too small (too much of a vacuum signal).
For this quick diagnosis to be correct, we are assuming you have a good engine combo to start with. Here is an example of a bad combo: low compression (7 point something), large cam (600 lift, 328 duration), and stock size valves. No carburetors on earth are going to make that run right. Keep in mind all parts of motor have to work together to get the results we want.
IDF style carburetors work really well if they are sized right to your motor and set up right. We have seen great power and gas mileage when your engine combo is good and your carburetors are correct for your set up. If you choose to go with a set of IDFs, we suggest you check the float levels before putting them on your motor. We hardly ever see them correct strait out of the box. They should be 30mm when open, 10.5mm when closed on a streetcar, and 11mm closed on an off road car. Make sure you don’t compress the spring in the needle when setting them.
We hope this article helps to steer you in the right direction of choosing the right carburetor for your engine set up.