Ok, my bad. I think I now understand what you are asking. I apologize.
When running two carbs online, most of the time you are using Holley 4160 carbs because two 4150’s wont physically fit in line. I don’t like that for several reasons.
1. The 4160’s use a metering plate and not a metering block like the 4150. That’s what makes them fit in line. That plate system is thinner.
2. That plate system makes it more difficult to tune the secondary side of the carbs. You can do it, but it is more time consuming and if you make a change that doesn’t work it’s harder to go back to what you had. That’s because for the most part you can’t drill and tap the plate for a brass set screw. You can close a hole you’ve made to big with JB Weld or something and drill it again, but again that’s a PITA if you want to make a quick change.
3. No matter what, if you want to make a change to the secondary side, you have to pull the front carb to get to it. If you want to change something on the primary side you have to pull the rear carb. That’s working to do work you don’t need to if the carbs are sideways.
4. You end up with the little float bowls with side hung floats. There isn’t much you can do with those floats to help with fuel moving to the rear of the bowls. The fuel will stack up at the rear and push the floats up, reducing flow if not shutting the needle and seats off completely.
5. Unless you run 4 primary bowls or maybe you can drill and tap the side hung float bowls (it’s been forever since I’ve even looked at those bowls so I’m going off at least 30 year old memories here) you put the fuel in at the primary bowl and the secondary side gets its fuel through that little tube that runs down the side of the carb.
I take all of that into account when thinking about mounting carbs sideways or in-line. So it’s not just one thing. It’s several.
You would be correct that a single carb, mounted in the front/rear orientation does have the same issues as mounting 2 carbs in line. The fuel runs to the back of the bowl, uncovering the main jets and lifting the float, shutting off the fuel. Same issue for the single and dual four set ups.
There are things you can do to address that in a single 4 like jet extensions. The bigger center hung float bowls have enough fuel volume to help a lean stumble from uncovering the jets with the extensions. If you love dragging the bumper for 300 feet then you have to do some other things. Guys have been modifying, reshaping and reforming floats for longer than I’ve been alive.
When you mount the carb(s) sideways you get the benefits of the bigger float bowls, easy access for tuning changes, secondary metering blocks and you can buy floats that are wedge cut on one or both ends to prevent the float from moving up on acceleration and shutting off the fuel.
If you get the single wedge cut floats (sometimes called “drag” floats) you have to make sure you get a left and right cut float when looking at the float in the bowl.
So the primary side (when looking at the primary float bowl head on) will get left cut floats and the secondary side (again when looking directly at the secondary side) will have right cut floats. If you mix them up, it will make the fuel slosh shut the fuel off because the cut is on the wrong side.
Or you can get the double cut (some call them “road race”) floats so it doesn’t matter which bowl they are fitted to. These double cut floats are also better for street driven tunnel ram stuff because going around a corner can cause enough fuel slosh to shut the fuel off and cause a stumble.
So the short answer is a single 4 has the same issues as in line carbs on a tunnel ram. Which led to my comment about some guys mounting their single 4 sideways. Again, I’m speculating here but I would think that was an effort to help with fuel control issues that couldn’t get fixed by the methods used when the carb is mounted front to rear.
I hope I made that more clear than I did before. Your inference that a single 4 would (and does) have the same issues as in line carbs mounted on a tunnel ram is correct. They do have the same issues. Evidently at some point some guys were turning their single 4 stuff sideways to help combat fuel slosh issues.
