Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

air filter leaning out afr

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Charlotte1:
Hello Everyone. Sorry for the long ish first post. I think its an odd issue that I'm hoping someone has had experience with.
My setup is
1972 Chevy C10. 402 BB Aluminum heads.Alum intake.10-1 comp.Custom hyd roller cam (220/228 @050. LSA 111). 1 3/4 headers. 700r4 trans with lockup. 14" idle vacuum. 17-19" light throttle( 1700 rpm) cruise vacuum. Custom Sean Murphy Induction, stage 2, 750cfm Qjet. 76 main jet, 43 prim rods , DA sec. APT adjustment. Sorry,I don't have any other build specs.
I have recently added an afr gauge.
The truck starts, idles and performs very well
I've have done the normal qjet tuning steps(apt tip in, spark plug reading). Tip in procedure showed success as well as the spark plugs look great.  APT screw is approx 4 turns up and idle mixture screws are approx 2-2.5 turns out. Sean Murphy indicates 3.5 turns up is normal for APT.
Initial tuning was done with the aircleaner Assembly removed for easy access to APT screw. AFRs are  idle=13.5-14.0 light cruise= 14.5-15.0 open throttle on primaries only to 4000 rpm=13.3-13.8 ish. WOT= 12.3-13.0
I then put my aircleaner assembly back on and all afrs(except idle) went lean. Basically add 1-1.5 point lean to the above readings.( light cruise went from 14.5 to 16.0)
Weird!!
My truck runs the oem 6-7 blade oem mechanical fan with oem fan clutch and fan shroud. My air cleaner assembly consists of a 1" riser , then 1" drop base, open element, 14" x 4" AFE brand, blue media type filter and chrome lid. 
I've contacted SMI about it and they have suggested it has something to do with the airflow through the element is effecting the float vent and float. They suggested to block off front of element to see what it does. I haven't done this yet.
I have fitted a 2" drop base, chevy dual snorkel type aircleaner assembly. With the 4" element, the top lid is raised/exposed above the base approx 1"( if that makes any sense.
I've never heard of an aircleaner assemble Leaning out a mixture. Usually just enriching it.
I'll try to post some pictures but havenet had any luck doing so yet.
Has anyone had the leaning out scenario happen? If so, what was done to rectify it?
Thanks Everyone!
Shayne

Cliff Ruggles:
Any particular reason you are running a drop base air cleaner?

Most move the lid WAY too close to the carb and can cause all sorts of issues, loss of power, etc.

Minimum 3" from the air cleaner gasket to the bottom of the lid, and even then with some "flat" lids that may not be enough......

Charlotte1:
Hmmmm.
Thats a great question! I’m not sure how I ended up there! I think it was partly do to the bases I was looking at had pre fabbed valve cover breather port.
I’ll take a look at that measurement. I’m thinking the open element air cleaner assembly that I had the original lean condition will be close. It is a 1” drop I believe.
But I’ll check and report back.
Thanks!
Shayne

77cruiser:
When I had my engine dynoed it lost 15-30 hp with an air cleaner on the Qjet.
No air cleaner  made the most power, contrary to what I've read & people have said.
Tried 4 different ones.

Charlotte1:
Yes. Air cleaner assemblies can be deceiving it seems. Engine Masters did a test regarding air cleaners and it was quite educating. They were using a Holley type carb though.
I can imagine that with the qjet secondary air doors, air flow could be very important.
I’m going to go back to my original other air cleaner base which is 1” drop. This is the air cleaner that originally caused the lean condition.That’ll raise up the lid to approx 3” from the carb air cleaner mount base.
But add some tape ,outside the element, along the front to sort of block air at the front of the carb and see what that accomplishes.
   

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