Gents, tests like the WOT to 7/8 roll-off yield symptoms that indicate possibilities, not hard conclusions. If a momentary surge results from rolling off to 7/8 throttle from WOT, this is a symptom of a lean condition because quickly lowering the slide a bit results in less air being fed to the engine, momentarily richening the A/F mixture.
This test works pretty well on carbs with direct mechanical lift. But vacuum diaphragm carburetors are devious, surly beasts, and the slight response delay of the vacuum operated slide can make things harder to read. Also weak spark can cause WOT power weakness or even breakup that resolves when the throttle is rolled off a little. Want to get the main jet as right as it's going to be? Make sure the ignition is perfect, valve lash and cam chain tension are right, and compression is good in both cylinders (>125 psi, <10% difference). Then find the smallest main that will run clean at WOT and go up 1 step. If you're working from the top down, revisit the mains after everything else is dialed in. You may wind up doing three or four rounds of change and compensation. Expect it.
There's no "perfect" jetting. Carburetors have to constantly respond to changing demand with a small number of simple components, and compromise is the name of the game. There are jetting combinations that work, combinations that work well, and combinations that don't work at all. Tuning takes patience, a high frustration threshold, and willingness to take a deep cleansing breath and walk away from the machine when you run out of the former and hit the latter.
Peace!
This test works pretty well on carbs with direct mechanical lift. But vacuum diaphragm carburetors are devious, surly beasts, and the slight response delay of the vacuum operated slide can make things harder to read. Also weak spark can cause WOT power weakness or even breakup that resolves when the throttle is rolled off a little. Want to get the main jet as right as it's going to be? Make sure the ignition is perfect, valve lash and cam chain tension are right, and compression is good in both cylinders (>125 psi, <10% difference). Then find the smallest main that will run clean at WOT and go up 1 step. If you're working from the top down, revisit the mains after everything else is dialed in. You may wind up doing three or four rounds of change and compensation. Expect it.
There's no "perfect" jetting. Carburetors have to constantly respond to changing demand with a small number of simple components, and compromise is the name of the game. There are jetting combinations that work, combinations that work well, and combinations that don't work at all. Tuning takes patience, a high frustration threshold, and willingness to take a deep cleansing breath and walk away from the machine when you run out of the former and hit the latter.
Peace!