Odd pilot circuit issue

[niznav13]

Greetings all,

I have a new to me, as of December 2021, 1979 XS1100 special. I have spent several months bringing it back to running condition, which has yielded great results up until recently. The bike starts beautifully, idles beautifully, does everything that is expected, except at idle the number 4 cylinder is not getting fuel through the pilot circuit. Outside of a stumble during off idle operation, and slight backfire in the mid range, its not obvious that anything is operating incorrectly. I am trying to come up with more diagnostic procedures to narrow down the problem, this is what I currently know through various tests and cleanings:

This is only happening on the #4 cylinder when being fed through the pilot circuit with the enrichment valves completely closed (choke off)

#4 fires with the enrichment valve fully and partially open (full and half choke) during a cold start. I have been using a temperature gun to monitor exhaust temperatures to determine when the cylinder stops firing. During warm up, all 4 exhaust pipes reach 350, then when the choke is off #4 drops to 150-170 degrees

As far as I am aware, it is the factory Mikuni carb, it has the flat head screw plug in the end of the pilot tower with the crossover tube feeding it, with a 42.5 pilot jet and a 137.5 main jet

The entire rack has been deep cleaned in pinesol 3 separate times during my quest to diagnose this

The crossover tube is very clean, I can visually see the pilot jet through the hole

Brake cleaner / carb cleaner shot through the pilot circuit, with both the jet installed and uninstalled, comes out of the air jet on the airbox side and also the orifice that the mixture needle goes into, indicating to me that the passages are clear

I have verified float bowl fuel level being correctly metered with clear tygon tubing attached the bottom plugs

I have installed new 3.0 accel coils and bypassed the ballast resistor

All four plugs are new and all get bright blue spark

#4 plug since being installed shows no color and is not remotely wet with gas any time i remove it

All 4 cylinders read 115+ psi of compression

I was fooled the first time I synced the carbs because the #4 cylinder reads the same vacuum as the other 3 cylinders despite the fact it is not burning fuel

When feeding off the main jet, all 4 cylinders fire and the bike is scary fast

All idle screws were set initially at 1.5 turns out from a light seat, if I continue to turn out #4 to approximately 4 turns, the cylinder starts to pop like its very lean and jumps roughly 30 degrees in exhaust temperature

So far as I am aware I have no massive air leak, since #3 run correctly I make the assumption there is not a fault in the fuel tee or vent region

Im the first to admit I dont know everything and I am sure there is something I have overlooked, if anybody has any input or thoughts I would greatly appreciate it. I adore my XS, it pains me to ride it while I know something is incorrect

Thanks for your time and wisdom

[LAB3]

About the only thing I can toss in is that I've used Pine-Sol as a carb dip before. The first time I used it I boiled the carbs in it for an hour and let them sit overnight and got "Good enough" results for an entire season but always felt there was a bit more there to be had after riding another bike that was the exact same year and model. The following winter I tore the carbs down again and gave them a 12 hour dip in Berryman's carb dip and got the results I was looking for! Pine-Sol is cheap and easy no doubt and not having to deal with toxic waste disposal is a plus but after using Berryman's I decided I had a choice to make in the future.

[jetmechmarty]

Without a carburetor in my hand, I can't explain where the orifices are, but your explanation says you're capable. You need an aerosol can of carburetor cleaner. Did you ever remove the pilot jet? It sounds like you did. You're going to have to remove the bank and clean the pilot jet and pilot circuit.

I have one of these filters right above each of the fuel tees between the carburetors. If it looks like this, it has to be a genuine Visu-Filter. The knockoffs will melt, leak, and maybe catch you on fire. These are $6.75 each at Aircraft Spruce. You can find it cheaper, just make sure it isn't a fake. It doesn't take much crud to plug up your pilot jets.

[Radioguylogs]

Is it possible some crud is stuck behind those 3 little holes where the pilot circuit exits into the venturi, just upstream of the butterfly? Maybe you could blow compressed air backward through the 3 little holes?

