One Black Plug

Hi Chums,
My normal riding is weekend stuff, so I am out on open roads using a variety of throttle settings ( if you get my meaning ). Recently, I have been obliged to ride in London where mostly the throttle is only partially open.
On both occasions the bike has started running roughly after about 30 minutes of this nonsense, and on getting home I have found that the left plug is very sooty, while the right plug looks perfect.
My first guess is a carb problem, but it has occured to me that maybe a coil is failing when it gets hot.
I have checked the air screws and they are both set the same, i.e. One and a quarter turns out. Then I took the carbs off and checked the jets and they are not blocked. Although a blockage would not cause rich running, obviously.
I didn’t check the jet sizes but I would hope they are at least the same. I have a mad theory that a previous owner may have fitted different size pilot jets ( I have seen this before ). Can anybody tell me what their jet sizes are ? Also, what about the needles ? I don’t think worn needles would be a factor at low throttle settings, but I’d like to be sure.
That then leaves a possible coil breakdown. Looking at the coils, I would say they are the originals, but I don’t want to start swapping them around as they are hard to get to on the 850T.

Any help would be welcome.

One thing to check on the carbs is whether the choke plunger is sealing correctly. It’s worth removing each one and checking there’s enough slack in the cable and that the rubber seat is ok. Other potential carb issue which might give the symptoms described could be a leaking float allowing the fuel height in one carb to be too high.


Thanks Phil,
I have lever operated chokes, not cables, but that is still something I can check. From the look of the plug it certainly looks like the choke is on. I’ll check that first before removing the carb to test the float is sound. Following that, I guess I can compare the float heights.


The bottom of the choke plunges are rubber to form a seal. As they get older they end up with niples. These can then play up by not seating proper and you have partial choke.
The other thing I would look at is tuning the carbs with a Gibson colour tune. These tune each carb in turn. Brilliant bit of kit I have used over the years. Also check carbs are balanced.

Hope you sort it.