So yeah...logic doesn't work here, AFR goes lean after attaching hoses and air cleaner can (1 Viewer)

FJ40Jim

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At 5500' the truck has just switched HAC from low altitude to hi altitude. It's a binary system, but altitude is progressive. At 4500 it was rich, at 5000 it switched to hi alt lean-out, which is actually leaner than ideal. It will be just right at 7500'.
 
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Following up: after pulling the carb to fix my secondary slow cut valve linkage issue, I struggled to get it running for the last month. In my mind, all I had done was pull the carb off, fix the bent linkage and slap it back on. It would fire up, run lean, then die. I pulled the carb again to check for crude in the idle circuit, and sprayed all the jets with brake clean. Didn’t find any obvious blockages. Put the carb back on, fired up and ran good with 13.5 air to fuel ratio. It was late on a cold night so I shut it down and left. Thought I had fixed it, but in the morning I buttoned everything up and started it, aannddd it ran lean and died. Took the FCS out and the brand new oring was gone, wondering if it got sucked into the idle circuit somehow.

Meanwhile, I got another carb to rebuild. Finally put it on last night and it is running great, it idles smoothly and I can adjust the IMS for lean drop, set it to operate around 13.2 AFR. Buttoned it up and still operating around 13.5 at idle. My vacuum is 16.5 -17 - great reading at 5500’ above sea level. Starts without pumping the accelerator - never had that before!

Here’s what I changed, I got gaskets for the base from Kurt at cruiser outfitters - put one on either side of it and I got another rebuild kit that had the paper gaskets for the middle insulator and installed those, I think both those insulators weren’t sealing even though they were new from 2 years ago. Also, my old throttle shaft weeped some and maybe that wasn’t helping. Time will tell if this one weeps. Lastly, and most importantly I used a magnifying glass on my old jets and they did have a tiny amount of crude - the lesson for me is even though I had a great stream of brake clean going’s through them and I could see light through them, they weren’t clean. It took a guitar string #12 pushed through them to really clear them out. I suspect that and the bottom gaskets were the main problems.

Will drive it later today and see if my old issues and running lean are completely gone.
 
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At 5500' the truck has just switched HAC from low altitude to hi altitude. It's a binary system, but altitude is progressive. At 4500 it was rich, at 5000 it switched to hi alt lean-out, which is actually leaner than ideal. It will be just right at 7500'.

Final update:
Before the carb rebuild, I had failed smog for high CO. With my new carb rebuild, gaskets on both insulators, new spark plugs, new air filter, snorkel fitted to air cleaner to get cool air to the intake, afr tuned at idle to 13.5ish, and new bushing on distributor, which I set slightly retarded, I finally passed emissions after several previous failed attempts. Now I’m the proud owner of a 5 year collector plate. No emissions for another 5 years.

Couple things I noticed, as @FJ40Jim alluded to, the HAC was leaning out my air to fuel ratio too much even though the truck is usually around 5300’ elevation. Also, living at altitude it’s hard to pull enough vacuum to get secondaries to open easily. Neither one bothered me for emission testing as I figured it’d help keep my total CO down. Afterwards, I followed Jim’s idea, and plugged the HAC lines. My AFR readings moved in the richer direction, which is what I wanted. Then I put a smaller, softer spring in my secondary diaphragm today, and finally for the first time, it opened, only about 1/4” but it’s enough to richen it when I’m on the highway - where before it was running 15-15.5 at 55mph, now it’s a smooth 13.7. Hope this helps someone else down the road.
 
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Driving wayyy better up the canyon in third, having gas coming thru the secondaries is sorta important I’m finding. @NeverGiveUpYota get a AFR, it helps a ton for tuning and getting things right....

Now time to rip out the smog for 5 years. The kit from Jim should be here...can’t wait to see how much better it drives then!
 

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