[Schming]

Welcome Neighbor !!!
I was thinking maybe the starter jet in #4 bowl may be clogged but the enrichener and exhaust temps negate that thought.
Seems like #4 is fuel starved while running on the pilot circuit but your confirmation of fuel flow also negates that thought.
Could it be too much air ? Maybe butterfly shaft seals on #4 are trashed. IDK but staying tuned (pun Intended )

[cajun31]

Greetings all,

I have a new to me, as of December 2021, 1979 XS1100 special. I have spent several months bringing it back to running condition, which has yielded great results up until recently. The bike starts beautifully, idles beautifully, does everything that is expected, except at idle the number 4 cylinder is not getting fuel through the pilot circuit. Outside of a stumble during off idle operation, and slight backfire in the mid range, its not obvious that anything is operating incorrectly. I am trying to come up with more diagnostic procedures to narrow down the problem, this is what I currently know through various tests and cleanings:

This is only happening on the #4 cylinder when being fed through the pilot circuit with the enrichment valves completely closed (choke off)

#4 fires with the enrichment valve fully and partially open (full and half choke) during a cold start. I have been using a temperature gun to monitor exhaust temperatures to determine when the cylinder stops firing. During warm up, all 4 exhaust pipes reach 350, then when the choke is off #4 drops to 150-170 degrees

As far as I am aware, it is the factory Mikuni carb, it has the flat head screw plug in the end of the pilot tower with the crossover tube feeding it, with a 42.5 pilot jet and a 137.5 main jet

The entire rack has been deep cleaned in pinesol 3 separate times during my quest to diagnose this

The crossover tube is very clean, I can visually see the pilot jet through the hole

Brake cleaner / carb cleaner shot through the pilot circuit, with both the jet installed and uninstalled, comes out of the air jet on the airbox side and also the orifice that the mixture needle goes into, indicating to me that the passages are clear

I have verified float bowl fuel level being correctly metered with clear tygon tubing attached the bottom plugs

I have installed new 3.0 accel coils and bypassed the ballast resistor

All four plugs are new and all get bright blue spark

#4 plug since being installed shows no color and is not remotely wet with gas any time i remove it

All 4 cylinders read 115+ psi of compression

I was fooled the first time I synced the carbs because the #4 cylinder reads the same vacuum as the other 3 cylinders despite the fact it is not burning fuel

When feeding off the main jet, all 4 cylinders fire and the bike is scary fast

All idle screws were set initially at 1.5 turns out from a light seat, if I continue to turn out #4 to approximately 4 turns, the cylinder starts to pop like its very lean and jumps roughly 30 degrees in exhaust temperature

So far as I am aware I have no massive air leak, since #3 run correctly I make the assumption there is not a fault in the fuel tee or vent region

Im the first to admit I dont know everything and I am sure there is something I have overlooked, if anybody has any input or thoughts I would greatly appreciate it. I adore my XS, it pains me to ride it while I know something is incorrect

Thanks for your time and wisdom

I had this same issue on a set of carbs I was rebuilding and it ended up being the pilot jet. They can look clean and still not be. I cleaned mine several times in my ultrasonic cleaner and I also let it soak in Berryman's in the gallon can. Nothing worked until I replaced the pilot jet. A key to pointing to the pilot jet is your mention that the carb fires ok when you have the choke engaged. The choke is bypassing the pilot. Mike made a good suggestion concerning those three small holes in the bore of the intake side. Those do play a factor on the pilot circuit. A few dollars on a new jet will probably fix the problem.

[Schming]

I had this same issue on a set of carbs I was rebuilding and it ended up being the pilot jet. They can look clean and still not be. I cleaned mine several times in my ultrasonic cleaner and I also let it soak in Berryman's in the gallon can. Nothing worked until I replaced the pilot jet. A key to pointing to the pilot jet is your mention that the carb fires ok when you have the choke engaged. The choke is bypassing the pilot. Mike made a good suggestion concerning those three small holes in the bore of the intake side. Those do play a factor on the pilot circuit. A few dollars on a new jet will probably fix the problem.

