Fuel or spark issue?

[Vdriveboater]

I had the carbs off, basic cleaning and went through them, since this was a new to me bike (14K miles '96). I also changed coolant, oil/filter and fuel filter. I also checked the plugs and didn't see anything strange. After getting everything back together, fired her up. It runs smoothly at very low throttle input (maybe less than 1/8), but any more than that and it acts/sounds like it's on 3 cylinders, whether it's in neutral or down the road. It didn't do this before I tore it apart. Giving it 1/2 or more throttle doesn't make any difference, it seems like 3 cylinders until the throttle is almost closed again. If it was something wrong with jetting it should change based on a larger throttle opening. The first time I put the carbs back on one carb was overflowing, so I took them back off and replaced the float valve with a used one (even though it looked fine) and it stopped that issue. I'm wondering if somehow the excess fuel made it into the chamber and somehow messed up the spark plug, even though it only ran that way for less than a minute? I suppose the easiest thing to do is to replace all the plugs with new and go from there?

[triton28]

Sounds as though one carb has a problem with the secondary fuel input operating range. This fuel input stage is governed by simple items: the needle jet; the jet needle; the throttle valve; the vacuum spring and the throttle valve spring seat.
First, if one has removed the needle jet holder and then the needle jet itself it is easy to install the jet upside down upon reassembly.
Next, if one has removed the diaphragm and the associated parts be careful not to block the air hole with the spring seat as this would block vacuum from raising the throttle valve.
All this is covered on pages 2-11 and 2-12 of Section 2, Fuel System, of the Motorcycle Service Manual.
Also, is the main jet clean on the offending cylinder? It will run fine on the idle circuit even if the secondary and main jet circuits are not up to snuff. I suspect the secondary fuel input operating range is the problem BUT, it could also be an air leak on that cylinder's carburetor rubber mount to the cylinder head but, that is a long shot IMHO.
Just some thoughts,
Dave

[Vdriveboater]

Sounds as though one carb has a problem with the secondary fuel input operating range. This fuel input stage is governed by simple items: the needle jet; the jet needle; the throttle valve; the vacuum spring and the throttle valve spring seat.
First, if one has removed the needle jet holder and then the needle jet itself it is easy to install the jet upside down upon reassembly.
Next, if one has removed the diaphragm and the associated parts be careful not to block the air hole with the spring seat as this would block vacuum from raising the throttle valve.
All this is covered on pages 2-11 and 2-12 of Section 2, Fuel System, of the Motorcycle Service Manual.
Also, is the main jet clean on the offending cylinder? It will run fine on the idle circuit even if the secondary and main jet circuits are not up to snuff. I suspect the secondary fuel input operating range is the problem BUT, it could also be an air leak on that cylinder's carburetor rubber mount to the cylinder head but, that is a long shot IMHO.
Just some thoughts,
Dave

Dave, I was very careful during re-assembly to ensure that everything looked and felt right, but of course it's possible that I missed something. Can I pinpoint the offending cylinder by riding for a few miles and then checking the header pipe temps with an IR gun? Are you thinking that the "bad cylinder" is running rich or lean?

[Nails]

Got a message that the server was broken, "Try again later". Did, and it got posted twice. So now I'm kinda deleting a duplicate.

[Nails]

Can I pinpoint the offending cylinder by riding for a few miles and then checking the header pipe temps with an IR gun?

I guess I would try to park somewhere in the shade to let everything cool down, then start (don't let it idle and get warm) and go (miss and all) for a little ways, and then shut it down and check the header temps. Just like you said.

I've done this with a spit test (spit on a fingertip and see if it sizzles -- the spit, not your finger). Sometimes it actually works.

But it might be easier to just pull a plug and look. I've checked high-end jetting by running that high-end way for a few miles, pulling the clutch, and hitting Kill. Then let it coast under my favorite Interstate shade tree, grab a beer from the cooler, and wait until I could get the plug out to check its color.

Are you thinking that the "bad cylinder" is running rich or lean?

It's presumably lean-dead.

