Holley Carb Troubleshooting and Rebuild - Things to Check!
Did some more troubleshooting on the carb. Found a few major issues.
1. The primary float bowl float was not adjusted correctly and was not shutting off the fuel. The tab on the float was bent down allowing the float to travel all the way to the top of the bowl without seating the needle. This allowed fuel to continually flow into the carb and richen it!
2. The power valve was the wrong size. According to Holley, the valve should be 1/2 the measurement of vacuum at idle in drive. According to my vacuum measurements, the power valve installed was opening too soon and dumping raw fuel in the engine when it was not ready for it, again richening the mixture!
3. The idle mixture screws were not set correctly. They were at 2-1/4 turns out. That is too rich for this particular engine setup. Using a vacuum gauge, I adjusted the idle mixture for maximum vacuum at idle. In my case, that was 1-1/2 turns out which is also the recommended Holley starting point.
4. My vacuum advance was not connected to the right port. It was connected to manifold vacuum instead of ported vacuum off the carb. This caused the timing to always be advanced at idle and then retarded under load as manifold vacuum drops under load! Totally backwards! Ported vacuum on a Holley can be found above the idle mixture screw on the passenger side of the carb at the metering block.
I bought this carb about 10 years ago as a "store return". I didn't do much to it then, assuming it was fine . I just bolted it on and drove the car, seemed fine then. I realize now that the temporary owner messed with the float and bent the tab too far and he also changed the power valve. I am checking online to see what the stock main jets should be as he might have changed those too! Word to the wise, never take anyone's word for it that a carb is "all set to go!" Each application is different depending on the vehicle and the amount that the engine is built. Use the proper tools like vacuum gauges and look up tables to adjust for the proper values for your application. There is no one size fits all!
Final word about power valves. Holley has built in an anti-backfire check valve circuit in all Holley carbs since 93. Blowing out a power valve by backfire is nearly impossible. However, the diaphragm can get old, cracked, and brittle so it is best to pull it and examine it if you suspect it to be a problem. To test it, turn all idle screws in all the way while the engine running at idle. If the engine stalls, the power valve is working properly. (keep track of the number of turns so you can put them back where they were).
I did complete rebuilding the carb and reinstalled it. I will have more video of making adjustments and test drives soon...Thanks!!