Assuming the float bowl is vented,
my guess is that the throttle blades should be up the transfers a lil higher, then the mixture screws reset to in the middle of their range. If the idle speed is too high, you will have to retard the timing to get it down.
Now you can start the tune, beginning with the WET fuel level.
I do this by clamping a rubber section of the fuel line. With the engine idling and the line clamped;
One of three things is gonna happen;
1) the idle speed will immediately begin drop as pullover becomes harder and harder, indicating a low WET fuel level.
2) the idle speed will increase, as the engine was in a state of being too rich, the fuel-level is too high
3) the idle speed will remain stable for say 15 or more seconds, before the speed begins to drop. This indicates that the WET fuel level is pretty daymn close. Let it be.
other tips:
If you have a carb with metering rods, they HAVE to stay down in the jets at idle. Make sure yours do.
Make sure your PCV system is working
Make sure your fuel pump is not sucking air at one of the rubber jumpers.
And, as already mentioned, make sure the tank is vented
I would normally recommend a fuel volume test, but since it seems to be fine in gear, maybe later.....
IMO,
900 rpm in neutral is way too high, and the 150 rpm drop tells me that 10* is too much idle-timing. Normally this indicates that throttle blades are too far open. but if you have air entering the engine from an unauthorized source, that will throw a monkey wrench into all your tuning efforts.
The correct throttle opening for your engine is to have the Transfer-slot exposure underneath the primary blades to be, about square. Make it so with the curb-idle screw and then leave that screw alone. At this setting your mixture screws should be out about 2>2.5 turns, at 1000 ft elevation or less.
If your idle speed is too fast, slow it down by retarding the timing, wait, did I say this already? Lol.
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Now if you still have that ugly 150 rpm drop into gear, go looking for an air leak. If your Carter is a 4bbl, make sure the secondaries are closed up tight but not sticking. You can test this by completely covering, and plugging, the secondary side with a shop rag, which should produce little to no rpm change. Check the brake booster charge-line, the vacuum advance diaphragm, anything plumbed to the intake. If still not found, check the intake to head flanges, including the valley side. I do that by sealing the crankcase and measuring the dipstick tube for vacuum, which should not be there. Do not let the pressure exceed 3>4 psi, or you will risk blowing out a seal, somewhere hard to replace......
When all else fails, go put your hand on the tailpipe, looking for a vacuum signal. If you find it, she'll need a valvejob. You might as well do this first, cuz if you find vacuum here, you just may have saved a couple of hours of hunting. Get the valve job done and start over.
