Shakedown breakdown ‘83 sputtering

hermanlifer

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All was going so well. About 30 miles into riding around aimlessly and making minor adjustments over the last few days I got a real problem. I’ve read and read post after post about similar problems but I figured I’d share my issue and eventually it’s solution.

Problem? Sputtering at higher RPM. I don’t have a tach. But if you open up the throttle with any gusto it will sputter and bog, in any gear. It happened out of the blue, running strong along about 45mph I decided to crank it up a little and about 50mph it sputtered. Backed off throttle and power returned. Limped home about 12 miles with a feathered throttle as it began doing this in all the gears at all speeds. If I upshift and drop rpm the sputters disappear so it seems clearly a carb issue. Did the hours of reading online, popped off the carbs and inspected. All looked clean, except a suspect diaphragm.
1983 Bs34. By the way.
Pinhole and slight tear?!! Could this be my problem? Did these old diaphragms only have 25 miles left in them? I’m going to try the plasti dip repair method tomorrow. We’ll see how that holds. In the meantime I can’t think of another reason why it would happen so abruptly like that. I understand jetting and air leaks and all but it’s been solid in the driveway and up and down my road during rebuild and testing and the short bits of road riding I’ve done since getting legal last Friday.
I inspected main and pilot jets and all looked clean, no junk or gunk this is essentially fresh build. New tank lines filter etc etc.

83xs650
Bs34 carb with stock jets
Original boots, not perfect but they passed air leak test with starter fluid.
K&n filters
Vacuum ports in carb manifold linked.
That’s all I can think of now. Long ass day.

Picture isn’t in focus but the light colored streak is the tear, but only a little pinhole of light is visible surprisingly. Both diaphragms seem to fall pretty slow too, so I’m not 100 on anything tonight. More questions than answers for sure.
 

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That tear sure looks like the culpret doesn't it. By the way ,you can easily check your diaphrams without stripping the carbs. Place your thumb over the top vent and lift the slide with your finger. Time how long it takes for each diaphram to hit the bottom ...should be in excess of 15 seconds any less and you have an air leak somewhere.
usual culpret is around the cap seal . I always run a smear of Vasoline around the rubber edge before closing the caps to get a perfect seal.

Before you tear into the carbs check that your battery is holding a good charge and your charging system is working ok. Spluttering at WOT is sometimes caused by low battery voltage robbing the coils and ignition system. Try running WOT with no lights or try turning on the flashers at WOT and see if there is any difference in performance. If none then that would pretty much suggest your carbs need attention.

Another test to check fuel flow at WOT in any gear is to apply a little choke . if there is any improvement in pick up then I would measure your fuel flow from the tank to the carbs and also check your float levels carefully.
 
The K&N filters could be contributing to the problem. A pleated type element like the K&N doesn't work well on these CV carbs. The pleats disrupt the air flow and a CV carb relies on smooth air flow to properly lift the slide. The problem is at it's worst in the upper midrange or when you apply heavy throttle, which sounds just like what you're experiencing. Try a test run with the filters removed.
 
Checked diaphragms drop speed with carbs mounted. Left side 13s, Right side 23s.
Checked battery voltage. 13 off. 13.3 running, 14.4 when revved. Brand new rotor.
Cleaned up fuse box connections, yeah I’m running stock box.
Running a bit rough so I popped off the K&N filters and ran it open. Idle smoothed our and revs smoothed out. Hopped on and sure enough no sputtering. Yesterday I couldn’t even pull out of the driveway without sputter, today I could get the front end lifting up in first and second. In interest of troubleshooting I popped filters back on right away and I’ll be damned but it ran just fine, maybe a little tougher but no sputtering, with them back on?!


Major difference between yesterday and today? Yesterday was 12 miles into a lot of start and stop riding in 90+ degrees Fahrenheit.
Today it’s cool and maybe 70 on a lightly warmed up bike.
Though yesterday I tinkered into the evening removing and replacing carbs so engine cooled off considerably and she still failed and sputtered.
So wtf did I do? Fuse box? Enough time to cool? Hot gas???? My tank is bare metal and my lines and filter are clear And they run near the motor. Hot gas wouldn’t atomize properly? Man I’m reaching! Haha.
 
You say you have K&Ns but are they genuine or a knock-off? I ask because some of the knock-offs have a large inner lip, a "stop" of sorts so you don't push them on too far when mounting. That lip can block the air ports around the outer rim of the bell mouth. Those ports feed your main, needle, and pilot jets air. Maybe you had the filters mounted a little differently before compared to now, pushed on more and blocking those air ports more .....

Uvs9R7Y.jpg


The air port blockage isn't 100%. The bike will usually still run OK at lower speeds because it can still get enough air into those ports. It's higher speeds and RPMs where the issues usually arise.
 
