ATV Florida Forum

what is the rule......too much air it runs rich and causes a black plug.....not enough air runs lean and white plug?....Am I correct ?

The easiest way to determine if you are jetted properly is to do a plug check for each "range" of the carb.
There are three ranges to adjust in the carb.
Low speed: Is off idle to 1/4 throttle position. The pilot jet (is inside the carb float bowl and is removable to adjust the mixture by changing the jet to a smaller or larger number size) or fuel screw found under the carb outside between the float bowl and the intake manifold.
The smaller number on the jet = a leaner mixture. The bigger number on the jet = a richer mixture. The fuel screw turned OUT is richer turned IN is leaner.
Old time round slide carbs use an air screw (on the outside side of the carb) turning the screw IN richens the mixture and OUT leans the mixture. The air screw is NOT common anymore these days.

Mid range: Is adjusted by the needle clip and is 1/4 to 3/4 throttle position.
There are slots on the needle where a clip can be moved up or down to change the mixture. The top position on the needle is leanest (letting the needle stay lower in the bore of the needle jet that it fits into. The lowest position on the needle is the richest letting the needle stay higher in the needle jet bore.

Top end: 3/4 to wide open throttle is the main jet. It is a removable jet in the float bowl. Just like the pilot jet, it is numbered and the bigger number is richer the smaller is leaner.

Checking jetting doing a plug check:
You will need to mark the throttle housing (twist throttle) or remove the top of the thumb throttle cover so that you can watch which part of the throttle opening you are in.
Using some gray tape place it on the throttle housing so that you can mark the position of the throttle. Mark the sweep of the throttle from closed to wide open. Then divide the sweep into 1/3's. Zero to 1/4, 1/4 to 3/4 and 3/4 to wide open. Then make a mark on the grip or throttle lever inside the housing (thumb throttle) so that you can make note of the position when riding.

Install a fresh plug and find an area with a slight incline about a football field long.
You will want to get into 3rd gear and hold the throttle in which ever area of the carb you want to check for the length of the field. Do NOT vary the throttle, you want to hit the kill switch and pull in the clutch, coast to a stop then pull the plug.
A tan color is what you want. A clean plug is lean and you will need to richen the area of the carb you just tested. A black or sooty plug is too rich and you will need to lean out the area of the carb you just tested.
You can do this check for al three areas of the carb.

This test will need to idealy be done in 55 to 65 degrees on a not so humid day. Hotter and more humid or colder days will be totally different as far how the jetting will be.
The hotter it gets outside the richer it will run. The cooler it gets the leaner it will run.
This should give you the basic understanding of how things work in the carb and how to check the ranges of the carb to be able to adjust the jets or screws.





Some indications that you may be too rich:
Power feels sluggish.
If the power seems worse when the engine is hot.
Putting on the choke makes it worse.
Dark plug readings.
Removing the air cleaner makes it better.

Some indications that you may be too lean:
Poor acceleration or it doesn't seem to rev out, sometimes if you back off the throttle slightly the revs will pick up.
Power feels better when you put on the choke.
Removing the air cleaner makes it worse.
Popping or spitting back through the carb.
Plug readings are white or clean.

Bigscrub gave you the whole 9 yards. In short...

Too much air = lean condition/white plug

Too much fuel = rich condition/black plug.   

You had it backwards. -Mark