air filter leaning out afr

[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.
   

[77cruiser]

If I remember right the salad bowl was the best.

[Cliff Ruggles]

The Pontiac Shaker assemblies used a very well designed air cleaner assembly.  The lower section dropped some and had a very nice radius guiding the incoming air into the top of the carb.  The lid had a corresponding rise and well designed curve to it.

Years ago I decided to install an RPM intake which was just over 1" taller than my factory intake. This required going to a 1" shorter air filter and moving the lid 1" closer to the carburetor. 

With that set-up the engine idled fine and drove perfectly.  However going quickly to full throttle there was a HUGE stumble/hesitation/bog, which cleaned up after a couple of seconds and the engine pulled hard to the shift point. 

I removed the lid and the problem went away.  Then I tried an X-Treme lid and SOB it stumbled just like the stock lid....hum?  I ended up making a custom air filter assembly for the Shaker opening and sealed everything else off running nothing over the carb at all.

I did some track testing and eventually went back to the factory iron intake and all the stock height air filter and Shaker parts.  It proved to be a very good set-up running just as quick as the taller RPM intake with all the "cobbling" to make it work. 

To this day I don't know why the XTreme lid didn't work as one would think it would equally as good as running nothing over the carb at all.......

[Charlotte1]

Hello Everyone
A small update. I have re installed the 1” drop base that I originally had noticed the afr change.
This air cleaner assembly definitely seems to lean out things. So I’ve tried an experiment. I put duct tape across the front of the air filter element. On the outside of the element. The tape is approx 7-8” long and covers approx 2.5-3” of the 4” element.
And funny enough, with that setup, it seems to richen the afr back to where it’s close to the readings where the air cleaner was off and tuned at. So not too bad.
I’m going to check to see if I have the hood clearance and if I do( I think I do), I’ll try a flat air cleaner base and a 3/4” raised base. With flat base, I might have to use my spacer.
It’ll be a few weeks to get the parts to me but I’ll post back with my findings.
 

[Frank400]

looking forward to hear about the results...

[Jeff K]

I use these.. They sell them on e-Bay. They keep the hot air coming off the radiator from going into the carb. The first time I saw one was on Zora Duntovs Corvette at the Custom car show at Pittsburghs new Civic Arena.. He was such a humble dude. Taking the time to answer questions, you could see he truly loved talking cars.