The Brass Idle Mixture Screw is set yo 1.5 - 2 turns out from “LIGHTLY” bottomed. The Steel Idle Stop Screw should be set far enough in to open the vacuum port to what we call “The Perfect Square”. The 1/4” line is problematic. I recommend no smaller than 5/16” Steel or rubber and no larger than 3/8” steel or rubber. Let me look through my spares drawers for a needle and seat.
A metering rod gauge is just that. No one gauge is better than another. As long as they are the same length. I’ve not had a problem with the kits supplied by Pete. I’ll be in the shop in about an hour and a half. I’ll let you know what I find.
As you and others have alluded to, you are running lean. You either have to much air coming in or not enough fuel.
I don't believe the metering rod would affect this. In my experience if not adjusted properly it can cause a rich condition at idle or a stumble coming off idle, but I don't believe it would cause a lean condition at idle.
Have you tried John's suggestion of spraying carb cleaner into the carb throat while running? Did it make a difference? How does it run off idle? If it runs OK off idle I wouldn't think the needle/seat/float level would be the issue.
You stated you have good vacuum. I assume this means the needle is holding steady? If so, I would think that rules out valve train issues. Good compression with steady needle would leave me to believe the valves are adjusted properly or close enough that they are not the issue.
You stated that you tried a second carburetor with the same results, so I would think that rules out the carburetor and fuel delivery in general. My efforts would be looking for vacuum leaks. I think you have ruled out a large leak so I would be looking for multiple small leaks. Intake manifold plugs not tightened or leaking past the threads, small crack in the intake somewhere, rubber hoses cracked, diffuser gasket leaking, etc..
Vacuum and compression gauges are both steady, all gaskets are new, all hoses are new, plugs are new and brass. I'm making a few changes to the fuel lines, and I'm putting a pressure regulator in line with the electric fuel pump and should have a new tach/dwell meter by this weekend. The engine starts, runs and takes the throttle great as long as the choke is closed. I have gone over this entire engine more times than I can count checking for vacuum leaks and find nothing, I would think a leak big enough to cause this would be easy to find. I have disassembled cleaned and readjusted everything 4 times. I can see missing something once or twice but 4 times is bad even for me. This will end up being something so stupid and right in front of me that I'll be ashamed to admit it.
I haven't run it since Sunday but I just went out and tried it after reading your post. With the electric fuel pump running with no choke and using starting fluid, once I got the timing of the spray down, it would start and run quite well. I didn't try the throttle because I was running out of hands. I have an ignition switch under the hood and I made a 4 inch long choke cable so everything is within reach. I'm just using clip leads on the pump, I'm going to hard wire it to the switch, plumb in my pressure regulator, change a few fuel lines around and see how it goes this weekend. I've been out of work over a month with a broken knee cap so my shop time has been somewhat limited too. Been back to work a few days but standing on the cement floor in the shop all day still leaves me a bit sore.
Pull the slow speed circuit jet out, and compare it with your old one. Glen and I think you don't have fuel supplied to the idle circuit. With the metering rod all the way down in the main jet, your fuel is from this jet for idle. I've found several new jets to be smaller than the original jets. While you have the jet out, pull the needle jet out and spray some carb or brake clean through to verify the passage is open. Your test by spraying into the carb makes me think this. I had a M38 with a brand new slow speed circuit jet that wouldn't run at idle either. Drove me mad. I went back in and found one of the two tiny holes had a flash or spec of brass lodged in the hole. Once removed, it ran like a watch. A mechanic from accross the street from my shop showed me the carb clean test. Stupid simple. John
At this point I'm betting it is something stupid simple, supposed to rain here tomorrow, if it does I'll pull it apart again. I'm getting pretty proficient at pulling it apart, just wish I didn't have to, oh well.
If I read your previous post correctly you had the same symptoms with a different carburetor in place. Is this correct? If so I have a hard time believing the issue is in both carburetors itself. It's possible, but I'm thinking you are overlooking something causing a vacuum leak. Like you said probably something simple. Keep plugging away and you will find it.
I did not spend much time with the other carb, but I rebuilt them both with the same kit and were adjusted the same way but I don't see how I could miss something 6 times (5 times with mine once with the other) but it is possible. Today I pulled mine off and apart, removed every jet, cleaned every hole with a wire and blew everything out with starting fluid and compressed air. The only difference is I replaced the new low speed jet with an old one that seemed to have a larger hole. I went back to mechanical pump, fuel to the carb does not seem to be an issue. Changed vacuum gauge to wiper port on intake. Started it up and vacuum at best idle was a little over 20, takes throttle excellent. This time it runs better with the choke open a tiny bit and the idle mixture screw makes a small but noticeable difference. Checked for vacuum leaks again and found none and while spraying starting fluid down the throat it runs with the choke open. To me it sounds like a float issue but just guessing at this point.
Question. Has the throttle flange ever had bushings installed? The worst mistake that can be made in the process is to bore all of the way through and through without stopping short of the interior to give the bushing a place to seat. If it has been bushed, look at the throttle shaft as it sits in the throttle flange and check for a circular gap all the way around the shaft on either side where it goes into the casting of the throttle flange. If there is a gap, there’s your problem. This gap will cause a disturbance in the airflow. The result is a high idle RPM, with the choke fully open, that cannot be adjusted out. The throttle will need new through bushings to close the gap and be useable again.