Calling Steve - Weber jetting ideas?

JFENG

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I’m a bit stumped on how to make the final jet changes on my 38DGAS carbs.

Here is the data: yellow indicates poor running condition
792853AB-C075-4BB5-9AAD-7FC73919A0C8.jpeg


One set of jets is perfect, except for a very VERY narrow lean spot. Zero hesitation or traditional bog as you open the throttle. But if you very slowly open the throttle there is a super narrow lean spot just about flat ground cruise (eg a very slight rise in the road). See the yellow cells in the table. Best lean idle is 1.5 turns off the seat.

One step richer main jet and 1 leaner idle results in over rich under all but very light throttle but no lean spot. Best lean idle is 1.75 turns odd the seat.

I’m hoping richer idle jets (55 or 60) and the 145 mains will fix it. The 55 idles are one turn out. A 60 idle will likely be close to only 1/2 turn out for best idle. Sound right to you?
 

Stevehose

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For kicks try opening your mixture screws 1/4 turn to see if that lean spot goes away. If so try a 60 idle jet. You could possibly lean out the higher rpms with a larger air corrector.
 

JFENG

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Stock mechanical pump without a regulator. I actually do not know pressure, but I also know the behavior is consistent so there is enough flow and float level is not changing.

But yes, when I install my new motor I will fit a pressure gauge and good regulator for the tuning phase.
 

JFENG

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Steve,
Update,

I went to a bigger idle (60) and small main (145) and got this.
0F0E6037-2DCF-4657-A975-181EFE76136E.jpeg

I drove it for 50 miles and this AFR was repeatable over pretty much any RPM between 2000 and 5500. It was very smooth and drove great. Had I not looked at wideband AFR data, I would have stopped right there. But as you can see the WOT is still rich by a full point.

So I drilled out my 190 air correctors to 1.97mm (5/64” drill). Now I get this:
D7FCC6A2-4A02-43AE-B5AE-3026168FFA84.jpeg
I tried hovering at that 15.5 lean spot, which corresponds to a slow accel in 5th gear from 65mph. St no point could I detect any toughness or hesitation or pinging. Car seems to have no problem with that very narrow lean blip.

How do I get rid of the blip so I can open my air correctors a little more? AFAIK, I either need an additional progression hole or a very slight chamfer on the throttle plate to uncover the last progression hole a tiny bit sooner.

How about changing the Emulsion Tube? It currently has an F57. I’ve looked and cannot find any data on AFR vs e-tube type.

Ideas??

John
 

Stevehose

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I don't know much about downdrafts but typically the air corrector tweaking is something you'd do for the last 25% of throttle - or higher. I would solder up the a/c's and and mains and redrill to 190 and 135 respectively, and drill out the mains if too lean until you can get to 12.5-13 after 50% throttle. I would keep e-tubes as is unless you can't make any headway on above.
 

JFENG

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That was what I was thinking too. Then I talked to the tech guy at Pierce. He said that the air correctors will affect the whole rpm range but mainly at large throttle openings. That’s why I tried opening up the main air correctors. I was surprised but I DID see the WOT lean out about 0.7 over the entire rpm range. So he might be right.

I just noticed there is a separate idle air corrector. I think I can use that to richness up the middle of the progression rangewithout adding gas to the main circuit. I don’t think DCOE’s have these as separate pieces.

At this point it seems like mainly an academic exercise of little practical value. But, maybe someday I can help someone else with 38/38synchronous Weber’s.

Thanks for the feedback.
 

Stevehose

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Yes the a/c is most effective towards WOT, other throttle positions is more affected by jetting. You really have to push it hard for a/c changes to be seen. I would still try the above.
 
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