Tree Farmer:

I need to append to your comments.

When doing a plug reading, full open acceration and nailing the kill switch will only provide an accurate reading for the main jet. This is only one of the 7 circuits in a carb. The main jet is usually the culprit but the pilot jet and the setting of the needle position can also cause trouble. Any signs of flat spots in acceleration and starting trouble are other symptoms with different causes. A too high float level or a sticky float needle valve can too cause rich plug readings.

Since we were on the topic I just wanted to elaborate a little.

Keith

 

Keith, my response addressed an aspect of the initial post, "what is jetting?"

"Jetting" generally refers to sizing a jet for near-stoichiometric fuel-air mixture combustion; usually, the main jet. At wider throttle openings, the main jet dominates mixture composition, analyzed by plug readings as described.

Regarding the seven other circuits you mention in a carburetor (Most ATV/motorcycle carburetors seem pretty simple; do all have seven circuits? For example, most automobile carburetors have "accelerator pumps", few ATV carburetors (if any) do.), few relate to "jetting," fewer still to analysis by plug reading. Thus, limiting the scope of my post to "jetting," I did not mention other carburetor circuits whose existence you correctly point out. I appreciate your contribution to the discussion, expanding into more comprehensive carburetor considerations. My limited response perhaps confirms your premise; the intelligence of those of us living in Appalachia is severely compromised by inbreeding, genetically guaranteeing our decadence. (Keith, I'm kidding! You already apologized to the citizens of Kentucky.)

"Speed & Sport's Carb Tuning Guide," at http://www.icnetx.com/speed/jet.html expands discussion of carburetor adjustment beyond the limited area of jetting and confirmatory plug reading.

Tree Farmer