Ok. Home from a trip, got the bike back out. Immediately starts rough and runs like shit. Sputter sputter cant hardly leave the driveway. Popped off filters, no change. Adjusted clutch, long shot, no change. Adjusted fuel mixture, and tweaked with carb sync screw. Sputters now happen at much higher rpm than before, I can push each gear up to its top end before the sputters start!. I rode across town and back with no other issues, I did take the screwdriver and tweak the carb a few times as I went. Starts back up hot with one kick. So its not done but I am on track, I mean i could ride this bike all day long with no issues as long as I dont get on that throttle too hard.So far i have only synced the carbs by sight and sound, I think its time to figure out vacuum syncing.
 
Progress is good! synch is mosty an idle area issue, if it's within a stone's throw it wont affect WFO.
 
Thinking now its got to be the diaphragm or the jets. I will order up some different jets and experiment, Any known sets for open 2 into one pipes that end at foot pegs?

. Went out again today and was needing that hard throttle to pull into faster traffic, no fun feeling like I cant rip up to speed when I need it.

Also, hard lesson learned: low on fuel, uphill with the way my tank is mounted equals running out of gas even on reserve....in traffic on a two lane country windy road. Luckily I got it off safe and only had to push a ways to get to flat and get some fuel. The old man who saved my ass with the gas can said I better only run ethanol free, How sensitive to modern fuel are these older engines?
 
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Another round of work, no joy.
Can’t afford to order any parts yet, so I’m doing the poor mans method of troubleshooting and trying to get her running. Especially cause I’m not 100% certain what the problem is, don’t want to throw money blindly.
So, I tried to repair suspect diaphragm, no change.
Double checked all wiring and put new plug wire caps on.
Test ride went well! No sputters, ripping up the road. Went back to the shop, handled some work for about an hour. Geared up for a proper test run and sure enough, sputtering terribly.
Decided that I’m going insane.
Attempted the dead cylinder method to tune carbs. Found an issue with left cylinder! Wouldn’t run by itself without cranking idle way up. Right side purrs like a thumper. Swapped plug wires, plugs, vacuum caps, and even the damn diaphragms to see if it followed. No change. Left side just won’t go like the right. Off with the carbs. Clean, re check floats. Compressed air throughout, reinstall this morning. Starts up easy, let her warm up. Begin dead cylinder method. Right side goes easy. Left side, goes easy too! Ok, try to set mixture screws, thinking I got it. Restart for testing and idle sounds great, less gallop more purring. But....still sputters off idle and even in neutral can’t reach higher rpm. Short test ride confirms unrideable. So back to square one.
Why did I get one mystery good ride yesterday?
The boots aren’t interchangeable so that’s one thing I couldn’t swap to chase bugs out, but today that suspect side ran well alone.
Next step compression test. I’ll have to acquire the tool.
What on earth would cause these intermittent sputters?
I did check that air filter wasn’t blocking intake, and I did check valve clearance and cam chain as well.
 
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Compression approximately 120 each side, probably better but that’s kicking over with an old gauge and the throttle wasn’t open .
Tested coil, rechecked every electrical connection. All is good.
Kicked over, runs nice on idle. Slowly apply throttle with filters off, looking into carbs, the slides are not rising just quivering a bit.
Done for today. Kicking old gal in worn out chuck Taylor’s I think I bruised my foot Hahahaha ahhhhhhh fuck
 
... Slowly apply throttle with filters off, looking into carbs, the slides are not rising just quivering a bit...

Apply more aggressive throttle, the slides should visibly rise. A bit of quivering is normal.

What did you do about the tear in the diaphragm?

Sealers, whatever, applied to the outer part of the diaphragms can stick the diaphragms to the sides of the carb body in there, keeping them from moving...
 
ignition???
Yeah, i suspect the ignition unit box, but dont have one to swap for troubleshooting. Inspected the guts and see no visually bad components, who knows what lies in the chips. Are they programmed? I run an electronic repair shop and have a degree in electronics. I could build this little circuit pretty darn easily.

I used a vinyl sealant, like a tire sealer. Just a dab and it made no change to the sputtering issue. Again, it was running like a top and then like a switch was flipped it began to sputter and wouldnt run at higher rpm. I love a good challenge but this one has me stumped.
 
I’ll re solder that board today. I read up on the rotor magnet, the rotor is brand new from mikes. I’ll see about a magnet and try that. Otherwise the pickup coil tests to spec, ignition coil tests good as well.
 
Replaced some diodes on tci board, they weren’t shot but I figured fuck it replace em while it’s open.No clearly bad solder joints, touched up most joints anyways.
As usual, fires up and idles great.
Still will not rev up. Popping and backfiring when I really go wide open. Looking at slides, they really don’t come up as I’d expect and I don’t see any fuel flow. I’m gonna see what I can do about getting some round slides on here. Even if they don’t cure my problem I’ll at least be free of the BS34 .
 
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