Carburetor Tuning Discovery

[algirdas]

I got phenomenal improvements in throttle response/control and eliminated accelleration flat/weak spots by changing one part of the accelerator pump assembly.
I replaced the spring on the accel pump rod with one taken from inside a retractable-tip ball-point pen. The new spring is the same length but about one-third the stiffness.
I 'stumbled' upon this idea while trying to eliminate a flat spot that occurred during aggressive acceleration from low speeds and from medium cruising speed. My thinking was most carb problems are RICH or LEAN. This acceleration flat spot could be either. The accelerator pump is to deliver fuel during SUDDEN accelerations. So if I disconnect the accel pump rod and ride without accelerating aggressively ,(smooth progressive rolling on of the throttle), the carbs should provide the correct fueling, not too RICH nor LEAN.
AND it worked. As long as I ROLLED the throttle on, even if I did it quickly, acceleration was smooth, NO flat spots.
BUT if I rolled on too quickly or whacked it wide open, HUGE flat spot/hesitation for about half a second then ZOOM.
So now I've got a flat spot during acceleration with or without the accel pump rod connected, BUT the flat spots FEEL different. The flat spot with accel pump disconnected is sudden/immediate, as if the key had been switched off, lasts for a moment and then the bike lunges forward-ZOOM.
The flat spot with accel pump connected is burbly, sluggish, and I hear the engine cutting in and out as it misfires.
Since running with accel pump disconnected, would, overall, put less fuel into the engine I surmised that when running with it connected the mixture was too RICH, and caused the flat spots during sudden or aggressive accelerations.
How could I make the accel pump inject less fuel during acceleration ? Smaller nozzles-Stronger return spring under diaphragm-less stroke length of pump diaphragm-Larger bleed off hole returning to float bowl- or weaker spring on accel pump rod.
I settled on the weaker spring idea since it could be done without dismantling the bike or the carbs, and I had a selection of springs on hand, still in the ink-pens, from an office building clean-out job.
Armed with needle-nose pliers and small tip screwdrivers I installed a clic-stic BIC ink-pen spring on my carbs - ZOOM.
Of course Murphy's Law dictates that "your mileage may vary"
Charles in Cincinnati w/ WEAK spring/STRONG acceleration.

[Lucky]

One of Yamaha's fixes for the stumble was to increase the pump rod length, effectivly doing the same thing.  thes was in a TSB from late 1982, i'm not sure which one, i'd have to look it up...
--Lucky

[Dave T.]

I have had little improvement with fiddling with the accel pump. Aiming the nozzels was most important, next was adjusting the limiter screw. I had the best results with backing the screw out pretty far, even as to remove it.

The best thing I did so far was to go to leaner main jets and keep the stock airbox setup with the vacuum operated flapper. I went to a 120 front and 122.5 rear main jets. When I had stock setup, when you "whacked" the throttle wide open, it would just cough and bog, instead of pull hard. This indicated to me overly rich mains. When I changed to these leaner mains, the stumble all but disappeared.

I am still plagued with a little stumble at 5000 rpm's, Especially when you unload the engine, like backing off the throttle and going downhill. I think this the time where the flapper is going from a 1/2 open to full open. I have to change my fuel filter about every 2000 miles, this helps a little. I may try to go even leaner, like 117.5 mains. Because you should be able to hold the throttle wide open and it should'nt bog. This would indicate a rich condition. My V will pull strong when I roll on the throttle, but will lose power when I "whack" the throttle wide open.

These are my observations. The pen thing is easy enough, I may try one of these springs and see what happens.. Good observation! ? :D