Ok, I'm having a bad week with nice stock L24 with SU's.  Runs fantastic until you get over 4-4.5K RPM, then it leans out and misfires.

Facts and Observations

• SM needles (been trying several others....)
• Four screw early carbs. Fresh and minty.
• New AEM AFR gauge in the exhaust. Never leave home without it.  AFR's 13-14 around town below 4K RPM, car pulls like crazy super smooth 800 RPM smooth idle, but crawls up to 15-16.5 or higher and misfires when you get over 4K. Doesn't fall on its face, just gets rough running and power falls off and has trouble getting to 6k RPM.
• Fuel deliverability confirmed. NOS stock pump and then with an NOS electric pump (mechanical out of the circuit), both behave the same way. Both produce buckets full of fuel when I free flow them into, well, a bucket. 
• Float level confirmed. Was a tad low, now perfect. No change in behavior before or after.
• yes there is oil in the domes. 
• If I pull the chokes on and get the jet tubes down 1/8 then it runs better and the AFR's stay near 15, but turning the mix screws down to 5 or 6 turns to get that same result makes the idle-4K range pig rich and awful. No surprise there. Not a solution.
• Tried another set of SM needles, (these dang things are typically too rich on a stock L24!!! Not this time) same thing. 
• Valve adjust and vacuum are normal. 
• NGK BPR^ES usual at 0.036.  All 6 look identical and lean-ish after a few hard pulls. 

I'm about to start sanding down the lower half of a set of SM needles to try to get it richer up top. Not easy to tell where to start sanding.

Suggestions? What am I missing?!?!?!?!?

Edited July 21, 20231 yr by zKars

Hmmmmm.   I wonder if a loose and floppy distributor cap could have anything to do with it. 

Can a spark mis-fire read lean just like lean can cause a mis-fire?

There. See? It was the loose dizzy cap. Guess it went wonky (sorry for the technical term) above 4k. 

I knew if I asked you guys would help me solve it! Even if only in spirit.....

So what did we learn today?  

-Misfires can cause a lean condition as read on the AFR gauge, just like the reverse. 

-Change a bunch of stuff and get the same result? You are changing the wrong stuff.

-Tunnel vision is blinding.

 

 

 

Edited July 21, 20231 yr by zKars

Glad you found it!

Well, that's what I get for not suggesting looking at your ignition after I read your post. I could have looked like a stud...well, yeah, that is going too far. 😞 

Of course for a sparky type, we usually suspect electrical first.

6 hours ago, zKars said:

So what did we learn today?  

-Misfires can cause a lean condition as read on the AFR gauge, just like the reverse. 

-Change a bunch of stuff and get the same result? You are changing the wrong stuff.

-Tunnel vision is blinding.

Love the mission summary.

I've heard it put this way... Misfires leave oxygen in the exhaust stream. AFR pics that up and interprets it as "lean".