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I had a leak just like that, would only idle with the choke on full until FULLY warmed up, and even then gave me issues. Was the seal on the butterfly control rod. Used some of that "3 in 1 oil" that had a nice long flexible pipe to drip oil on each of the surfaces until I found the one that sucked in oil. Of course with how straightforward and cheap the seals are, may just be worth it to replace all. I replaced most of my philips with hex head while I had it apart, also had to replace one of the four screws holding the plates in the butterfly valves as I had it a little worse for wear by the time I got it out If it does appear to be those seals, here is some info: http://www.yamahaclub.com/forums/index.php...ost&p=80253 If there is no air leak, then maybe just keep cranking the pilot screw out on the side that isn't firing? Mine had to come pretty far out to start firing that cylinder, I think around 5 is when it started purring to life.

could be a bunch of different things, but all fairly easily correctable. first thing you'll want to do is completely clean the carbs, top to bottom. then, make sure the floats are set to the correct height. finally, if the pilot screws are shot, you'll probably want to replace those with new screws. oh, and then balance the carbs so that both are pulling the same amount of air in, operating synchronously. here's what I had to do to my 81 XS400 Special II to get her running this spring (again one where the previous owner had it running reasonably well teh previous year). I had the same problem, where it didn't have much power and only wanted to run on one cylinder: replace an o-ring on the butterfly valve rod to fix air leaking in and making the right cylinder run lean / bike only want to run on choke when cold. ($10) clean both carbs with shop air (90 PSI). (free) replace battery. ($$$) replace pilot screws (combined with float levels, fixed left cylinder not wanting to fire at idle / general performance). ($15) adjust float levels. (free) synch carbs. ($5 for some hose and ATF fluid, had the yardstick) ...which is why I recommended those things

wouldn't hurt to consider it may also be too much air at idle, since the choke increases the amount of fuel in the mixture artificially. leaks around the carb holders if they're cracked the whole way through, ratty air filters letting too much air in, a few less common things, etc etc...

yea I only have about 10 miles on the 26mm setting, I suspect 26mm may be a tad too rich for me as well.

Ahh, I fed you incorrect information Mine only has about 12.7V across at 1250RPM, at 2500RPM you should have between 14 and 15VDC. Apparently at idle it doesn't really charge teh battery, just keeps it from discharging. Make sure you find your correct float height. Diff sources have slightly diff measurements for diff years, mine being an 81 special II (XS400H) has a measurement between 25.7 and 27.2 mm depending upon which source I read if I remember correctly, I just set it for 26mm and man did it make a difference. New battery, correct float heights, new idle screws = wonderful bike The way I've read to set the idle screws is to back them out until the bike stops increasing in RPMs, then turn them back in until the RPMs drop just a hair, then turn them back out just a tiny bit, apparently leaves them a little on the rich/safe side at idle. Do that for both sides, dropping the idle via the adjustment screw as necessary, until both are dialed in nicely. Mine has a little burble, maybe something is just a hair rich, but I figure better a hair rich than a hair lean. Plugs look happy, bike runs happy, so I'm happy

Finally had a chance to work on the bike today, sure enough with the new battery, holding 2500RPM results in 14.5VDC! One thing I noted was that at 1250RPM, the charging system is putting out much less, looked to me to be about 12.7VDC, and the reading would peg out at 14.2 or whatever from about 2000RPM up, but below that it would ramp down to a low of 12.7V at 1250RPM? Sound about right? Also, the battery was shipped dry. I filled it with the electrolyte, and let it sit for a couple hours as per the instructions, topped up electrolyte. Then put it on a small charger ($35 for a new 1.5A max, battery minder designed for smaller batteries like this) for a day or two (recommended at least 10 hours). The next day, the electrolyte level in one of the cells was very, very low...where the hell did it all go!?!? I filled it back up with electrolyte instead of distilled water, as the instructions didn't mention wtf to do in that case...who knows, who knows...it seems to work fine now, I probably ruined it, $60 bucks for a good napa battery...sigh... Still don't have the left cylinder firing correctly at idle, but that'll go back in my original thread...

I asked my pops and that was the first thing he said too...The guy I got this off of said the battery was less than a year old, but...it doesn't seem to want to hold a charge. I've been blaming the POS charger I have...but all signs seem to be pointing towards the battery! Thanks for sharing your experience. Will replace and update the thread with my findings.

