Okay, carburetor gurus. Got one for ya.

My 1980 XS650 has been running great until recently. It's basically stock with a stock bore big fin cylinder, a Mike's 2 into 1, stock Mikuni BS34's and pod air filters, stock TCI electronic ignition with Iridium plugs. I'm pretty familiar with motorcycle carbs, messed with a bunch of them over the years. These BS34's are almost exactly like Stromberg CD's as used on British cars and I'm pretty familiar with those, but this problem is whipping my ass.

While riding recently, I noticed at WOT the engine would start breaking up. It was definitely throttle opening related, the engine would pull smoothly all the way to redline unless the throttle was wide open. But off the bottom everything was fine. Then on a short hop I noticed it ran like crap just off the bottom for a few seconds, then cleared up and ran great. The next time I fired it up, it ran perfect at idle but would die as soon as the throttle was cracked. This was (is) so bad that it's unrideable.

Pulling the choke out to the first notch gets rid of the flat spot and makes it rideable so that tells me that for some reason the pilot and progression phase is lean. So I figured the progression holes in the carbs (it's a lot like a Weber or Dellorto sidedraft that way) were probably clogged with something or other. So I've been through the carbs twice and verified that the progression holes, the idle hole and everything else are clear. Still the same problem. (BTW, it's possible to kill the engine by screwing the idle screws all the way in but cranking them way out doesn't seem to have much effect on how it runs.)

Here's the strange part: if the engine idles for 30 seconds or so and you open the throttle the first time it revs right up, then after that each time you try to open the throttle the flat spot comes back worse and worse until it just dies. If the engine idles for 30 seconds or so, again the first time the throttle is opened it's okay then each successive throttle opening gets worse, like the progression and pilot phase are running out of fuel.

Valve adjustments are correct, the plugs are clean, it has the correct type pilot jets, I've been through the carbs twice to clean everything out, fuel flows freely from the petcock, I've raised the wet fuel level as a test, all to no avail. The carb boots are not leaking vacuum but the throttle shaft seals are. Could this be the problem or am I missing something else equally simple? FWIW, Strombergs with bad seals will idle erratically but won't hesitate like this. But they also don't have the progression hole system these carbs do.

H'ep an old fart out!

stretched diaphragm or pin hole in one. Theses are CV carbs so cutting back the throttle closes a plate not the slide like most bike carbs thus lessens the slide flutter. Never get the diaphragms near alky or carb cleaner! it dose funky things to them. Strip'em down and clean all the passages with cleaner and compressed air.

If you find them gummy inside grab a old Tupperware bowl and a big bottle of Pine-Sol from the dollar store (get gen pine-sol not pine cleaner) soak carb over night wash in hot water the gooey gunk will wash out. then clean with carb cleaner and air blast.

Then adjust the valves to the loose side of tolerance, Dan

I'm having similar problems on my XS650. I had it running great earlier in the week then all of a sudden it's started running like crap. Idles fine, thats about it. It's as though something got stuck in the jets. Though I haven't torn mine apart yet, so it very well may be debris.

I'll make sure to check the diaphragms on mine as well.

I checked the diaphragms carefully, they look fine. The accepted test for them is to raise the slide all the way up then block the oblong air hole with a finger, it should take at least 15 seconds for the slide to drop all the way down. On each of mine after 30 seconds they were still halfway up, they went to the bottom after I took my finger off the hole. The valves are on the loose side; I always do that because on engines like these the valve clearances will close up, not open up. Like PHeller, the bike started doing it suddenly all on its own but there's nothing clogged. This has really been weird.

I think what's happening is this: the shaft seals leak, meaning the throttle valve flaps are closed too far so that the engine will idle properly because it's getting extra air at the shafts. This means the progression holes get ~no~ vacuum at idle because the flaps are closed too far and fuel dribbles back down through the passages into the bowl. I have shaft seals ordered, we will see.

