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.


Honda Etched On Brain
Top Contributor
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Michigan, tri-state area
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.