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!

 

Any update on this issue?

 

Not yet. Did compression and spark test again which both were fine. To me this sounds like carb issues, so I will give it back to the carb guy and see if he can figure it out. Som you have any thoughts, Timbow?

 

I am concerned with the color of the fluid. Have you taken a good luck into the fuel tank? Just want to make sure that once the carbs come back all clean and ready to do, that something from the fuel tank does not get into the jets and that is that. Putting an in line fuel filter from the tank to the carbs is a good idea. Unless you know that the tank is spotless and the fuel is clean.

Do you have any pics of how gummed up the carbs were when you first started? Getting the little holes clean can b a pain the neck. There is a very small jet in the bottom of each float bowl that is missed by most folks cleaning the carbs.

 

Did I miss the part where you changed the oil because it had gas in it?

 

The gas tank has been thoroughly cleaned with vinegar and is all clear metal when taking a peep. A brand new gas cap and petcock have been fitted and I also added an inline filter. I found an old photo of when I did the disassembly. I've just taken the bike back to the carb-guy, hoping he can sort it out.

 

If that last pic was after the clean, not to sure how good of a clean it was? There is still a bit of deposits even on the surface of that carb, so you have to wonder what is inside the jets, emulation tube ect.

 

Oh no, Timbox. That picture is before cleaning. Thought that was what you asked.

 

So I just got the bike back from the carb guy. He located the issue but was unable to solve it properly. Carb #3 is the criminal. To be more precise, the idle system is clogged in it's little canals. He spend much time soaking the body and blasting ports, and did gets some tiny snazzy bits out, but when applying gas and air pressure to pilot jet, he could still not get it to deliver the same amount of gas as the other carbs can. Therefore, when idle, the #3 cylinder still lags as it's running lean. When revving it comes more alive. He's compensated the lag/lean with the mixture screw so it's more out than the others. The bike is running, but it's not optimal. Should the whole carb body be replaced?

 

Soaking the carb in what type of solvent?

The internal passages (the "little canals".....I like that!) get "clogged" when fuel remains inside those passages for an extended period of time, the fuel itself evaporates and leaves behind what were previously dissolved solid matter.......hard as a rock. It takes a very aggressive solvent to even begin to loosen up that type of material, and then the use of air pressure to help blast the material out of the passageways......and it may take multiple cycles of soaking-in-solvent / air pressure to finally break up the clog and remove the material.

https://www.xj4ever.com/inside your carbs.pdf

Notice all the right-angle turns within the internal passages......that also makes it really difficult for air pressure to exert full pressure thru the openings that you are trying to use to "exhaust" the debris past....

 

Hi Len! I believe he used regular carburettor cleaner in abundance. So much that he could sink the affected area into it. Exactly how that went down, I'm not too sure about. Do you have a recommendation for a specific solvent? Berrymans doesn't seem available here in Denmark. If I were to give it another shot myself, are there any mechanical way to unclog those passages (canals ) ie certain string or something like that?

I can't open the link you've posted. Maybe I need to do it from my computer