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while riding an event this weekend my bike was surging at 1/8 throttle position.In sixth gear on a gravel road(transfer section) 1/8 throttle pos.bike was surging consistently, main was good, pilot seemed to be correct.could this be a function of the needle?rb'd head and carb with c-12 32:1.
thank you
CB

Assuming proper float level and a clean carb, I would try one size richer pilot and see if it improves. If not see what happens when you raise the needle one notch.

Does this not sound like a leak somewhere?

Specifically, surging is not a 'function' of any carb circuit, but I doubt that's what you meant. Your reference to the needle is because of the 1/8 throttle position?

Surging generally results from a lean mixture..it being lean due to something errant...something wrong..not a state of tune.

No? Yes?

yes at 1/8 position.There seemed to be no indication of a lean mix at w/o.Pilot appeared to be good based on the response of the air screw settings.This only was happening at 1/8 t/p,1/4-full appeared fine.Am I even close to a good guess?Could the kips be modulating?For what is worth I pulled the plug color was normal,no aluminum stuck to the electrode.Not boiling over.

I wouldn't think 1/8 throttle in 6th gear is going to be even close to where the KIPS comes on.

I would think that any leak that caused a surging condition at 1/8 throttle would also effect air screw response.

A bit more on the 'surge' part? It was with power? On the 'up' part of the cycle, did it actually pull? What would you guess the RPM range of the oscillation was?

Mike started at a good place...to start. Check the float level, clean the carb. At least give it a good shot of compressed air after a dousing with BraKleen if you don't have a can of Berryman's around. That with the carb in pieces to get to as many passages as you can.

Anything lead up to this? Like....'I just put new reeds in....'?

As far as the good guess part...you're right about the throttle range and what part in the carb works at that opening, but the needle (itself) isn't going to cause anything like that to happen.

surge would appear after fully accelerating 1-6 up to point where torque taper's off then throttle reduced to a cruise of sorts .I would guess a speed of about 50 mph.RPM I really could'nt accurately guess a range of a few hundred perhaps.It would pull for a second or two then not.The engine would'nt bog or stumble it just had this pulsing feeling.

What was the ground surface? Any washboard? Is there a different surface on which you could repeat the accelleration/cruise procedure to find if the same thing happens? Asphalt maybe?

Hard packed gravel road,no washboard,yes there are a few asphalt roads that have potential near me.Your point brings up another observation being the "surging" was not noticeable in the farm fields. Maybe due to a higher rolling resistance? Something tells to start with the obvious and go from there. I most certainly thank you for your help
CB

Here's why the washboard question......

I experienced that a few weeks back in Idaho. Nice gravel road, it had washboard I could see...but I couldn't feel it. No bouncing, bumping, or shuddering. I took note of that particularly, the period of the bumps was such that it felt very smooth. I thought, 'Nice suspension!!'

BUT...I DID feel something that could be described pretty much like what you're talking about. At the time I thought something was wrong with my back brake..I would noticebly slow down..then speed up. It was cyclical. Yep..you could call it 'surging'. Your guess of 'a few hundred' RPM is what brought it to mind. That's a quite narrow range.

While I couldn't feel the washboard, I could SEE it just fine..and sure enough, it was on the washboard sections (generally on curves..acceleration bumps caused by skipping differentials and unloaded inside wheels..the sorts of things washboard comes from) that I could feel this weird, cyclical, pulsing thing going on.

That isn't going to happen on asphalt..something smooth.

Maybe?

Excellent observation I will give the asphalt a try.

I couldn't of made it up if I hadn't experienced it. I don't know what it IS, either. A conspiracy between the suspension and mother earth, a confluence of disparate factors that serendipitously conjoin to screw with your head.

OMMMmmmmmmm OMMMMmmmmmm

Maybe its just???

Flat-slided, modern fat-piped, PV'd two strokes don't part throttle cruise very well like this...You may be pulling your hair out for nothing. Try it up an incline where it will need this 1/8th to pull the hill, and see if it goes away. They like it under load, but hate that narrow line of coming on/off load. The older 2T streetbikes had the exhausts tailored around helping this problem....not making max hp

Yep.

Something that hasn't been mentioned is what the bike sounded like. The condition noted (hi speed, 6th gear, part throttle) is going to result in some yippee-skippy in any 2T that's jetted half-fast (four-stroking). You could feel that as a 'surge'.

AND...in that situation the throttle is very finicky, too. It takes a fraction of a red hair's diameter of throttle movement to make the bike jump.

Fuzzy's prolly right.

was most noticeable going up hill.The bike was running nice and crisp had good pull.Typically I try to jet the bike for conditions on race day.The jetting numbers fell in line with prior experience.Not that it means much a 150 m 40p cek-2 70 deg.at 30 rh c-12 straight @32-1.220 rb head and carb 607 reeds woods pipe 1000' elev. for a little more insight on the machine.

'Up hill' takes care of the no load part, I guess.

It wasn't 4-stroking, then?