Troubleshooting: Cylinder not firing

Hi Y'all!

After a complete overhaul of my XJ700s, I'm having trouble with cylinder #3 not firing. At first I could barely start it and it couldn't really idle, so I sent it to a carb-guy for synchronization. I can now start it and have it idle and it responds well to throttle, but cylinder #3 is not firing and exhaust pipe isn't getting heated and can easily be touched.
I have spark and have tested with different spark plugs. I've also tested compression which is good too. I then tried spraying starter spray into vacuum port on intake manifold #3, which made the cylinder fire. Oil smells of gas, and when I did the starter spray test today, I noticed that a mixture of gas and engine oil comes out of the exhaust (see photo). I'm leaning towards carb issues, but I'd like to hear your take on the issue.


Thanks!
Nikolai

 

There could be a blockage somewhere on that carburettor, either somewhere in the passageways or in one of the jets or even in the float height setting.
It's a bit hard to diagnose over the forum. Did the person who synced the carbs just do a bench sync without having the carbs on the bike?

 

That's what I was thinking too. Is there a specific passageway or jet I should hold as suspect? I did the bench synch meticulously myself, but I'm not perfect and the carbs are sensitive. The carb-guy synched the carbs with them installed on the bike. I called him and explained the issue, and he said that he of course will re-do them if they are the cause. I just want to make sure and throw it out to you guys before I do that.

I forgot to mention that the valve clearance is good too.

 

I don't think there is a specific passageway, but any one of them could be blocked with crystalized old petrol.
Have you sprayed any aerosol cleaner through any of the passageways and watched if and where it emerges within the carbs?
Spray some through the air jet on the carb mouth for a start and see what happens. It should come through into the jet nozzle (or whatever it's called), that big one in the middle of the carb with all the holes in it.

 

When I did the carbs, I did the whole "Church of Clean" and had each carb open. I also had them through ultrasonic cleaner. I did spray with carb cleaner aerosol too, and used tiny strings to get into narrow areas. I could have missed something though. What you're mentioning is with the carbs off, on the bench, right?

 

Yes mate, off the bike. It sounds like you've been thorough.
Long shot but are the diaphragm needles all at the same height and carb 3 not letting enough air or petrol through?

 

That is one thing i'll make sure to control. Thanks!