very quick stuble off idle question

Just totally stripped, cleaned and reassembled an old set of 26's and fitted same to the bike. Synced right on the money, floats correct. Adjusted air mix screws to optimum idle for each and returned idle back to 1,100 using the thumb screw. Just checked the timing and adjusted to right on the "F" at 1,500 RPM.

Perfect @ idle and 2,000+RPM all the way up to 7,000 BUT SMALL HESITATION and STUMBLE at pull away unless I crank upto 2,000+ to launch.

Do I need more/less air on the air mix screws. Do I need next bigger pilots?

Thanks.

Per FSM: from point of highest idle speed, turn all four air screws in a small equal amount. Re-adjust idle speed to 800-1000 rpm [with the main idle adjusting screw].

Have interpreted this as being desireable to have a very slightly enriched mixture to minimize stumble when pulling away from idle. (Perhaps remotely akin to function of an accelerator pump.)

Patton,

Thanks...I'll go out as soon as the rain stops and give that a whirl. I'll try 1/4 turn in on each. I'll report back asap.

How far are you out on the pilot jets?

I have found that there is a point (about after 2.5 turns) where any more out doesn't make the mixture richer, but actually leaner!

I think this is probably pretty well documented in your manual (I have VM24's) but I haven't looked at that thing in a LONG time.

With the mods I have I'm going to have to go to a bigger pilot, and I have the exact same problem as you.

I think these things are jetted a "shade" on the lean side from the factory anyway. I bet nobodies bike would mind a slightly larger-than-stock pilot jet followed by the proper adjustments.

WG can comment on that, though...

Ive got the air mix screws on the air filter side of the carbs so, my understanding is that the further out they are, the leaner the mix.

I had them each adjusted for fastest idle, somewhere between 1-2 turns out on each.

i have now each one a 1/4 turn back in, to richen slightly. Haven't riden yet as it was weeing down last night. Will try this revised setting this afternoon and report back.

The pilots are 17.5's and the mains are 118's.

I'm assuming that if all's ok north of 2,000 RPM then my "problem must lie in the air mix screws?

I discovered my stumble problem source by accident. You can try this.

Take the bike somewhere really dark, and idle the engine. Look for spark flashover.

on mine, three of the four were showing rings of light where the shell surrounds the ceramic. These plugs had less than 4000 miles on them. I had some fire bleedover on a wire as well.

One of the brand new plugs I bought to fix the problem had the same problem.

If the fire isn't at the tip of the plug due to path of least resistance bleedoff, you are going to notice it under loading.

I dumped my impedance caps at about the same time, replacing them with lawnmower miniboots.

The stumble in my case was electrical. It might be in yours as well.

Thanks everyone for all the help. Problem resolved.

Took the carbs back off and replaced the 17.5's with 20's. No more stumble.

So, for those who care, 26's with mix screws at 1.25 turns out, 20 pilots, 117.5 mains, Needles on middle (#3) slots.

Thanks again all the help.

Post edited by: BARNEYHYPHEN, at: 2007/08/01 15:34