Can anyone tell me how to choose the primary jets by using a vacuum gauge. I read that this is the best way, but I don't know if I'm supposed to use full throttle or part throttle and at what speed and so on. I have a T bucket with a stock 283 and 450 cfm Quick Fuel 4160. Thanks.
The main jets should have absolutely no effect on manifold vacuum what so ever. The main jets are the primary control for the air / fuel ratio under steady state operating conditions. If you are afraid of it running rich or lean, learn to read the plugs, watch for black tail pipe smoke, or install a wide band O2 sensor. You can set timing with a vacuum gauge, however it is very easy to dial in to much advance by relying solely on the vacuum gauge and you end up with a detonation problem. Since that QuickFuel is essentially a rebadged Holly carb, that means that your power valve is operated off vacuum. The power valve opens under low vacuum conditions (WOT, or close to it) to provide extra fuel during peak engine loads. This extra fuel flows through the power valve channel restrictor, not the main jets. Holley recommends determining the power valve size by using a vacuum gauge. Their procedure is as follows: 1) Hook a vacuum gauge to an intake manifold vacuum port. 2) Warm up the engine and note the vacuum reading at idle. Automatic transmission vehicles need to be in the Drive position, while manual transmission vehicles can be in Neutral. 3) Divide the vacuum reading in half. The number will determine the correct power valve. You can also set your idle mixture screws with a vacuum gauge. That procedure is as follows. Again, this has nothing to with the main jets and only effects the idle fuel mixture when the engine is running off the idle circuit, and not the main jets. 1) Install a vacuum gauge to a full manifold vacuum port. 2) Turn the screws all the way in until lightly seated. Next, back them out 1 full turn and restart the engine making sure the choke is completely off. While at idle, back them out 1/8 turn at a time, alternating between the 2 or 4, until highest vacuum is achieved. You secondaries may or may not be vacuum actuated, depending on the carb setup. If they are, they can be adjusted, but you won't need a vacuum gauge to do so. That is done with good old fashion trial and error road testing.
The "better" way for jetting selection is a wideband O2 sensor at least for a stock street driven engine. I have read, (here) though never tried, the claim that a mechanic's vacuum gauge can be used for jetting. The idea as I recall was to jet up or down until highest manifold vacuum is achieved, at a steady level ground cruising speed of around 50 or 60 mph. Whether this works or not I can't say, but always keep in mind that jet size also has a direct effect on the wide open throttle air fuel ratio. Tailoring the power circuit after jetting is finalized (to compensate for this) can be a little tedious. It's possible to be jetted perfectly for steady cruise conditions though the result may be too lean under power, upon acceleration. Very lean fuel mixtures are OK when cruising in high gear but under power will roast a valve very quickly. Reading spark plugs is an art form and modern fuels have now made it very difficult for the average DIY. Give it a try, take some pics, let us know what you observe.
Thanks for the replies. In this video from Summit he talks about using a vacuum gauge around the three minute mark. They also answered my question right at the beginning, don't know how I managed to miss that.
If you do try the highest vacuum at part throttle cruise method, it works best if you can set up a throttle stop. With one, you will be at the exact same throttle position each time.
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