J, my partner was having problems with his KZ wanting to stumble and not idle right. Some research told him the pod filters are bad about blocking half of the oblong hole at the top of the carb. He relieved the filter to open it back up and that solved most of his problems.

It sounds like you probably have more issues than that, and I am by no means a bike guy, but I thought I would throw that out there.

A couple of the guys on xs650.com mentioned the same thing. The pods I'm using were specifically designed so they won't block the hole. It's weird how it started all of a sudden, I didn't fiddle with anything or etc. and I have been using those same filters all along.

Here's another one. My father was having a problem with one of his mowers. Went through the carb several time, and finely replaced it with a new one. Still the same problem. It ended up being a wad of crap at the fuel pickup in the tank.

And one more, change the plugs. Remember that "carb" problem I chased in the RX-7 for a month that ended up being the plugs.

Curmudgeon wrote: Here's the strange part: if the engine idles for 30 seconds or so and you open the throttle the first time it revs right up, then after that each time you try to open the throttle the flat spot comes back worse and worse until it just dies. If the engine idles for 30 seconds or so, again the first time the throttle is opened it's okay then each successive throttle opening gets worse, like the progression and pilot phase are running out of fuel.

That, to my mind, is a classic example of the carburetor slowly running out of fuel. Gasoline into the carburetor is restricted, and you're gradually running the bowl dry. For when you do this (run the carb dry, slowly) you get these exact symptoms.

Though there are a few other things to check or consider.

One is to use a good lighted lupe, and inspect those passages. Don't count on them being clear just because you see light or can squirt carb cleaner through them. I have seen some that had a crystaline substance on them, restricting them. Seems to be tied to ethanol content. Requires mechanical removal of the stuff, with a properly sized jet pin.

Second is ignition. As in ignition breakup. Yahamas are notorious for this. Rev the engine up, placing it under load, and the ignition starts to break up. At low load, the ignition works just fine. Doubt you've got this, but a ride with the ignition light hanging over the handlebars helps you.

One way to trouble shoot an ign problem is to close the plug gap by 1/2 if the problem is moved up in the RPM then the ign is weak. I'm still betting on clogged passage or petcock. If its a vacuum operated petcock see if theres a PRIME position as this will turn it in to gravity flow (like the old days) a crack in the vacuum line could be pulsing the diaphragm in the petcock at speed. Heck 80 might even still have a screw off bowl on the petcock might check it too for smegma.

Nope i looked no cleanable bowl but it is a vac operated unit so try prime position. Theres a filter stack on the inside of the tank that could be getting clogged and or tank vent in the gas cap. With the Ethanol in the gas it can corrode up the air holes or swell the gasket.

Weak spark occurred to me too. The Iridium plugs are supposed to be the perfect solution for this engine but you never know... since the ignition was designed around standard plugs I plan to stick a set of them in it and try it again. I have had Bosch platinum plugs create a helluva bad cold driveability issue with two different car engines, maybe this is similar.

The petcock is a purely mechanical piece, it's a custom replacement one specifically designed to do away with the old vacuum diaphragm mechanism that never worked properly. It has two plastic filter tubes that stick up into the tank, one for main fuel and one for reserve. There's a strong fuel flow from both positions.

Foxtrapper, you would not believe where Mikuni put the pilot holes, they are at the TOP of the venturi. There is a whole series of cast in passages with lead and aluminum plugs going up the side of the carb from the pilot jet passages.To go through them 100% with wire or jet pins would involve basically destroying the carburetor. I used to think Weber was bad about doing that until I looked closely at this setup, I still scratch my head over it.

Diaphragm Test for BS34

No experience, but I have heard that iridium plugs can fire far too easily, with a thready spark, and therefore not ignite the mixture well. Again, no experience, just hearsay. But, remembering the days of old where the fix for a weak ignition was to widen the spark gap, it's got a pretty good basis.

I'm not disagreeing that you're seeing good flow out the petcock. But the problem may be downstream of that. A float sticking on the shaft, a needle sticking in the bore, internal fuel line damage, etc. As a semi-pointless example, my lawnmower. Gas flows great out of the tank, but it's darn slow actually filling the bowl. If I let it run out of gas, it takes a few minutes to actually fill the fuel bowl.

