Geralds57

I’m working on a 67 390hp after a carb rebuild and it idles well but after making final adjustments and it’s warmed up, I noticed a drop in idle speed as soon as I installed the air cleaner. I’ve tinkered around with the idle mixture screws and it doesn’t make any difference, it still rises or drops. Stock carb and new filter assembly. It will drop or raise 100 rpms when installing or removing the lid. The car runs well at driving speed but I’m wondering if it’s starving for air then or when it’s at full throttle? Any suggestions?

DansYellow66

Might be a PVC issue since installing the air cleaner causes the air supply line to the valve cover to pull filtered air from within the enclosed filter/carb inlet area rather than the open atmosphere. But not sure how it could make that much difference at idle.

6D2148

Does it happen just setting filter on, or as the wing nut gets tightened?
Chip

Geralds57

As soon as the lid goes on.

Geralds57

It's very odd. I can hear the idle difference immediately either way. I thought about the PCV also, so I'll look into that today.

jim lockwood

Putting the lid on forces air to pass through the filter element and that slightly restricts air flow. That causes a mild choking effect and your idle mixture is almost certainly a little richer.

If you can, put the lid on the air cleaner first and then adjust the idle mixture.

427/42566 coup

What Jim Lockwood said

DansYellow66

Adding to this it’s possible your mixture screws are set just a bit too much to the rich side and the bit of air restriction is pushing it into a rich roll off of idle rpm. But I’ve never had a carb that was that sensitive to a rich idle mixture.

Geralds57

I drove it to get it warmed up today, so I have the mixture screws at 1 full turn. I tired them at 3/4 turn but the engine didn’t like that and while I was doing that, I saw coolant dripping from the top radiator hose, so that shut me down on the carburetor. I had to repair the hose leak after it cooled down. Back on the carb tomorrow.

DansYellow66

That doesn’t sound to be on the rich side so still a bit of mystery. If you have timing dialed back it might be that the slight amount of rpm loss When the air cleaner lid is installed is being further accentuated by a lack of timing causing it to drop further.

6D2148

Check for loose screws and or bolts.
Chip

dplotkin

Put a tach/dwell meter on it and record the drop. You may be hearing things and it would be good to know the actual extent of the drop in rpm.

Dan

Geralds57

I wish I was hearing things but it’s real. Tach drops plus the sound.

DSR

Do you have a stock air cleaner? If not Is it touching your throttle linkage? Is your air filter extremely dirty or restrictive?

SWCDuke

Does the engine have the OE air cleaner? I think it's an open element type with a 14" x 3" element. The air flow at idle is so low that it would require very sensitive lab equipment to measure the pressure drop across the filter. Something else is going on.

Duke

Critter1

Just a guess but I once owned a 65 396 car that had an incorrect aftermarket Holley carburetor on it. The float bowl vent tubes were cut straight across instead of at an angle and both touched the air cleaner lid. The same issue occurred. It ran richer with the lid installed. The air cleaner lid is at an angle so the tubes weren't totally sealed but it still had an effect. There were C shaped marks on the underside of the air cleaner lid.

Geralds57

Original air cleaner base. Original carb. Nothing is touching linkage. Vent tubes are not cut, they have correct angle. Filter is new. It makes no difference if the whole assembly is removed or just the lid. Too many things going on today to get back at it.

Guru of the wrench

Fir st thing I learned at GM training center in1962 in carb class was fuel air ratio.For every gal. of gas you need 10,000 ga. of air. Very same thing happened with my 67 427 390 when I first bought it. Had a clean looking paper air filter and RPM,S would drop when cover was pup on. Bought a high performance S T P air filter and that fixed it. Started my own auto shop in 1965 and took many week long classes at GM center, best training you can get, and all free.

Guru of the wrench

Well did you get it fixed?

Geralds57

I finally got back on it. I changed the pcv and the air cleaner. It is a little better but I haven’t had the time to drive it yet. Too many priorities and bad weather.