HAVE A QUESTION ABOUT SINGLE CARB V-TWINS

[CORSCO]

A few weeks back, I installed jet kit/pipes on a XV1700 Road Star. This week the customer came back complaining of it running too rich.
I brought the bike in and hooked up the EGA to it.

Seeing as how the bike has only one carb, these were the readings I was getting...

F. CYL. had 2.2% CO and 200 PPM.
R. CYL. had 8.2% CO and 2000 PPM.

I tried it with the AIS disabled and enabled. With it enabled I got lower readings of course, but the same variance. Compression was 140 in both cylinders. The bike only has 4000 miles on it.
I thought possible carbon on the intake valves on the rear cyllinder so I ran some AMSOIL through my fuel bottle then ran Combustion Chamber Cleaner through the vacuum port on the intake tract. No help.

When at half throttle, you could hear the rear cylinder missing or loading up. The jet kit came with a larger idle jet, so I installed the stock jet back in. It brought the rear cylinder down to reasonable levels but leaned out the front.


Today, another XV17 came in for a jet kit with 1900 miles. I installed it and the readings were the same. Also, our other tech did pipes and a jet kit on a VTX1300 and came up with about the same readings, also a single carb. Lean on the front, way rich on the rear.
After I did the XV17 a good customer brought his VTX13 in for a jet kit. I EGA'd it stock first. The front and the rear were close enough together. EGA'd again when done and again, both were the same.
Now, on both the VTX's the valves were done. The other tech did a full service plus the pipes/jet kit. I did the jet kit and valves on mine. The customer had just installed the pipes himself.

So, my QUESTION after all this backstory is...
Why would the rear cylinder be so much richer than the front cylinder on a single carb model?

I had fans on the bikes while running. The readings were the same at initial start-up and after the bike warmed up so the rear cylinder getting hot theory does not apply here. I can't wrap my head around why this would be.
Any insight???

[dieziege]

Maybe it is intentional, to keep the rear cylinder from overheating?

Intake manifold design is a PITA anyway...it is hard to guess where the fuel/air mixutre is going to go if you have on carb (or injector) feeding several cylinders if the fuel isn't very well mixed with the air when it leaves the carb.

[BuzZz]

Since the rear-runs-hot thing doesn't seem to be the problem here, it sounds like some sort of flow dynamics problem.

These new pipes that have been installed on these bikes... is the rear pipe notably shorter than the front? Routed with less sharp bends and restrictions?
Some other factor increasing the gasflow through the rear jug?

That's about the only straw I can grasp at....... am I ever glad I don't have to deal with customers....