Hello, I seem to have a problem with my charging system. At idle, 2000 RPM and everywhere in between, my bike just isn't charging the battery. So, I'm trying to figure out where the problem lies... I've got the Haynes manual (terribly inadequate), the XS360_400 manual from the Service Manual link (not terribly explicit), and the 82 XS400 Seca manual (great, but not terribly applicable). From crossing these references against each other, I've come up with the following for an 1981 XS400H: Any bike, with the engine running at around 2000-3000RPM, you should have about 14.5V across the battery. I do not, too low and drops as the battery discharges... Next, check the alternator. You have a stator and a field coil. The stator is three stationary coils, depending upon which manual I check it says they should be .49 or .72 Ohms, roughly +/- .1 Ohms. So, roughly, .4 to .6 Ohms, or .6 to .9 Ohms. Mine is roughly in that region, at .7 Ohms...as in, OK with one manual, NEEDS REPLACED according to a diff The field coil is the coil that swings around, generating AC current which the rectifier cleans up into DC. Mine is roughly 4.1 Ohms. Depending upon the manual, either 4 Ohms or 4.5 Ohms, again with a +/- range that basically means OK in one manual, NEEDS REPLACED in a diff manual Next, if you have a mechanical voltage regulator you can run some tests to find out if she is A-OK. Mine is solid state, so according to the Haynes manual "it is likely the problem"...thanks, that helps a lot. According to the 82 Seca manual (by which time apparently Yamaha had combined the rectifier and the regulator into one unit) I should be reading less than 1.8V in parallel with ground through the voltage regulator, through the field coil...at least I assume I'm supposed to leave the regulator plugged in, the Seca manual is less than explicit. I read about 11.8V with my battery at about 12.3V (a little low I know). So, voltage regulator problem? I bought two used off ebay, and all three show 11.8V across, so I'd guess that the problem is actually me understanding the manual...or I that just bought two bad solid state voltage regulators? The final check is the rectifier, which is the most straightforward: I just set my meter to the diode-check setting and make sure I got a reading around 700 on each of the single diodes, and 1400 on the two in series (.7 volt drop = normal). Rectifier is A-OK. So did I get burned on two bad solid state regulators? Looks like the wiring is the same as on the older XS400, but the electronics are newer. So I could theoretically take an old mechanical regulator and use it in place of my three bad solid state regulators? Or is my field coil bad? Or is my whole alternator system out of spec and needing replaced? I'm stumped!!!

1/4" drillbit did the deed perfectly for me if I remember correctly. Nothing I did could get my right cylinder to idle correctly, always way too lean. New screws and bam, nice light tan sparkplug. Might want to double-check you have 14.5 or 14.7 or whatever the recommended voltage is across the battery terminals while you're idling. My voltage regulator died, I wasn't getting a charge and it idled progressively wierder and wierder as the battery slowly died before I figured out what was going on.

Well, first problem is only one cylinder is firing. Did the new plugs help? If the left side is completely black, she's running too rich, no air leaks on that side likely haha. A couple things affect how rich/lean the bike runs at idle, easiest thing to check/adjust first is the float level. New idle mixture screws can be purchased from mikesxs.com (look for parts for the 650, our shared carb is the BS34). Doesn't sound like you have an air leak in the boots, but wouldn't hurt to coat them in silicone as a short term improvement. Carb cleaner will get rid of some varnish, but if you have any ports plugged what you're probably going to need is access to shop air. Nothing like 60 or 80 PSI blasting through a port to knock out anything crudded up in there! Don't leave out any details of what you've looked at or changed, anything could be a clue. Is your battery fully charged, and the bike's charging system working? If the battery is a bit low, perhaps the right side is having hard time firing reliably.

if i had to guess, I'd say you're getting way too much air and not enough fuel. take the carbs off & blow everything you can find out with shop air, wouldn't hurt to check the float levels. easiest way to see what is happening during idle is to just take the spark plugs out. are they white? tan? black? wet and black? that means, very lean, happy, rich, omgwtf not firing at all rich. could be an air leak maybe? take a look at the plugs, that'll give you a start. maybe feel (close not touching) the exhaust pipes to make sure that both sides are running. I'm no expert, I'm just (finally) starting to get mine tamed. Now its a voltage regulation problem for me