Adjusting my QFT...

[lippy]

First, I'm clearly no expert on carbs. But I have a QFT also and I've recently spent a lot of time learning and tuning. Here are some observations/opinions, but there are folks on this forum that know a lot more than I do.

- If you have a hot cam with overlap, the AFR meter will do you more harm than good until you get to higher RPM/load. Cams with overlap allow a lot of air and unburnt fuel through the exhaust at idle and the meter will read lean at low RPM/part load. If you then adjust the idle screws to get a "correct" reading, you actually will make the car rich at idle and this will throw off your tune across the entire range.
- As others have said, set your idle screws to achieve fastest idle and richen a bit if necessary to get rid of stumble.
- If you can't get the idle set correctly with the screws (unlikely), you may want to experiment with small changes in the idle feed restrictors.
- Go to larger squirters to address stumble when accelerating.
- Check your vacuum at idle and ideally when driving. My vacuum at idle, with the idle mixture screws set for fastest idle and vacuum, is about 7.5". When you open the throttle fully you drop the pressure differential between the atmosphere and the intake manifold and open the power valve. Higher numbered PVs open earlier and lower ones open later. I experimented and used a 4.5 and it works fine. You don't want it to be too close to idle or cruise vacuum or it will begin to open too early.
- The number on the PV dictates when it will open. The size of the PV restrictors controls how much fuel is given once it does open. If you are lean at WOT you can try to use larger PV restrictors.
- The idle circuit controls the amount of fuel given during idle. The idle circuit + main jets controls the amount of fuel given during cruise. The idle circuit + main jets + PV restrictors controls the amount of fuel given at WOT. I experimented with main jets after setting the idle to get me close on cruise and WOT, and then changed PV restrictors to richen up my WOT.
- The high-speed air bleeds (HSABs) control when the main circuit comes on, and also affects the mixture. Larger HSABs will make the main circuit come on at a higher RPM but will also allow more air into the carb, making it run leaner at this point. Since you need more air then fuel, my observation is that the engine is less sensitive to small changes in HSAB numbers. I put slightly larger HSABs in to make my mains circuit come on a bit later.
- I've learned the hard way that reading the plugs and feeling the engine is probably better than relying solely on an AFR meter.
- Although modern carbs are very adjustable, they are still pretty coarse instruments. Just try to get things reasonably close. You'll have a hard time dialing it in perfectly.
- Make one change at a time and keep good notes. Also mark down your baseline settings. This way, if things get to far off you can always to back to the baseline.

As I said before, a lot of the people on this forum are far more knowledgeable than I am on carbs, so feel free to correct anything above that's wrong.