Running rich on one cylinder

Hi, new to site looking for some ideas
Bike is Yamaha 1984 XJ650 Maxim american import fitted with pod filters and aftermarket exhausts

Problem is No 4 plug is fouling after about 50 miles, and looks black and sooty, not wet . All others OK light brown.

Ignition;-
Changed plug three times to ensure its not just the plug
Replaced Plug caps
Checked that the spark jumps about 1/2 inch on all cylinders

Fuel
Checked fuel float level same in all carbs
Fuel jets all adjusted to 3 turns out
Cleaned out all carbs
Synced scarbs

Compression
Checked Compression - all cylinders similar at 150 psi

Oil
No sign of smoke from exhaust on start up or on closed throttle after revving so don't think it is valve stem oil seals inducing oil to the cylinder
Not using /burning oil noticeably

Bike runs Ok before fouling, No 4 seems a little cooler than the others which would tally if the cylinder was doing less work

Any ideas guys ?

 

first try switching position of plug wires move 4 to 1 and 1 to 4 see if you get the same running cooler exhaust.
could be bad spark plug or dirty plug wire connection

float levels ? did you do a wet set or just measure the hight of the float. float height is just a starting point
Setting the fuel levels

black sooty and dry can also be a lean condition

 

Sticking choke or bad plunger, main jet worn out or reamed out during cleaning, needle in slide hung up or not seated completely, needle/seat has slight leak. Most common rich issues

 

Have you tried a colortune plug? One thing you state your mixture screws out 3 turns , have you turned in the cylinder running rich a half turn? The mixture screws do not have to be set the same .

 

Thanks for the ideas guys , I have now stripped the No. 4 carb and checked for anything obvious . I did a static bench synchronisation check also (they were only very slightly out) . Float levels checked again (measurements) and did with fuel level (wet set) all ok and in spec according to the manual.

Did happen to notice a little white smoke from the side exhaust after running idle for 5 mins so kind of suspect it could be oil issue (leaking guides/rings) although compression is good and doesn't smoke on the overrun which would indicate that it was a valve guides leak issue...

Next steps,,,,,
1. ( from xj550 comment) I'll switch over plug leads 1&4 and run for a while to check exhaust temp's and plug colour
2. (From Colin ) I'll ensure the choke (enrichment device ) is not sticking
3. (from Jetfixer) I don't have a colour tune but may invest in one to see visually what is happening, I 'll try leaning off the mixture a little on No4 also to see any difference .

Here's hoping ...I'll let you know now I get on .

Cheers

 

Update ...think I have solved the problem
1. plug leads swapped from coils - no difference , and checked primary and secondary resistance , all in spec
2 choke mechanism all ok , stripped and rebuilt carb , (again !) all ok
3. Carbs all checked in balance and all mixture screws the same tuns out (3)

Well after all this messing about it was still misfiring a bit so checked the plugs again , all sparking ...........but.... when I checked the resistances one was open circuit ,, and this was a new plug !

Anyway replaced that one with one of my old ones and now running fine--result!

 

I've seen new plugs faulty lots, they are so mass produced now a days it's not uncommon.
Good stuff, shows how thorough you have to be, can't even trust new parts some times

 

Agree, and quite surprising. I'll be checking all plugs from now on before fitting . Just bought 4 new NGK iridiums for my other bike and checked the resistances before fitting them - all 4100 Ohms so all good

 

Spark plugs should only have resistance if they are resistor plugs. They should also only be used on ignition systems designed to use resistor plugs. If you add resistor plugs to existing resistor plug caps your total secondary ignition resistance will be well out of spec.

 

Yes that is correct , obviously they just have resistance when they have a have a resistor built in . Resistors are also in to distribute the power evenly. Only way to get Electricity to flow evenly to " ALL " of your Plugs is put in a Resistor. In reality you don't need them, but because Power must distributed evenly, they put the Resistors in them.
The xj650 I have is fitted with resistor plug caps also and appears to run fine . However would be interesting to see if there was any performance difference without any resistance caps and plugs ....

 

Not an expert, but I think with the XJ's it is important to keep total resistance of the plug and cap within spec. Others can weigh in as to the reason. Our XJ have electronic ignitions (TCI box), but they are a far cry from "computer controlled."

My XJ550 originally had 10k plug caps and specified non-resistor plugs.

When my plug caps were bad, I replaced with NGK 5K caps (all that I could find at the time), but now use equivalent NGK spark plug with 5K resistance. Total resistance is still 10k. I clearly noted the change the cables plugs, the inside of the side frame covers and manual so whomever my estate passes the XJ too will know about the change.

 

Yes thanks tend to agree , I'll check out the specs and adapt to suit I think .

 

Got a good question for you tech guys ... the Plug cap resistances in my Manual states UK models 5.5K ohms for the cylinders 1-4. ....and for US models it states cylinders 1 and 4 is 5K ohms but for 2 and 3 it states 10 K ohms why would that be ? Why 10K ohms for the middle two cylinders on US models ???

My bike is a 1984 US import with a later YICS engine...which spec would be applicable ?

 

misprint in manual

 

Well I have checked 2 manuals now and they state the same ??

 

haynes manual states 10 k but it is incorrect.

 

Odd as the genuine service manual from Yamaha States the same as the Haynes manual

 

haynes got its info from the FSM .