07-25-2015, 08:59 PM
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CC Member
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Join Date: Jan 2002
Location: Cooper City,
FL
Cobra Make, Engine: Contemporary Classics, red white stripes
Posts: 139
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Not Ranked
Cool.
I've had pretty good luck with this approach.
0. Start with a good carburetor - preferably one that is brand new and matched to your combo.
1. Make sure the fuel pressure is correct, the carburetor is level, no leaks, and the floats are set correctly. If any of this is wrong, it will throw everything else off. Also, don't forget that many problems are diagnosed as a carburetor problem, when it is actually something else.
2. Set the timing. If you have the timing off, it will act like the carburetor is misadjusted. I like 16-18 initial, plus ~20 advance, so that total advance is 36-38. Of course this varies from engine to engine. Newer, more efficient combustion chambers require less timing.
3. I prefer to run w/out a choke. If you are running with one, disable it while you are tuning the car. Deal with one variable at a time. You can adjust the choke later.
4. Set the idle screws for max vacuum at idle. As you lean it out, it will increase the vacuum. Stop leaning it out when the vacuum stops increasing. Take your time, and go in and out with the idle screws until you get the feel. As you screw them out, the engine will run slower (and vacuum will decrease) As you screw them in, it will run faster. Find the point where it stops increasing as you screw them in, and don't go any further. Make sure they are set the same on both sides of the carburetor.
5. Drive it several days, just working on the idle screws. It runs on the idle circuit 100% of the time, so get this right first. The main circuit (jets) add fuel. It doesn't replace the idle circuit.
6. Drive with a vacuum gauge. When you gradually give it gas, you'll see your vacuum drop. At some point, your engine will stumble. You want the power valve to open right above that point. The stock power valve is usually 6.5, which means it opens when the vacuum drops to 6.5. Above 6.5, it is closed.
7. Run with a vacuum advance distributor on the street. When you are not under power, the power valve is closed - which means the carburetor is running leaner. As you lean the carburetor, the fuel burns slower, so you need more advance. If you run w/out vacuum advance, it makes the exhaust much hotter. This doesn't hurt drivability, but it isn't fun driving a car that always runs hot.
8. Tune the main jets with an O2 sensor. The O2 sensor will tell you your AF ratio at cruise and at power. Max power is somewhere around 12.5. You want it to lean out to about 16 at cruise. If you are too rich at cruise, you'll foul plugs. Add fuel by going up 3-4 jet sizes. Remove fuel by going down. You can do this on a dyno if you want, but you need to find a shop that knows what they are doing.
9. Here's the fun part. When your car is running bad, what does your O2 sensor say? Is it lean or rich? Your O2 sensor will tell you what's going on.
10. At this point, it should be running pretty good. You can fine tune air bleeds and the accelerator pump circuits, but you usually don't have to mess with them to get a car that is enjoyable to drive. You fine tune to get rid of that annoying flat spot over 6,000 RPMs, or perhaps a bog when you leave the starting line.
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