This is a quiz since I finally found the solution. So put your best diagnostic skills to the test and see if you can figure this one out. It took me a couple of weeks off and on with the aid of a wide band O2 sensor.
Engine is a 306 mild build 212 @ .050 comp cam with 9.2 compression.
Total timing not including vacuum advance is 35 deg. 15 for vacuum advance.
Rev. limiter set at 5400.
CARB
New out of the box Quick Fuel 600 VS Slayer series. I like this carb. It is very tunable. I live at 5,000 ft. which complicates the issue. Just about all air bleeds, jets and restrictors where changed more than once. Granted engine is over carbed for the rpm range it runs in.
SETTINGS/SYMTOMS
It likes to idle at 12-12.5 AFR at 16" manifold vacuum. Idle screws at this altitude are 1/2 turn out.
As the engine heats up the AFR at idle seems to wander from 11.9 to 13.5. I have a 1/2" heat spacer under the carb.
Idle transfer circuit restrictor jets were changed to allow for altitude and is running from 13-14 AFR.
Main Metering jets were decreased 3 steps for altitude. 2 steps were also tried.
Power valve restrictors were reduced from .055 to .051.
Primary main metering kicks in at exactly 2,000 rpm. Air Bleeds were increased in size to what Quick Fuel recommended for the altitude.
As the engine gets into main metering sometimes the AFR runs correct from about 13.5 to 14.5 and other times it stays rich at a pretty much consistent 11.2 AFR. At times it appears to be heat related, (cooler rich, hotter good), but not always.
During light acceleration, less than the PV opening vacuum rate, may go lean to 16 or may stay below 15. During WOT AFR is about 12.2 at the beginning and goes to about 13.8 as rpms increase. After this AFR may remain rich or be correct. The plugs look good but were pulled after I arrived back home each time after running in idle transfer for about three minutes to get home.
It took awhile to get to the point were the symptoms were identifiable due to necessary altitude tuning in the idle transfer area, air bleeds and main jets.
Quick fuel was called four times on the issues but only recommended main jet and air bleed changes. They were nice and professional but I could tell they kind of thought I was nuts. I began to think the same thing to.
I also hooked up my laptop to the wideband and recorded a graphed session for 15 minutes when the main metering was running rich. It confirmed the gauge was correct. This was a tough one. Although rare, this condition could exist on most Holley based carbs and you may not know it if you don't have a wide band sensor.
Consider all options. Good luck.
Added: One rule, because carb issues (circuits) can be inter-related you must get the issue dead on, not in the ballpark.
Quick Fuel Carb Diagnosis
Threaded Mode