But he states,

"Brake cleaner / carb cleaner shot through the pilot circuit, with both the jet installed and uninstalled, comes out of the air jet on the airbox side and also the orifice that the mixture needle goes into, indicating to me that the passages are clear"

[cajun31]

But he states,

"Brake cleaner / carb cleaner shot through the pilot circuit, with both the jet installed and uninstalled, comes out of the air jet on the airbox side and also the orifice that the mixture needle goes into, indicating to me that the passages are clear"

I stand by what I stated. I would replace the pilot jet with a new one. I too sprayed cleaner and a good burst of air through them and until I replaced the actual jet nothing solved the problem.

[bikerphil]

You could swap pilot jets with another carb and see if the problem moves. Sometimes tapping the carb while it is running may clear the problem. Spraying Windex or water on each header is an easy firing condition test.

[cajun31]

You could swap pilot jets with another carb and see if the problem moves. Sometimes tapping the carb while it is running may clear the problem. Spraying Windex or water on each header is an easy firing condition test.

Good idea and easy enough to do without spending more then a bit of time.

[niznav13]

Greetings all,

I thank everyone for their thoughts, I return with more information and the solution that has apparently worked, but for what actual reason I cannot say.

I neglected to mention a few things in my original post. The carbs had many problems when I first got the bike. During the initial take down and deep cleaning I discovered the #4 pilot jet was stripped to oblivion. I got it out using easy out extractors, cleaned it and installed the new pilot jet from the rebuild kit I ordered. Upon installing the new pilot jet, I discovered that the hole was cross threaded and ruined my new pilot jet before my brain caught up to me. I then, apparently, successfully re-tapped the hole and was able to get one of the old, newly cleaned pilot jets to thread in and seat to the point it was visually the same depth as the other 3 pilot jets, and tightened snug.

Thus began everything I wrote in my initial post.

So I went out on a limb, and decided that since it seemed to react positively to the idle mixture being dialed almost completely out, I would go up to a 45 pilot jet. It seems to have solved the problem I was having. But it doesn't make any sense and I don't know why. I have tried multiple other 42.5 jets and they react the same as the problem one.

I will continue to experiment

[cajun31]

Greetings all,

I thank everyone for their thoughts, I return with more information and the solution that has apparently worked, but for what actual reason I cannot say.

I neglected to mention a few things in my original post. The carbs had many problems when I first got the bike. During the initial take down and deep cleaning I discovered the #4 pilot jet was stripped to oblivion. I got it out using easy out extractors, cleaned it and installed the new pilot jet from the rebuild kit I ordered. Upon installing the new pilot jet, I discovered that the hole was cross threaded and ruined my new pilot jet before my brain caught up to me. I then, apparently, successfully re-tapped the hole and was able to get one of the old, newly cleaned pilot jets to thread in and seat to the point it was visually the same depth as the other 3 pilot jets, and tightened snug.

Thus began everything I wrote in my initial post.

So I went out on a limb, and decided that since it seemed to react positively to the idle mixture being dialed almost completely out, I would go up to a 45 pilot jet. It seems to have solved the problem I was having. But it doesn't make any sense and I don't know why. I have tried multiple other 42.5 jets and they react the same as the problem one.

I will continue to experiment

sometimes when you have to get aggressive in removing a pilot jet you can sometimes damage the seat the jet contacts when seated. The way you have described your ordeal that and other things may have occurred that are slightly blocking flow through that orifice. The slightly larger jet doesn't make that big a difference. If you ever take the time to tune the carbs by really dialing them in with a color tune or other method you might find that the pilot screw adjustment for the damaged carb will be different from the others.

[Radioguylogs]

Doesn't it seem that using an oversized jet would be a band aid for a restricted gas passage?

Here's a random thought. Is it possible there is the tip of an idle mixture screw broken off in the carb? Can you see light through the screw hole into the venturi? I realize the cylinder responds to 4 turns out on the mixture screw, so maybe this thought is not useful.

Just trying to help.