One cylinder has problems (sooty deposits)

Recently I pulled my 82 XJ650RJ out of storage and am tuning it up. I moved a couple years ago and the bike sat (with gas in the tank & Carbs) for a couple of years. I drained & cleaned the tank, Had the Carbs cleaned by a local shop who also changed oil, trans oil, new tire and other essentials.

The problem is that the power was not what it should be, so I pulled the plugs to find cy's #1-3 were fine however #4 was covered in soot. Consulted the trusty manual which stated it was ignition or fuel/air mix running rich. So.. I pulled the carbs again, checked the floats, all ports were fine. Had the coils tested at a local shop and they were fine as well.

Also noticed cy's 1&2 were running hotter than cy's 3&4 by 180-200F. I have a Digital IR temp gauge. With the bike at idle, I can pull #4 ignition cable with no noticable drop in RPM. The bike idles fine at 1200RPM and has new plugs. SO.... What could be causing misfire in #4 cyl? Any Ideas ?

 

It's either too rich because of some reason like a leaking float valve or not firing due to a bad sparkplug, cap, wire, or coil. The resistors in the cap frequently go bad and the coils crack. You need an ohmmeter to check the cap and wires and a spark tester to check for a good spark and a Colortune to check the mixture. I'm betting the mixture is way too rich and will show up yellow on the Colortune.
There's also a chance they mixed up the jets when the carbs were cleaned.
BTW, I wouldn't trust the shop to tell me the coils are good. The 19 year old high school dropouts working there probably don't even know how to test coils.

 

Thanks for the reply. I have a good meter and will check the wires and cap. I have never used a colortune. Can I get one at any auto parts store? I assume it is a sparkplug with special features.

I hate to pull the carb bank again but guess the only way to see if they mixed up the jets would be to compare to one that is working correctly. I'll rule out ignition problems first. The coils were replaced (by me) a number of years ago, however since then there has been only about 10k on them. The ignition cables are perminantly attached to the coils. One fires cy's 1&2 the second CY's 3&4. Came labeled and everything. I have a feeling one of the coils is a bit weak.

The guy that tested the coils right in front of me was one of the mechanics at a small Yamaha dealership. He seemed petty honest because I was prepared to get another set. He seemed to think the problem was the carbs. I still think it is ignition, But then again I work on computer systems and only hack at cars & Bikes.

 

There's a lot of discussion on here about Colortunes so just check the other messages. The plug cap resistors go bad and can be confusing to check but cyl 1,4 have a 5K ohm resistor in them and 2,3 have 10K ohm resistors according to the manual but they can all be 5K. Older ones tend to read 8K or be open. You can get replacements from NGK at most bike shops. The coil winding is 11K ohms from end to end so with both caps on it can read either 21K or 31K from end to end but it's better to take the caps off and check everything separately since the wire ends can get corroded if they got water in there and you wouldn't know it unless you take off the caps. THe stock coils tend to be too weak but if it fires one plug it should fire the other one too unless the wire or cap is bad or the case is cracked.

 

I pulled the caps and checked the resistance this weekend. The problem cyl was reading just below 5K ohm so I replaced it and made sure the cap was snug, cleaned the plug, fired her up and took a 50 mile ride (to town and back), there was an improvement in performance and only slight deposits on the plug. Ordered two new ones today since the rubber cover between the wire and cap was suspect.

With the other postings about the color tune, I am more inspired than ever to get one of those things and try it out, I bet it could be useful on mowers, chain saws and all the other power tools in the shop (with the right adaptor)

When the new caps come in I will let you know if it solves the problem, otherwise I am back to the coils or the air/fuel mix in cyl 4.

I never thought to just yank the plug cap off the ignition wire, but sure enough it does! Could be a simple fix for those with a similar problem.

 

If you know where the pilot mixture screws are I would suggest oening the #4 screw about a half turn and see if that cylinder fires better. Yes I know that is a richer mixture but if #4 doesn't idle then the soot could be from the intermittent firing of #4. A lean mixture of fuel too lean to burn will eventually burn when enough fuel is introduced. This then makes for a rich mixture leaving behind the soot. Best way to find out is the colortune. Second best way is to pull the plug and see if it is dry or wet. Wet with is too rich to burn. Dry is too lean.