Originally Posted by Big_Matt
I have a Holley 600 square bore on my 302, without going into to much detail, are they easy to tune yourself? Of is it better to find someone to do it?
I hear they are considerably easier to tune than the Carter thermoquad, but would it still be possible to do a reasonable job if a beginner was doing it?
No reason why a beginner shouldn't be able to tune a Holley carburetor, but it is like anything else, it helps to have someone who has done it properly before show you how. The basic parts and related adjustments are fairly easy. The most difficult thing is actually understanding what to change and why and how to gather results of changes and analyze them so that you know whether you're making the kind of progress you want for the job.
On an engine or chassis dyno, life is fairly easy because you typically have a wide-band O2 sensor that tells you what the A/F (air to fuel) ratio is while the engine is running. The dyno system typically plots a graph of the data points taken over a period of time as expressed in engine RPM. So you end up with a readout/printout of what your A/F is throughout the low to high RPM range of your engine. Generally, you want to set your A/F at about 13.2:1 for maximum performance. A slight bit more or less depending on your atmospheric conditions can make a bit more power for your combination, though it may be difficult to obtain good repeatability of some peak numbers as your engine heats and cools slightly differently between pulls, more or less time expires between pulls, etc. Repeatability of a few HP is considered about as accurate as you're likely to get without a lot of experience and very precise measuring/calibration, etc.
If you don't have a dyno, life gets a bit tougher, but still very reasonably possible even for the novice tuner. You need to have basic tools. Particularly a vacuum gauge, a tach and (if using points) dwell meter, a timing light and at least one spare set of sparkplugs. It also helps to have a nearby racetrack or at least someplace where you can mark out a known distance (doesn't really matter how far) and a mate with a stop watch to keep time. You don't need the mate to get the basic tune right, just to improve your "tune for better performance" if that is what you want.
If you just want to ensure that you're not running too lean or too rich, you'll need your sparkplug wrench and a box of new replacement sparkplugs for your engine. Pull the number one plug and measure its gap after you get to your private stretch of road/track. Gap your new plug to the same dimension and insert it. Fire up and run a pass to your known distance--full throttle, switching off the engine as you cross the "finish line." Once stopped, pull the plug and read it. One of the best ways to read it is to look at the heat absorbed into the plug by counting the discoloration of the threads. The obvious sooty for too rich reads of the insulator will assist you in getting the basic A/F sorted, but using the "heat rings" will get you fine tuned. An alternative is to use an exhaust gas temperature gauge if you have one.
Make your adjustments and insert a new, clean sparkplug and run it again until you get your mixture just right. Once you're sure that you have the right mixture, you can modify your timing for ET using your mate with the stop watch to see improvements. Once you get too close to tell whether your gaining or losing performance using the not-so-highly-sophisticated stop watch method, you need to take it to the track or to a chassis dyno to make any reasonable improvements. If traction is a problem, accelerate up to a known speed of say 50 km/h before you cross your starting line. If you have your mate riding with you while timing, be sure that he can see the start/finish line easily. Work it out ahead of time so that he says go when you get to the line and wait for him to say stop, for you to cut off the engine.
It only takes about 3-4 passes to get it fairly close with some experience. As your day gets hotter or cooler, it can have some notable effect on time, but probably not enough for the basic tune-up needs to matter. You can work out the basic tune up before doing the timing tests so that you keep them as close together as possible. Also, wait for your engine to come to complete operating temperature before each pass so that you will be consistent in your "environment" all around.
The modifications to your carb are basically going to be jetting for a double pumper and jetting and vacuum secondary spring (rate) changes for a VS carb. There are many other tuning bits that you can modify, but you probably want to get started with these and as you better understand how each adjustment affects your engine, work on them. Just remember that it is better to be on the rich side than on the lean side. If you feel that the engine is too fat (rich), lean it up one jet set at a time. If you feel that it is too lean, add two jets (numbered values) at a time until you feel that it is either getting close to too fat or is losing time if being timed. If you just added two jets and the time dropped off, back it off one jet to see if it recovers, and another until it improves as long as it doesn't lean out.
Once you get your A/F where you think it ought to be, add a degree or two of advance (keeping an eye on the A/F on every pass!) to see if your ET/MPH picks up. This is something that is rather difficult to do without the benefit of a timeslip, though.