Surging on deceleration

[1980mcneil]

I'm going to try to make this questions short. And thanks for the great information as always guys.

I rebuilt my carb and I have a assortment of all kinds of jets that were handed down to me when I bought bike. I have no idea what Jets were in it before the year down. Now that everything is back together, the bike runs great other than it surges while decelerating in gear. I rev up 1st through 3rd say, I let off the gas completely and while decelerating the bike surges every second or 2.  Not lagging like it's gonna die.  Surging or lunging like little bursts of power the instantly go away.  Assuming I have no air leaks, because the bike ran great less than a week ago. I can test that later. But if it indeed is because of jetting what would cause that? Since there's no throttle on the bike at that time I would guess it's the pilot jet. Right now I'm running a 58 pilot jet second needle from the top and a 165 Main. I'm at sea level essentially and between 70 and 80 degrees.
According to the jetting chart in this forum at my altitude they recommend stock settings. I'm One Clip leaner on the needle and one size smaller on the main jet. I've heard most guys are running that and I might be going into a little bit of elevation tomorrow.
I have a pro circuit to platinum pipe comma a power Dynamo ignition, a reed spacer and FMF silencer. I also have the decompression valve that goes on the cylinder wall not the one on the top of the head. I'm told this does result and slightly lower compression but I'm not sure if that will have much of an effect on the jetting.

I really have no idea, but if I were to guess I would say it's a lean situation. And yet the stock pilot jet is supposed to be on the rich side and now it's even warmer outside...  I'm going riding tomorrow after work and I'm hoping to get in one pilot jet change before we leave and then I'll be stuck with it for the night.   Advice?  I have a 55-60-62 pilot, and the 58 installed now

[1980mcneil]

I'm still getting the surging.  I guess I'm giving into the fact I might have an air leak.  I messed with the throttle cable recently. (It's not stock) maybe that is the problem.  I know the cap on the carb has a gasket, bit what about where the cable comes through?  I have a tiewire that keeps the cable in the carb.  Can air leak through there or through the cable?  How would I test for air leaks on the intake side?

[1980mcneil]

I would really appreciate someone chiming in here.  I have a little bit of an update...

Long story as short as possible...  I believe my surging was caused by a high idle screw adjustment.  The weather has really heated up here in Wa, so I pulled the carb out and leaned out every jet and needle to one setting below stock. 165/55/2nd clip.  During that process I had unscrewed the idle almost all the way.  Rode the bike and surging is gone and bike ran pretty good.  except it wouldn't idle...
"no problem, just screw this idle in and good to go".  Screwed it almost all the way in for it to idle, started riding and Surge is back!!

I'm hoping with this information you guys could point me in the right direction or at least confirm the direction I plan on going since the plan is to go leaner and I don't want to melt the old girl.  I'm guessing the bike won't idle with the screw out because it is still rich and needs the slide further open to get enough air to idle.  So my move is to go even leaner on the pilot.  With a leaner idle it shouldn't need as much air and idle screw to run.  Is this logic sound? I'm only at about 500-1000ft.

Side note. ( the bike always seems to run rich.  since I got it, it has always had pretty bad goop coming from the tailpipe and header silencer connection.  The bike has lower than average compression that i believe is mostly due to a cylinder decompressing valve (not the head valve)  although I've read that compression has little to do with jetting i'm inclined to think differently.  If the vacuum of the engine is the same as stock but there is less air being compressed it seems you would definitively need less fuel in the cylinder.

[dave916]

Think there is always a little surging on deceleration a big single, if you have other issues air leaks /crank seals  reeds etc it will make matters far worse

Needle diameter will have a big effect on what pilot size u use

I  have been trying to reduce any surging{not that it bad} on my supermoto over the past few weeks /i have tryed a lot of differant setting differant needles ,pilots, slides,     setting rich on pilot {slide high} with idle screw  all the way in,  nothing seems to vary it that much


Even fitted  a twin entry boost bottle to help  getting a smooth low end




 

[sandblaster]

Here is something that is not often discussed.
Once you get your jetting sorted out check out... Flywheel size...
Yes, it will affect surging...
A lighter flywheel makes the surges smaller but quicker in frequency...
It's much easier to control.
I'm using a Power D system and I used the lightest flywheel which is 1 lb lighter then stock.

And if I'm entering a corner to fast and I'm concerned about surging.... I pull the clutch in and use my rear brake just a tad more...

[motorman455]

I dealt with surging for a few years until recently. This year I replaced the carb with a JD 41mm keihin and it completely disappeared. Not sure if it's his needle design or what but the surging is gone.

[1980mcneil]

I do have the power Dynamo already for my dual sport kit.
I ordered some leaner Jets for both the pilot and the main.
He'll be here on the 12th and I'll give an update

[dave916]

I do have the power Dynamo already for my dual sport kit.
I ordered some leaner Jets for both the pilot and the main.
He'll be here on the 12th and I'll give an update

In Theory running rich on pilot /idle screw in full/  which in turn would have slide slighty open should work good as it gives case compression a bleed off on closed trottle
Seems to work ok for me anyway / I like to run rich on pilot / due to alot of overrun on a supermoto