[triton28]

Vdrive;
I always have to double check everything I do to the carbs as while not complicated they do require consistency and sometimes I am in too much of a rush and make an error. Usually my second guessing myself catches most mistakes.
Yes an IR gun is the easiest way to determine which carb is not like the others. Lean is the condition I am thinking of.
You and Nails describe a tried and tested method to run the cooled off/cold engine in the erratic operating range, taking the temperature of the headers and then cancel the ignition power and allow the engine to stop.
Nails also adds the mobile main jet test method which involves removing a plug and that described operation is the definitive way to test for main jet fuel/air ratio under active conditions to my way of thinking.
I know back in the 70s when the "Colortune" system came out I tried it and while it does work, by then I had become engrained in the test procedure Nails outlined as it just seemed easier to do and besides, I got to go for a ride.
Keep at it, you will find that AHA moment and probably will chuckle at what it was,
Dave

[Vdriveboater]

Can I pinpoint the offending cylinder by riding for a few miles and then checking the header pipe temps with an IR gun?

I guess I would try to park somewhere in the shade to let everything cool down, then start (don't let it idle and get warm) and go (miss and all) for a little ways, and then shut it down and check the header temps. Just like you said.

I've done this with a spit test (spit on a fingertip and see if it sizzles -- the spit, not your finger). Sometimes it actually works.

But it might be easier to just pull a plug and look. I've checked high-end jetting by running that high-end way for a few miles, pulling the clutch, and hitting Kill. Then let it coast under my favorite Interstate shade tree, grab a beer from the cooler, and wait until I could get the plug out to check its color.

Are you thinking that the "bad cylinder" is running rich or lean?

It's presumably lean-dead.

Thanks Nails, I'll give it a try. As for spark plug observation, I would think that it takes quite a few miles on a four stroke to change the look of the plug for analysis. I'm thinking the header temps will be the easiest/most obvious indicator.

[Nails]

the mobile main jet test method which involves removing a plug and that described operation is the definitive way to test for main jet fuel/air ratio under active conditions to my way of thinking.

I'm building (well, mostly just planning) a backcountry touring bike: a thumper d/s with mods to haul water and camping crap -- a carbureted XT350. I'm considering adding a lambda sensor to the header and rigging a circuit board (plans on the web) to end up with an on-board EGA. The Big Plan (just talking, mind you) is to make it easier to do simple carb adjustments for elevation. I'd basically camp at a range of elevations with bunches of jets and stuff, taking lots of trail rides and the time to find the best jetting. Then make a chart for myself, like when to turn the pilot screw vs messing with the main jet. Call it "semi-closed-loop" fueling. Having the ESA available (just push a button for a spot check) could also help field diagnosis in the middle of nowhere.

(Note that the XT350 has one cylinder and two carbs. It's a bitch to tune, but then works super well: the small primary CV carb provides nice throttle snap and the secondary CV provides high-speed flow -- opens at about half throttle.)

As for spark plug observation, I would think that it takes quite a few miles on a four stroke to change the look of the plug for analysis.

You're looking at soot, which deposits immediately. Give it a few miles to become unambiguous.

[Vdriveboater]

The issue went away with new NGK standard spark plugs. Looking at the previous plugs, I think #2 had an issue that was caused by the "flooding incident" on that carb the first time I put the carbs back on, with #2 apparently having an issue with the float needle (even though it looked good after thorough inspection). I replaced that needle with one I had removed from my previous XII ('94) and the flooding stopped. Other than some burbling at very small throttle/no load, she now runs beautifully. I backed out the pilot screws slightly thinking that richening might eliminate the burbling, but it didn't, but it's just a minor inconvenience. Riding the XII back to back with an '07 Yamaha RSV, I still give it the nod, especially in low speed handling with a pillion on board. I'm 5'6" 155 and with a 5'8" 179lb passenger, the RSV top heaviness can be alarming at times during parking lot maneuvers. Without question, the XII is better for around town with its lighter weight, low center of gravity and quick steering, whereas the RSV tracks straight and true on the highway and its suspension feels tighter and a bit more controlled at higher speeds. Both are great bikes in their own right.

[Nails]

My bike had an extra sparkplug gasket (the metal seal), which impaired the plug. In addition to all the road crud that accumulates in those deep wells. I recommend taking a good look down there.