If it's not one thing, it's another -#1 carb fail

[KA1J]

I've been trying to figure out why the XJ11 has been running rough and today I pulled #1 plug wire & found nothing changed, pulling the other three made the bike stumble so I knew cyl 1 wasn't firing. I switched plug wires at the coil and that made no difference, #1 had no change. I then replaced the sparkplug with a known good one and that didn't make a difference. I removed sparkplug 1 & smelled the compressed air coming out & air it was, no gas smell whatsoever.

I just rebuilt these carbs with new float assemblies and replaced the throttle shaft seals, did a lengthy job of cleaning everything in carb cleaner & made sure all orifices were clear. I have in-line filters (octy) so nothing would have gotten in to block the fuel flow. I rapped on the side of the carb body & bowl with a rubber mallet and nothing changed. It's just not getting any gas to #1 cylinder.

Lovely... Tomorrow is supposed to be beautiful weather into the upper 60's & clear skies...

Yup...

[MPittma100]

Carb Fail

If you have not already, you can absolutely verify no gas in the carb by loosening the drain screw. No fuel indicates a stuck float in the closed position.

I had a new (brass) float do the same thing in the same (#1) carb. Not sure if was hanging on the bowl or gasket, or it was made wrong or bent. It looked fine?

My fix was to remove the bowl (with carbs still installed on the engine) and put one of the old floats back in. This was on a 79 Special. It is a little tricky to do while still mounted to the engine, but it can be done.

MP

[KA1J]

MP, a good idea,

As with so much else, the solution was just not clear...

I have allen caps on the carbs so I needed a shorter 4mm wrench. Had
to get one of the shorty packs like a folding knife and then undo the
bolt holding them in place and with that I could reach inside and
remove the screws without removing the bank of carbs. I drained the
bowl first (so there was fuel in the bowl & it wasn't getting to mix
with the air) & when I removed the bowl, the float was not stuck. I
flipped the float up & let it fall back down a few times. I held the
float up and started the bike & then readied the bowl underneath. I
released the float and saw the fuel flowing down into the bowl below
& then pushed the bowl up in place and within seconds the cylinder
was firing perfectly and the bike was running like an electric motor.
Still holding the bowl in place I removed the spark plug wire to #1
and it stumbled and the bike ran like it had been running. I put the
screws back in & it's perfect now.

There was a tiny bit of grayish sediment of some kind in the bottom
so maybe something had wedged in the valve & when the float was high,
the buoyancy of the float & the particle was enough to keep the valve
closed tight. I have no idea. Maybe all I needed to do was drain the
bowl and the weight of gravity would have pulled the valve down and
if there was a particle in there binding, it would have flushed out
with the gas. Then replace the drain screw and it would have done the
same.

Regardless, it's working and I can get back to working on the X gas
tank.

It sucks when more than one thing happen at the same time and you
have no idea it's multiple things awry.

[MPittma100]

These things happen

I believe that is commonly termed as a "GREMLIN". They have a tendency to come out about this time of year (Halloween). Glad you were able to figure it out.

Like you say, sometimes you just have to take it back apart and for whatever reason it starts working like it is supposed to?

Another way to find a dead cylinder is either with a spray bottle with water or a heat gun. Either will show a cylinder that is not firing. Or if you are brave enough, you can always lick your finger and (very) briefly touch each pipe.

A tip for future dead cylinder diagnosis: Since each coil fires two cylinders, a coil failure will cause the loss of 2 cylinders rather than one. Looking down at the coils (under the tank) left coil fires cylinders 1&4. Right coil fires cylinders 2&3.

MP

[KA1J]

I believe that is commonly termed as a "GREMLIN". They have a tendency to come out about this time of year (Halloween). Glad you were able to figure it out.

Like you say, sometimes you just have to take it back apart and for whatever reason it starts working like it is supposed to?

Another way to find a dead cylinder is either with a spray bottle with water or a heat gun. Either will show a cylinder that is not firing. Or if you are brave enough, you can always lick your finger and (very) briefly touch each pipe.

A tip for future dead cylinder diagnosis: Since each coil fires two cylinders, a coil failure will cause the loss of 2 cylinders rather than one. Looking down at the coils (under the tank) left coil fires cylinders 1&4. Right coil fires cylinders 2&3.

MP

True dat!

One of the differential diagnosis I needed to exclude was that perhaps there was a failure in the dynacoil. I switched the wires from 1 & 4 and the problem remained at #1 so I knew it was confined to there. I was just praying there was compression at #1 as I already rebuilt the top half 3K ago.

So glad it's back to its ballsy self again!