KZ550-F1: Running Rich and Poor Fuel Mileage

Quick Update: Went with 35s and got them installed. Way too lean. A quick 2 mile test run ended up stopping right at that point for temp reasons. Pulled two of the plugs and they were almost snow white, just a bit too hot.. A bit of cooling and a bunch of tape on the air filters later and I managed to get it home with the engine still moving.

Going to try out the drilling method before going and buying new jets again. Almost $60/set will get insanely expensive pretty quick.

The mixture will richen up as the motor warms up, not get leaner.

I'm aware of that. Here's a pic of what just over 2 miles (with a fair bit of traffic) did to a brand new plug though. That, and the engine was noticeably warm just to be around. There's air cooled warm and then there's "I could throw a fan on this and replace my furnace" warm.

It's got a bit more brown now that I've blocked off some air filter to richen it up a bit.

Plugs don't tell you what throttle position is not jetted correctly. Some positions can be really rich while others can be lean. The plug just gives a sort of average. To really know what's going on you have to have a throttle position correlation to engine behavior to know what to change.

For instance, if you are at 1/16 throttle steady on the highway, and you start to get some hesitation, but idle is ok, then you are probably lean on the pilot circuit. This can be fixed with a larger pilot, or possibly raising the needle with a shim. The correct choice will be based on what happens at larger throttle openings.

There is a lot of overlap on what different actions will produce, so I like to look at the whole range of throttle positions before changing anything. Without a wideband gauge, some of it will be guesswork. The best tool you can have is to make meticulous notes while doing a test ride. Even with a wideband gauge, I'll have a full page of notes correlating throttle position to engine behavior.

Estimating throttle position is difficult to do while riding. A piece of masking tape and a pointer on the grip comes in real handy for data taking. That would be my first suggestion to eliminate some guesswork and start to make repeatable tests.

Dynojet has some good notes on how to do proper jetting tests.

Valves adjusted?
Steve