Lean condition under light acceleration
The lean condition is encountered under light acceleration or while engine load increases at constant throttle (climbing a hill).I have a wideband in the car which is aiding in tunning the carb. In addition to the gauge indicating a lean condition (16 to 17 AFR), I can feel it in the car as well.
If you think about this, it will become apparent what is happening, and is actually an excellent test, because you can watch it unfold.
on the flat;
The metering rods are sucked down and everything is fine.
Your butterflies are somewhere up on the transfers.
As the load increases, vacuum drops, and the transfers should be supplying more fuel. But with the power-piston sucked down,sooner or later, the the circuit stalls and cannot keep up with the demand. So it goes lean.
You can sometimes prove this by examining where the mixture screws are set. A) At idle,if your rods are too far down, then you will have to go richer on the trims in compensation. B) If the rods are too far up at idle, the trimmers will have to be set leaner. Interestingly, set this way, the engine goes lean off-idle. Because the trimmers are always alive, and they are set lean..
What I sometimes do is;
Set the mixture screws in the center of their adjustment range. Then, put the throttle up on the fast idle cam at about 2000 rpm; exact rpm not important. Now, since you have an AFR sensor, using the powerpiston height adjuster, crank the rods up or down to get a good number.
Then see what happens back at idle.
You might have to go back and forth a couple of times. And you may figure out, that you can only get so and so close.
When this happens it is usually because your butterflies are not in the right place up on the transfer slot, at idle. So ,adjust the idle speed up or down, up should go rich, and down should go lean. When you think you are getting close, but the idle speed is outta whack, use timing to reel it in.
Then start over at 2000 on the fast idle cam.
If you haven't thrown a bunch of timing at it, you should get really close on the first go-round. Most guys try to run way too much timing, and the engine likes it, and there is nothing inherently wrong with lotsa timing...... it just makes setting the carb up a lil tougher .
I'll help you;
Make sure the secondaries are closed up tight but not sticking. And you gotta run; a PCV plumbed to the proper port on the carb, and if you have a brake booster, I run it off the common plenum, not a single runner. Make sure the Vcan is on the sparkport. Then;
with a stock 318/360 set your Idle-timing to 10/12 and see how close the first go-round gets you.
