The procedure for adjusting the idle screws on a Holley requires a little finesse. Before starting the engine you turn the idle mixture screws clockwise until they are fully seated, and back them out 1.5 turns. This is the recommended starting point for tuning. My Holley 4150 has 2 screws on the primaries, some have 2 more on the secondaries. Start the engine, let it warm up, and then you can begin adjusting the idle mixture.
Adjustment can be done 2 ways. First, turn one screw in until the idle slows and then back it out until you get the highest vacuum reading. Then, back the other screw the same amount. If the idle increased you should lower it back down and repeat the procedure starting with the other screw. The second method ignores the vacuum reading and you adjust the idle mixture screws to get the highest rpm.
The problem I ran into was that turning the driver side screw had no effect on the idle speed or vacuum. In fact, I could turn it all the way in which should cause the engine to stall, but it didn't. The passenger side adjusted just fine though. Holley says ineffective idle mixture screws are indicative of a blown power valve, which would dump extra fuel at all times. I'm not sure that a blown power valve would only effect one side of the carb but it's an easy replacement.
Holley power valves are rated based on the vacuum that they operate. The stock valve is a 6.5 which requires an idle vacuum of 13" or higher to function properly. The correct power valve # should be half your idle vacuum, otherwise it could open too soon, and in worse cases open at idle. In my case, I'm only getting around 9" of vacuum at idle so I've known for a long time that my power valve is wrong, so this was a good time to replace it. I ordered a new 4.5 power valve from Amazon for less than $7. A week later it's $13.84, WTH Amazon?
Since I had to remove the primary metering block to replace the power valve I figured I would drop the main jets down from 71 to 70 since my spark plugs looked a little dark.
Here's the primary metering block from my Holley 4150. You can see the power valve screws in from the back side of the jets. What I wasn't expecting to find was a 6" hair coming from one of the passages. It looked like a strand of hair but it could have been a fine synthetic fiber. I tried to gently pull it out but it was stuck. I determined it was the port for the idle mixture screw and when I removed the screw the hair pulled right out with a big clump at the end.
The screwdriver is pointing to the passage where the hair came from. The idle mixture screw is removed and screws in from the side (the bottom in the picture). When I straightened out the hair it was about a foot long. I can't imagine how it got in there and I don't know if it came from Holley that way. I have removed this metering block once before and it seems I would have noticed this hair before.
I swapped the jets and the power valve and buttoned up the carb. After priming the engine fired right up but I could tell right away that nothing had changed. Fudge. It still wouldn't idle and the driver's side idle mixture still had no effect.
At this point I felt that the carb must have a blocked passage, especially since I already found some junk in there. I'm not too confident in my carb rebuilding skills so I felt it was best to just buy a cheap spare carb and rebuild this one later. I decided to buy an Edelbrock 1405 Performer 600cfm for just over $300 shipped.
When the new carb arrived I began removing the Holley. I started by disconnecting the fuel line and draining the fuel. When I pulled off the Mr. Gasket chrome fuel filter I noticed something wrong--it rattled when I shook it. If a filter rattles then it's loose inside which means fuel is leaking around the filter material. I looked in the outlet end and saw something was blocking the passage so I used a paperclip to try and pull out the obstruction.
Several plastic/rubber chunks came out, WTH? The one piece is curved so it's probably the seal around the edge. This kind of junk would cause total havoc with the smaller passages in the carb. When I drained the Holley fuel bowls more junk came out.
The larger pieces in the center of the blue cap are about 1/8" in size. No wonder the engine wouldn't idle. So the Holley needs a full tear down and rebuild. I'm glad I bought the new Edelbrock.
