Occasionally there are questions that present themselves from natural causes, mostly due to differences in machine set-up. Since I am leaving Hot Cams, the guy that will be answering the tech questions here asked me to do a Faq Page on jetting. I'll post it here and you guys can tear it apart or add to it or tell me what you think.
Generally speaking, jetting can be quickly diagnosed by the following questions:
1) Low end performance
A) A lean condition is typified by surging and popping back thru the carb when steady light throttle is applied. It will cause popping during high rpm deceleration thru the exhaust (quicker when moderately lean, rapid when very lean, once every now and then when correct), once the rpm has dropped enough that the air speed thru the idle circuit is slow enough for it to pick fuel up that is. It can be quickly diagnosed by adding a slight application of the choke or starter enricher (if the popping stops, it’s lean). The intake will have a “droning” sound at steady throttle or under light acceleration loads in low gears when it is too lean. So, the question is; “Does it pop out the exhaust during deceleration with a closed throttle from high rpm to idle, while in 4th gear or above?”
B) A rich condition can also cause surging but will more than likely be very sluggish to respond to throttle inputs too. A quick diagnosis is to run the machine up to high rpm and listen for the occasional (once every now and then) pop out the exhaust, if no popping is heard, it is too rich on the idle circuit or fuel screw. If turning the fuel screw in does not remedy the richness, take a step down on the pilot jet and try again. So, the question is; “Does it pop out the exhaust during deceleration with a closed throttle from high rpm to idle, while in 4th gear or above?”
2) Midrange performance
A) A lean condition in the midrange is somewhat hard to diagnose. The biggest single giveaway is the sound of the engine. Usually when the engine is lean in the mid, it stutters and stammers and just doesn’t want to run right when it is VERY lean. When it is a little lean, it will cause the engine to labor slightly to run (it won’t sound happy), the engine will sound strained to maintain rpm, and the droning sound from above is easily heard thru the intake. It will also most certainly have a hesitation during sudden throttle application. This triggers a sound to be generated from the carb like the engine is sucking for air right before the engine starts to accelerate. A light popping will also be heard from the exhaust during steady throttle. When the jetting in the midrange is correct, you can accelerate thru the midrange with light throttle application or rapid throttle application and no hiccup or hesitation will be felt or heard. A method of diagnosis for a lean hesitator is to pull the choke on just for an instant and then stab the throttle the instant the choke is closed. If it takes off, the needle may be too lean or the pilot jet too small. Start with the needle, by lowering the clip one notch and feeling for the result. So, the question is; “Does the machine accelerate slowly through the midrange without any hesitation or hiccups?” Listen to what he says closely and go from there.
B) Rich conditions in the midrange are easy to diagnose. Under normal circumstances, a richly jetted midrange will make the machine very slow revving and unresponsive to small throttle position changes. If it’s the size of the pilot jet causing it, you will get no popping during deceleration no matter where the fuel screw is set. If it’s the needle causing it, you will be able to get it to pop during decel by adjusting the fuel screw. So, the questions to ask are: “Does it pop out the exhaust during deceleration with a closed throttle from high rpm to idle, while in 4th gear or above?” And “Does the engine react instantly with acceleration to small throttle changes when applying the throttle lightly?” (works best when in 2nd gear so the throttle changes can be felt immediately). If those 2 questions are answered with “No” and “No”, it’s most likely rich on the pilot circuit first, one step down on pilot jet size then repeat the test. If the answers change to “Yes” and “No”, lower the needle (raise the clip one position). Keep changing until you run out of clip positions. If it’s still needle rich, go to a leaner needle.
3) Top end performance
A) Lean condition on top is a little harder than the midrange. If it’s VERY lean, it just won’t take throttle. If it’s a little lean it will most likely rev quite good but, may SEEM to struggle to rev all the way out. Typically speaking, in a 4 stroke, it’s pretty hard to discern the differences from a little lean, just right, and a little rich. About the only way is to learn how to read spark plugs or find a lambda sensor to use. The question to ask is; “Will it take throttle past 1/2 throttle to ¾ throttle?” If the answer is “No”, it’s lean.
B) Rich conditions are fairly easy to discern. If it won’t accelerate or accelerates very slowly between ¾ throttle and WOT it’s rich. The question is obvious.
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