'78 XS1100E, '80 carbs low rpm funk

[gtem]

All,

An update on the '78E with '80 carbs situation: I think I've almost got it. So the current setup is the gentler 5IZ7 needles with 120 mains, I went down to #40 pilots, and am finally getting a decent color on the plugs, pending more testing. The last remaining issue was cyl #4 kept getting a sooty plug. I checked and rechecked everything, tested the choke circuit for leakage, etc. The rest of the carbs had the idle mixture screws set out to 3 turns out with the #40 pilots but #4 would get sooty even with just .75 turns out. Finally I pulled the mixture screws from cylinders 3 and 4 to compare, and did some flow testing with WD40 through the idle circuits and saw that #4 was flowing more past the idle mixture screw even though it was turned in tighter! Upon disassembly of #3 and #4 idle mixture screws I found that the o-rings are all pretty compressed into a tapered conical shape (the seat that they are pressed against). Well it seems that at some point when messing around I had reversed the direction of the o-ring on #4, and somehow(?) that was causing problems. Or maybe my "flow testing" fixed something. I don't know. But I swapped the idle mixture screws on 3 and 4 and the rich condition on #4 seems to have been resolved. The bike has the cleanest idle it's ever had, I've never been able to lug it down to 2000rpm like this before. I'll be fine tuning the idle mixture screws a bit more, I think I'm a tad lean at the moment (set them to 2.5 turns out). I'm also considering repacking the RC pipe with some looser type fiberglass wrap to help it flow a bit better (hopefully without getting too loud).

[gtem]

A further update on the '78E with '80 carbs and RC 4-1 pipe (#40 pilots, #117 mains, 5IZ7 needles),:

I had started to get sooty plugs again, I had about lost my mind! Well I finally thought to take a peek, and realized the crusty old baffle had collapsed inside of the exhaust, restricting flow quite a bit. Re-welded the baffle and repacked it, I'm now back to clean plugs. The baffle in the pipe has roughly a 1-3/4 to 2" ID, perforated metal surrounded with fiberglass packing. When the bike is warmed up it will do some light juddering from a 3000rpm roll-on but then cleans up and pulls nice and hard to 90+mph Some backfiring when coasting down in gear, that seems to go away when I put choke to the halfway on position. It doesn't like to sit and warm up on choke, I almost immediately take it fully off and just bounce the throttle lightly until it holds and idle about 30 seconds-1 minute after starting the bike.

So it seems like I'm back to some low-speed/transition circuit tuning. A check with my colortune shows a nice blue at idle and it mostly maintains that when revved, there is a moment where it looks a bit orange, but it's hard to simulate accelerating against a load when just doing static revving. Below are photos of plugs #4 and #1 with the idle mixture screws turned out 3 turns with the #40 pilots.

Does the sooty base ring imply a rich idle/low speed mixture? I'm not experienced enough yet to have that dialed in feel to tell a lean stutter or backfire from a rich one. Bike doesn't smell too gassy or anything at idle, idle feels very nice and smooth. Before, the plugs would be totally sooty including the entirety of the ground strap. They're staying remarkably clean now.