1982 XJ650 Cylinder #3 not firing until warm

I'm having an issue with cylinder #3 on startup. The bike starts fine and idles but only on 3 cylinders. It will rev up but sound very boggy until you put it in gear and try to start riding. With any load , the engine will stutter and almost die. The #3 exhaust pipe will be warm/hotish but not nearly as hot as the others. After about 10 minutes of reving and try to get it to go, cylinder 3 will spring to life and it'll run like a dream all day. This only happens on cold startup after sitting overnight. Last summer I completely rebuilt the carbs following the church of clean as closely as possible. I have confirmed good spark and the plugs are clean. All gaskets and rubber carb parts are brand new. The carbs were synched using a yics tool a month ago. I've spent hours playing with the pilot needle to get this cylinder to fire reliably but it's at a position that once it is running, the bike runs very well so I don't think it's the mixture setting. I'm really at loss on how to proceed at this point, anyone have any advice?

 

Have you done the valve clearances? It would be good to get a Gunson colourtune to check what is going on in the combustion chambers. Check the plug ends too to see what colour they are.

 

Id bet the choke passage in bowl 3 was missed, therefore running lean u till its warm enough, otherwise a slight restriction in #3 pilot circuit.
Valve clearance issues tend to get worse when warm... Its starving for fuel
You have to be able to squirt cab cleaner in the top, coming out the bottom, theres an itty bitty jet in there that gets clogged easy and no one remembers to check.

 

Franz, I haven't done the valve clearances yet, I'm not so sure how to adjust shimmed valves and I don't have any of the tools for that. The 1,2, and 4 plugs are a bit black (probably because iv'e only been idling it for a while now) and the #3 plug is significantly cleaner but a little wet.
Colin, I really only use the choke to get it started initially, I usually turn it off a second or two after it starts as it doesn't run well with the choke on for some reason. I think it over-richens it and doesnt want to run.

 

https://xjbikes.com/forums/threads/bigfitzs-airhead-valve-adjustment-with-pics-parts-i-ii.116006/

The jet in the float bowl is easily missed I forgot about that. Valve clearances check the above it is excellent. Is your air filter clean? Incidentally watch how you turn the camshafts with the Yamaha tool bolted to the head as you can damage the head if the cam hits the valve depressing tool.

 

You should be able to warm it up a few minutes on part choke, mess with it 1/4-1/2 so it stays running at round 2000+ rpm. If it wont do this, you have an issue with your choke circuit.

 

Note, only full choke to start, then sliwly reduce choke as engine warms till it can be shut off.

 

So I went out and tried to run it on the choke for a while. It would only stay running with the choke if I held the throttle cracked open a bit, but cylinder 3 was definitely firing while the choke was on. However, when I took the choke off cylinder 3 stopped firing and I still had loss of power. I held the choke open and cracked the throttle for a little longer and eventually cylinder 3 stayed firing and was good to go. I think I'm going to have to pull off the float bowl and see if something is blocking the pilot jet or that choke jet.

 

REPLACE THE SPARK PLUGS ....I just pulled my bike out of hibernation was only running on 3 cylinders ,put in my spare set of plugs problem solved . The other item unthread the plug cap and snip a 1/4" off of plug wire ,rethread cap back onto wire and see if this improves situation.

 

I don't think the plugs are the issue. They are pretty much brand new, I've been fighting with this since I finished the bike in October and now I'm trying to really fine tune everything in before riding season starts.

 

You still could have a bad plug have you at least swapped plug ,since 3 is the one not firing swap number 1 with 3 and see if there is a change.

 

Just went and checked, flipped the 3 and 4 spark plugs. Still dead on 3 so the plug seems fine.

 

I had the same issue with mine. Swapped plugs, wires, tried everything. It was the carbs.

 

And you knew it was the carbs because you ruled out spark. Check the simple things first.

 

I won't have much time this week to get the carbs/float bowls off as I have a couple big exams on friday. I'll probably get around to it on saturday, but in the meantime does anyone have a good strategy for fishing out any debris that might be stuck in the pilot circuit? These carbs were pretty trashed when I bought the bike so it wouldn't surprise me if there was still some crud left in the passages. The jets are all brand new so I'm pretty confident any blockage would have to be past the jet. I did notice last night that the rubber boots connecting the air box to the carbs are pulling off of the carbs on 2 and 3. I don't think that would necessarily cause my issue but I figured I would throw that out there.

 

Not on the airbox side,
and good carb cleaner in all passages, tip cleaners (for a mig or torch) work, or use copper wire and twist a few strands togeter till you get the size you want... Kinda flimsy tho

 

I'll give that tip cleaner idea a try and report back when my exams are done. Thanks to everyone who has given me advice so far I really appreciate the help!

 

So I pulled my carbs today after I got home and started looking for an obstruction in my #3 pilot circuit. Turns out, there was a small brass chip left in the bore of the #3 cylinder pilot jet. Picked it out with a torch tip cleaning wire and put everything back to together. All cylinders were firing right away, so problem solved! Thanks again to everyone who offered their help. I guess moral of the story is, don't assume your brand new jets are free of obstructions.