Semi-generic carb theory question

Pat D

XS650 Addict
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Joppa, MD
So a little back story. I've had my '74 a little over 10 years now, riding semi regularly for 9 of those (did a quickie teardown/restore/paint/polish when I first got it). Been following the forum all that time, and have gleaned scads (technical term) of information and help over the years on this and that. Bike runs like a champ, pulls hard, runs steady state fine, but has always had a temperamental idle. I've practically memorized the carb guide, they've been cleaned to a fare-thee well several times, most recently last weekend. Bike is in tune re: timing, timing chain, valve adjustment. Floats are set properly, I shoot for about 1500 rpm idle hot to keep the battery up and trying to avoid the stalling that bedevils me. Sometimes it just dies. Bike "appears" to be all stock beyond the small details I've added cosmetically- stock air boxes and filters. Mufflers are aftermarket, they appear to be the "Dunstall replica" product on Mikes site, they're reasonably quiet with a wee bit of "bark" when you get on it. They've got what appears to be fiberglass wool in the chambers- they are for sure not open or majorly "free flow". Best quality idle has the mix screws about 1 3/4 out. Spark plug insulation is brown, not tan, so I don't think I'm lean.

Other than the intermittent stalling (I go out of my way to avoid red lights!), I've noticed that after idling a minute or 2, if I slam the throttle open, it'll want to die- if these carbs had an accel pump I'd say it wasn't working. Sitting on the center stand in the driveway idling, same deal. I can open either carb butterfly at the carb, if I slam one open to say 1/2-3/4, it's like I pulled the plug wire- as I let off, it starts firing and revs fine. Both sides will do this independently of each other, or as noted above, together if I snap the throttle open quickly. Now- it seems to me based on the many years I've been doing carbs in automotive applications, if the floats were set too low or there was a fuel flow restriction, the bike wouldn't pull hard to 7k through all gears. Too high and I'd have spillage out the vents. Since there is no accel pump, I can't see how I'm momentarily flooding it. I can't find any evidence of a vacuum leak around the carb boots or airbox connections.

I have not pulled the idle jets to verify they are stock, I have no reason to believe they aren't, but getting them out to check might be an issue. I did replace one bowl b/c someone previously had totally buggered up the idle jet and it was never coming out, the replacement bowl did have the correct idle jet for my '74- I was able to get that one out. So I guess the question is, is what I'm experiencing a "characteristic" of these vacuum slide style carbs? As noted, I consider myself fairly competent with automotive carbs (updraft, side draft, downdraft, 1923-1980, never played with any SU's), but maybe I'm missing something here. I sure would love to be able to sit at a stoplight for 2-3 minutes and not worry about the damn thing stalling or dying if I blip the throttle- frantically kicking that sucker while the cages pile up behind is no fun.
Any thoughts are welcome and appreciated.
Hmmm. Too much info? Too little? Would a video of what I'm experiencing help elicit a comment or 2?
Have you synched the carbs? When I got my XS running, synching the carbs was the Holy Grail for smoothness, idle, power and responsiveness. I used the dead cylinder method.
Carbs are synched via the dead cyl method. All other tune up specs spot on. I'm starting to look to the coils, they're original, and they've gotten hot in the past. Problem seem to kick in after about 10 mins of riding/idling. New coils on the way.
Yes, it could very well be electrical related. Your plug caps look original. Check them too. They came with a rather odd ohm rating of something like 8 or 9K. When they go bad, that resistance starts to climb and eventually can start choking off the spark. It might push through OK at higher RPMs but could struggle at low RPMs. Might explain your stalling at idle. Personally, I just replace the caps with NGKs, pretty much the standard of the industry. You can go with 5K ohm caps but if you have points still, you could use zero ohm caps (no resistance). In fact, that might be the better choice because the non-resistor NGK BP7ES plugs are getting hard to find. With zero ohm caps, you could run the still readily available resistor version, the BPR7ES.

I usually change the plug wires too but if I don't, I at least clip like 3/8" off the end of the old one to get to some fresh wire. I do this on both ends. Then, before install, I prep the ends of the wires like so to insure I get good contact .....

Plug Wire End.jpg
5Twins, I think you are spot-on. I put the new coils on and had no spark on one side afterwards- I had power to the new coils, the issue was somewhere between the points and the coil, a bit of connection cleaning brought the spark back. I also determined that the battery was down a bit, so a perfect storm of small issues combined to foil my riding. I'm still dealing with a weak charging issue, I'm going to start a new thread on best "next steps" to get that resolved. Regarding your comment on the spark plug caps, given that my wires are original and approaching 50 years old and I'm running the stock points, is there any reason to not consider replacing them? Seems like anymore I'm weighing the age of electrical components against the questionable quality of "new" parts coming out of China, and trying to determine which way to jump- this applies to every motor driven toy I have!
Like I said, I usually change the plug wires too. I switch to silicone wires. I use the kits from EMPI for the VW Beetle. You get 5 wires, enough for 2 1/2 twins, lol .....

EMPI Plug Wire.jpg

To protect them, remove the protection sleeve from one of your original wires and cut it in half. It covers practically the whole wire and you don't need that much. You only need it about in the middle of the wire where it exits from under the tank .....

Update. Installed new coils to replace the 50 year old originals, disassembled and cleaned the ignition switch contacts, and cleaned all the connections associated with the coils, points and condensers. Damn! Runs like a new bike- will idle all day long, the electric starter actually fires it off, and one kick when cold gets it going. I'm going to assume I had a really weak spark before, and was basically "drowning" it when I snapped the throttle open, once the revs were up I was getting enough out of the alternator to get enough spark at speed where I had no issues. I will no longer fear red lights and stop signs! After the coils and contact cleaning I did install new plugs and no resistance caps, but didn't notice a difference- it was the coils and clean contacts that made all the difference. Huzzah!