Stuck a new set of plugs in it today, went with standard BP7ES non-iridiums. No change. The only thing that seems to help is to crank the idle speed up to around 2k (spec is 1k-1.2k) which means I am moving the throttle plate over the progression holes about .030 or so. Once I do that, the flat spot goes away and it's rideable except of course the idle is way too high.

I had the float bowls off the other day and turned on the petcock just to see, it flowed well. Made a big mess.

Did you adjust the idle speed with the idle screw or with the pilot screws? Have you done a carb sync lately?

I have a feeling you might be chasing two separate problems if the throttle shafts are still leaking vacuum. Personally, I'd put diagnostics on hold until that problem is addressed. No point chasing your tail and potentially adjusting around the leaks if you're going to be changing that pretty soon anyway.

This link is a good read. http://www.gixxer.com/forums/showthread.php?t=159718

Hokay, got the shaft seals in today. No more vacuum leaks, tested 4 times. I had to turn the throttle stop screw in about 2 turns compared to the original setting to get a decent idle and resynced the carbs. Had to set the mixture screws again, nowhere near where they were. This tells me yes there was a nasty vacuum leak. But still the same frickin' problem. It's getting worse, the bike won't rev over ~4k RPM now.

I did some more sort of random testing, there is an air bleed jet for the pilot system. If I stick my finger over it while it's idling, the bike dies. That indicates to me the pilot system is working.

So I am beginning to think I may have an ignition problem instead. The pickups and etc either work or they don't, I did each cylinder in turn and put a plug on the wire so I could see spark, when the engine tries to die it still has spark. That of course does not mean it's happening at the right time.

It seems this engine has a dual element pickup. From what I can find, the TCI uses the signal from both pickups to determine engine RPM and then it advances the timing. So I pulled the ignition control unit, it looks and smells fine, don't see any fried parts-except one.

There is a Zener diode in the pickup circuit which appears to 'separate' the two pickup signals. (Zeners allow voltage to flow one way only until a certain voltage is reached then they allow two way. Standard diodes allow only one way flow, they pop/explode if the voltage threshold is exceeded.) The one in my box looks like it vomited, there's all kinds of crap at one end and the lead looks corroded. It's visible at the right side of the board, there's four white/yellow 'easter egg' diodes, then two gray resistors, then my fried Zener diode, then two capacitors.

Some Googling shows that this has been encountered before and causes various problems. 12V Zeners are $1.49 a pair at Radio Shack. Fingers crossed.... TCI boxes are not easy/cheap to come by and replacement ignitions are certainly a $$ alternative.

Oh boy, electronics now.

The boy can dance, sing and carry a tune in a bucket!

I would cross my fingers for you, but people would look at me funnier than they usually do.

Good luck, hope that solves your problem.

Can't dance (ask any of my exes). Sing, only in the shower. Tune in a bucket; I can't keep time on the cymbals! But I can trash electronics with the worst of them. I've let the magic smoke out of MANY parts over the years.

Been known to do that a time or too myself.

Mmmmm... nothing like the smell of freshly roasted resistor, is there?

With the over wellming switch to EFI theres boat loads of aftermarkey ign boxes out there now but not sure if you'd find the proper pick ups for an xs650. You'd have to do some work for sure to make crank pick ups. You can find used Dyna 2000's for around $100

There's a couple of aftermarket ignitions for an XS. The most popular uses a mechanical advance from the original point setup, that system had the points on the end of the camshaft. Since mine never had points to start with, I'd have to convert it to the mechanical point advance setup and then install the ignition system. That means I'd lose the crank trigger TCI system, which I hate to do if I can avoid it. It would be a lot like ditching a digital crank trigger ignition for an olde tyme distributor setup.

PMA+PAMCO=perfect simplicity with a little bit of electronics attached.

Glad I don't have a TCI.