1967 122S Wagon, B20E/M41 with Dual HIF6 Carbs, choke on forward carb only. Forward carb rebuild with SU23 Rebuild kit, rear carb recently rebuilt with SU-23R kit. Throttle-shaft seals are new and good, both carbs have BAL needles

After rebuilding my rear HIF6 with a full kit (sans my missing needle valve) I find that I get the same annoying behavior that I hoped to fix with a rebuild kit.

That is, my rear carb will idle and balance fine, but when I do the plunger test (the spring loaded thingie that lifts the piston) The engine always stumbles and dies immediately. This happens regardless of how rich or lean I set that carb... which really makes no sense at all.

To complicate things, the engine has several symptoms of an over-rich setting. Run-on/dieseling at shutdown, some black smoke at the tailpipe, some soot on the damper piston, poor mileage and high fuel consumption.

I believe that I've got the forward carb mixture just right. Low smooth idle, idle increases briefly then drops when I try the plunger test. According to my research that is the perfect response for a proper air/fuel mixture. However the rear carb always acts poorly. I generally assumed that it was a problem requiring a rebuild kit, but now that I've rebuilt it with the SU23R kit (same as the forward carb) I'm no longer sure. The rear acts like it never gets enough fuel regardless of the jet position/richness.

I suspect that the rear carb may not be getting enough fuel. I can't think of any other reason that the rear carb would *always* acts like it is lean on the plunger test.


When I do have things balance and the richness matched the engine will not perform at all under load. It idles fine until I try and drive it, then it just falters and idles like crap.

Any comments or advice?