GT550 Sooty spark plug

[paulnz]

Happy new year to everyone.

I have a 74 GT550 that runs well, but tends to foul the right hand front cylinder plug (wet). Changing the plug all good and runs well. I took the bike for a good (hard) run down the motorway and when back home looked at the plugs, two are nice and brown and the one (that fouls) is black and sooty.

Any suggestions most appreciated, many thanks. Paul

[Jimroid]

Possibly low compression, HEAVY FLOAT( are those brass?),loosing a little bit of trans fluid?

[Alan H]

High fuel level in the 'rich' carb?

[pearljam724]

Dark brown to light black is what you want. Too much black carbon presents the cylinder running too rich. If the plug is very wet, you may have flooded the cylinder prior to running it or have a petcock that leaks a little during idle times. You can easily test that. Float levels may be slightly off compared to one another. Your jet size's may be incorrect for your exhaust also. If the plug is very sooty or too wet at times. Just drop the jet size. Read the plugs religiously on a 2 stroke. It's critical to monitor the crank seals condition via plug readings. Too much idling and excessive low Rpms causes carbon build up. Including exhaust, head and exhaust ports. You could also have oil check valves that leak at stand still causing excessive carbon or some people add too much oil by adding premix into addition of using the oil pump or add too much oil as a premix. You have to take that info and investigate. Slightly rich is fine, so is slightly lean. You just want to avoid too much carbon build up and very lean plug readings. Very lean plug readings on a particular cylinder is a good indication of a cracked, swollen or torn crank seal pertaining to that particular cylinder.

[Jimroid]

I have seen brass floats leak and that will make a very rich cylinder.

[pearljam724]

As mentioned above cracked floats or pinholes in floats can cause a rich cylinder also. Take float and shake it around for the sound of gas inside. Or boil the float in mildly warm water to check for bubbles rising from microscopic cracks. I would start by adjusting the air/fuel mixture on the cylinder that is reading too rich. Just turn it a 1/4 turn at a time. Take it for a ride, read plug again to see if that improved the reading. You have to open the throttle to 3/4 of a turn running down the road for several hundred feet or more. Hit the kill switch at that moment, then immediately read plug without restarting bike. You won't get the plug reading your looking for following idle or excessive low rpm.