68 HEMI GTS
Well-Known Member
The stumble needs to be worked out with pump cams, squirter, and obviously proper arm adjustment.
I've got a fun one for you carb guys
- Thread starter 67Charger
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Sweet5ltr
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May want to try a different accelerator pump-cam.
Paul_G
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FYI, They say a good starting point for the idle mixture screws is 1 1/2 turns out and that is correct for the starting point. I have found that they generally end up between 3/4 turn and 1 turn out to hit the sweet spot.
Sweet5ltr
Well-Known Member
Yes, not sure if you have a Wideband 02 gauge, but typically 13.5-14.5 AFR is the target @ idle.
67Charger
Well-Known Member
UPDATE:
I screwed the idle screws in about 1/2 to get 13.8 or so at idle, and actually backed the throttle plates off a little too. The car drove fine, though definitely a bit leaner across the board as expected. The off-idle stumble was not as bad, and the bog at 5" was tolerable. I was really happy... ...until this morning. 45°F and it was a bear. Way too lean everywhere and the 5" bog was worse than ever. Barely cleared up at WOT. This thing does NOT like the 40° difference between early morning drive to work and late afternoon return home.
I screwed the idle screws in about 1/2 to get 13.8 or so at idle, and actually backed the throttle plates off a little too. The car drove fine, though definitely a bit leaner across the board as expected. The off-idle stumble was not as bad, and the bog at 5" was tolerable. I was really happy... ...until this morning. 45°F and it was a bear. Way too lean everywhere and the 5" bog was worse than ever. Barely cleared up at WOT. This thing does NOT like the 40° difference between early morning drive to work and late afternoon return home.
Sweet5ltr
Well-Known Member
And now you’re seeing why many of us have transitioned over to EFI ;) If you really enjoy driving the car often it’s one of the best investments.
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67Charger
Well-Known Member
My 67 Charger with a 496 is EFI...
Carbs can work. I'm just going to keep tinkering to get it to perform at least as it is supposed to. It will never be self adjusting like EFI, bit I've driven a 440 with a 3310-1 for years and it was light years better than this one.
67Charger
Well-Known Member
So, if it runs so much better in warm air than cold, is it safe to assume it wants more fuel overall? Seems the cold, dense air is amplifying whatever is going on. I'm still running around with the leaned out idle and no other changes.
In the afternoons, it starts and idles fine, and in runs good with only a mild stumble off idle and a small (but still sustainable) bog window.
In the mornings (38°F this morning), I have to pump the gas to leave a stop light with out killing it, and any acceleration dropping it below about 10"Hg makes it bog unless I go WOT, and it may be the accel pump that is feeding it though the transition.
In the afternoons, it starts and idles fine, and in runs good with only a mild stumble off idle and a small (but still sustainable) bog window.
In the mornings (38°F this morning), I have to pump the gas to leave a stop light with out killing it, and any acceleration dropping it below about 10"Hg makes it bog unless I go WOT, and it may be the accel pump that is feeding it though the transition.
Sweet5ltr
Well-Known Member
Seems like you need to start trying different pump cams, they’re fairly inexpensive. I never had any issues in cold weather and double pumpers, that’s typically when I drove carbureted cars the most. If it has an aggressive camshaft and the majority of the transfer slot is covered to maintain the idle speed, you’re going to need to drill a 3/32” hole in the primary / secondary throttle plates. By the way, looking back, I had a very similar issue with my last OOTB 4150-carburetor during the tuning phase, it was the pump cam. It ran pretty terrible with the factory supplied cam from Holley.
hangn0ut
Well-Known Member
Your secondary blades need to be cracked. Even if it's not a four corner idle set up, the secondaries add fuel at idle so the fuel doesn't go stale as some drivers may go months without getting into the secondaries.
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