Just an FYI post:
I thought I would be responsible and add J-Tubes to my old school Barry Grant (Holley) carbs. I have ran these carbs for 20+ flawless years. These were on my original Merc 420's then on my 600HP 454's before I went Holley EFI. I just put them back on my new Merc 420 builds with 500 EFI cams. I decided to put my handheld AFR gauge on to check my air/fuel ratio. To my surprise I was dangerously lean. 14.5-15.0 AFR. I had to add 6-8 jets larger to get them down to 12.6-12.8. Everything was linear and I only added primary jet 74-76 to 82. I also went from a 6.5PV to a 8.5 PV. I have a bung in every pipe so I have 4 banks and all were consistent. Also picked up 3mph.

I been saying this for along time too, the J tubes lean holleys way out, the truck crossover tube richens them. Its strange too how float level changes will lean out afrs but that circles back to j tubes, no tubes vs truck tubes and WHY it changes the afrs. Clearly it affects whats happening in the float bowl, Smitty

It makes total sense. J-tubes are in the line of draw and creates negative pressure scenario in the float bowl, the truck crossover has holes in the top for a "ram air" affect and pressurizes the float bowls and normal vents are the in between. Just like the carb/air-box tubes on my snowmobile. It richens up when I pull them off for those -5° nights you get stuck out in every once in a while.

I just twist the tubes around 180°, so the open angle face is towards the venturis. Any flooding would enter the carb and intake as per USCG requirements.

That and having any elec fuel pumps shut off with zero oil pressure. Solves a lot of potential issues.

180° would be directly over my float bowls. Why don't I just install them correctly and jet accordingly?

The standard carb vent tubes are cut at an angle...and face the bowls.

Turning them 180° makes them face into the bore of the carb.

My bad if you thought I was speaking to moving the J tubes 180°.

If you have to change jets...likely the airflow is disrupted enough to affect atomization.

It actually affect fuel in the bowl which affects jetting. If you have a carb with like a 96 mj and j tubes then take them out, you need like 88 mj to get it to same afr, if you put the "offroad" truck crossover tube (which would disrupt air flow far more then j tubes, you need like 82 Mj to get same afr fwiw, Smitty

So the air pressure change in the J tube from air passing over it is changing the pressure in bowl, pushing less fuel through the jets.

Check.

I suspect...any airflow disturbance on the boosters is going to affect their ability to draw/atomize fuel.

Bottom line...AFR gauges with good O2 sensors are worth their weight in gold.

Thank You Smitty.

years ago we had an issue with one of the modified boats. it would start to run lean in the top rpm. this was not the case on the dyno so after lots of staring and head scratching some one can up with what sounded stupid. the boat had scoops that were open in the front and the back and the carbs sat up slightly in the scoop. the idea was close off the rear of the scoop. after closing off the rear with tape all the problems went away. turns out the air flow going straight thru the scoops was messing with the vent tubes and causing a pressure change in the bowls and causing a fuel flow problem. air flow in and around a carb can cause all kinds of strange issues.

The J tubes really screwed mine up. Once I got them out, the carbs were much easier to get right. I was approaching a 100 main jet and still going lean!

Gone!