1976 Running Rich


XS650 Addict
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Pine City, MN
I just picked up a 1976 C that had been parked in a shed since 1997. Got it running decent, the stock pipes were rotten so I bought a cheap pair of 17” long megaphones, look great, way too loud for my taste (old guy I guess). I put the small sink strainers in the ends of the mufflers, fit perfect, look like they belong. They quiet it down very well. I cleaned and rebuilt the carbs, the kits I got came with 135 main jets and 27.5 pilots but I put the originals back in. I put new NGKs in (7s) and have put a 150 miles or so on it. It started missing on the left cyclinder, pulled the plugs and it’s running rich on both sides, the left one was fouled and I couldn’t clean it enough to bring it back, I put the old champion in that came with the bike, fired right off.

Stock motor
Stock air box w/foam
Aftermarket short megaphones
Sink strainers

Could the sink strainers be restricting enough to make it run rich? It’s very torquey, great for riding around town or back roads. I like the way it runs, should I re jet a little leaner? I didn’t look at the stock jets, I suppose I should although nothing on the bike look like it’s ever been apart. I’ve read that it should be 122.5 mains?

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Yes, it's possible you've plugged the mufflers up too much. Take the sink strainers out and try it, see if the black plugs go away. That's the problem usually with short mufflers, they're not big enough to run quiet. If that does cure the black plugs then I'd recommend different mufflers. These Commando mufflers from YamahaXS650.com are wonderful, about the best muffler available for the 650 at the moment ......


They look good, flow well, and sound great (a bit louder than stock but not too much).

Yes, the stock main jet size for the '76-'77 carb set is a #122.5. Stock pilot should be a #25 of the VM22/210 type. If it's running this rich with the stock size jets then something is definitely amiss. If it's not the plugged mufflers then it could be in the carbs. Are the floats set correctly at 25mm? 24mm or lower would give a higher fuel level and make the bike run rich. Also, were the carbs all gummed up when you cleaned them? If so, that gas varnish can eat away at the jets and make them bigger (richer). In particular, it can eat away at the needle jet. This controls the midrange and that is where you spend most of your time running. Did you move the clip on the needle? Stock setting is in the middle slot (#3 from the top). If you're set lower than that (in slot #4 or #5), that will make it run rich.
I like those mufflers, price is right too. I have a CM400 running with shorties and sink strainers, that runs fine with them, I could put the shorter megaphones on that (I don’t really like the looks of the shorties, but they were available locally when I put the CM together and have worked fine) and get the commandos for the 650. I’ll pull the strainers and see if the plugs clean up. Do these motors like NGKs or Champions better? Seems like different motors have preferences.

I need to check the jets that are in it, I have pin gauges to measure them. The carbs were surprisingly clean, they had 1997 gas in them, it had 1/4 full tank as well, both cleaned up well.
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I pulled out the sink strainers and rode around town for 15 minutes or so, definitely too loud for my taste. Definitely revs more freely. What about packing them with the stainless scrubbers? I believe they are Emgo mufflers ~17” long, riveted together, I have seen posts on others packing them, looks like perforated tube an a perforated disc about 1/2 way down the length, something like the attached, but chrome


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I pulled the baffles, these were packed with packing cloth, pretty dense, and it was in the back half only, not sure if it was pack that way or if it pushed back there. I repacked with SS pot scrubbers, 6 per side. Still pretty loud but it is a little quieter and lowered the tone, sounds nice and not quite so obnoxious. What about a disk midway, attached the the perforated disc that is part of the baffle?
Sounds good. As 5t says, do double-check your float levels. You can have an otherwise perfect set up but if fuel level is to high you'll run rich.
I cut 3 3/4” holes in the sides of the baskets and put them back in, sounds awesome. That should allow considerably more flow and it tones it down a little more. I just put new plugs in, I will try to get the majority of miles on at highway speed, I realized that when I fouled that plug most of the driving I’d done was around the block to test things.


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I got out and rode about 10 miles or so, some in town, some on the 2-lane. This is really the first time I've spent any time on the highway, wich leads me to believe my rich condition is in my pilot and needle jets. It pulls super strong up to about 4000 rpms then kind of runs out. I have to say, I really like having the torque and wouldn't want to lose that, but now it is making me think I should bump up my main jet. It is stock now, but it does feel lean, no pops, but sort of "strained" sounding, if that makes sense? I've never owned one of the XSs before, how do they typically make power? Do they like revs?

I thought re-jetting 650 carbs would be simple. After all, it's just an old time twin cylinder bike for crying out loud, lol. But no, it's not. You must change everything, mains, pilots, and needle clip position. In your case, yes, you will need bigger mains. The '76-'77 carb set came with the smallest mains put in any 650. They're fine if the bike is all stock but quickly outpaced once you start modding things. With your home-brewed sink strainer baffles, I can't say exactly how big you'll need to go but I can tell you about the biggest this carb set will take is a #132.5. Above that and you start getting upper midrange break-up under heavy throttle, even with the needles leaned a step.
The carb kits I got came with 135 main and 27.5 pilot. I am guessing no point in putting the 135 main in, 125 or 130? what about the 27.5 pilot? It is awesome on the bottom end right now. Where do you buy your jets?
Yes, a 135 would be too big for your carb set, but a 125 is only one size larger (jets come in 2.5 number increments) and probably isn't enough. I'd start with a 127.5 then try a 130. The stock pilot will work fine until you lean the needle a step. Then you'll need the larger pilot. And by the time you increase the mains a few sizes, you will most likely need to lean the needles a step to clean up the upper midrange to main transition area.

I use only genuine Mikuni jets. You can get them from Niche Cycle.
I haven’t got my jets yet, still running my strainers with 3 3/4” holes in each one, straining the sound. Went out for a ride, it was spitting and sputtering around 5,000 rpm’s. I choked it and it didn’t really have much effect, or was inconsistent, didn’t cure the issue. I pulled the plugs, light brown to tan, not horrible looking, not fouled. Still runs great below that though. I’ll try the 127.5s when I get them. Thoughts?
Got my jets, ordered the wrong ones with the large round, turned them down on the lathe, can even still make out the numbers.

127.5 - slight improvement but still falls off a little after 5,000
130 - pulls great to 4,000, then it blips a little til 5,000, then starts to pull again, kinda done at 6,000
I would guess this is the “stumble” due to the needle/needle jet being rich now?

Feels like the top end could go another step up, I am thinking I need to lean the needle by a notch before I try that.

I hadn’t done the spray test so I did that before I go any further, that was good, still has the stock air box including foam, all in good shape.

Yes, lean the needles a step. But remember, that will lean the idle to midrange transition area as well and may create a flat spot there. If so then the larger 27.5 pilots will be needed to "fix" that.