1985 xj700n running on 3 cylinders

On the plus side, I am learning a lot about this bike.

I recently rebuilt my carbs thinking my rough running problem was fuel related, but now that I have the bike back together I have discovered that the reason for the rough running was cylinder 4 not participating. The block was not getting hot on that side. Cylinder #1 is running just fine.

I thought this was a spark issue, but I tested the plug outside the engine and it makes spark. I also checked the #4 carb, and it fills as it should. The other three cylinders are running fine.

So far, I have pulled and tested the resistor in the cap, and it is showing 10 ohms.

I pulled the #4 wire and grounded it to the block - the bike runs exactly the same with the wire on the #4 plug as it does grounded to the block.

I have read quite a few of the references others on the site have prepared concerning ignition problems, which is why I am as far along as I am, but I have run out of ideas on how to solve this one without just wading in to parts swapping.

Any ideas?

 

If you are getting a good spark, then the problem is fuel or very low compression. I had the same problem when I got my bike and found that if I opened the pilot screw to almost 5 turns I got ignition. Therefore, I was not getting enough fuel or I had an air leak. Fortunately, I found that the intake gasket for that cylinder was sucking a lot of air and when I removed the intake I found that 1/2 of the gasket was gone. Made and installed a new gasket and it solved the problem. Have you checked for air leaks on the engine side of the carb--gaskets and o-rings? take some propane while the engine is running and go around the intake and o-rings and see if you get any response. Sometimes simply adding enrichment (choke) will work. Unless that cylinder is badly low on compression your problem is probably an air/fuel problem. I am assuming that the valves are in specs and you did a good bench sync of the carbs and you have a stock air filter.

 

How deep a cleaning did you do? Did you split the rack and replace all seals and pull/check the pilot screws/seals as well? The carb bowl is filling and maintaining a full state (I'm assuming a good flow when you open the drain port), but perhaps you still have a clog somewhere.

 

Typically the plug caps should read 5K or 5000 ohm +- 10% not 10, K i presume, Get a new cap or swap from another cyl see if problem moves.

 

I definitely have fuel, because the #4 plug comes out wet after I run the bike for a few seconds. I will test the compression in the am and see if that is the issue.

I will also see if I have an intake air leak.

Not church of clean, but every passage with carb cleaner and compressed air. I may have to pull the rack again if I try all other possibilities without a solution.

I may have made a mistake on the resistance read - there are settings on my meter that would require I multiply the reading. I will check that again.

I'm thinking that the swapping thing may be how I can rule out an electrical issue - if I can swap #1 wire onto the #4 plug and vice versa, that should let me see if it is an ignition problem. Don't know if it will reach, but I can remove the coil from its mount and rig a solid ground for the primary and then try it.

Any chance that I am dealing with a weak secondary and/or plug wire that can't jump the plug gap under compression? Does that ever happen?

 

Generally a dual coil will fail on both leads, however one cap higher resistance will cause one cylinder to go weak and also "supercharge" spark on the other, this will eventually cause coil failure.
Also check #1 cap, if the resitor failed to short, 0 ohm this will cause same effect, as electricity follows path of least resistance, "supercharging 1 and weakening 4, eventual coil failure.
Simply swap the leads 1 to 4, to test they fire simultaniously as they are same coil (one cyl TDC compression,other TDC at valve overlap where there is no fuel and therefore does nothing though its sparking)

 

Also, though its better, typically a coil with 2 wire feed doesnt need to be grounded on the coil frame.

 

The manuals list a 10 K ohm resistor for the #3 and #2 caps, which is incorrect. It's not unusual for someone to not know of the error and fit those caps anyway. It's not as common, but does happen that sometimes 1o K caps get fitted in the wrong locations.

 

Just because a plug fires outside the cylinder does not mean it is firing when under compression and with fuel. I've had that happen. Did you try a new plug or at least exchange plugs? Also, if your plug is very wet it could indicate that you are getting too much fuel for the plug to fire.

 

Ok folks, with your help I got her fixed.
I flipped the leads on the 1/4 coil and discovered that 4 was still not firing, so on the fuel side.

After a close inspection, I discovered that I had pulled the airbox boot on #4 too far out when I put the carb rack back in (I push them into the airbox when pulling the rack, as suggested by one of the many wizards on this site). About half the boot had the second ridge sticking out, when it should be inside the box to make the seal. Corrected this, and #4 fires happily now.

I never would have believed that a leak that small could richen the mix that much, but it did. My years of muscle car and boat motor shade tree mechanic school does not translate as much to these classic bikes as I would have hoped.

Thanks for all the help, Tabaka, Nuch, Colin and Moe. Great site!

 

You're right about that.
The mix was leaned out to the point of incomplete combustion, which is where the soot came from.
More air = more lean. And since the XJ was delivered already running on the lean side it doesn't take much to lean it out too far. A foam air filter is enough to make it run like crap.

 

It was one of the first things I did... replaced the "performance" red foam filter (that the previous owner put in) with the stock paper one.

As a side note, that red filter is not bad at scrubbing parts after a good soak in carb cleaner!