Bought a 2005 S40 about a month ago with ~4000 miles on it.  I'm now noticing that at low speeds (not necessarily low RPMs), the bike jerks under acceleration.  Not sure how else to describe it other than uneven acceleration, even though I'm holding the throttle steady.

I did the easy stuff: changed the oil and replaced the air filter.  That didn't help.  What should I try next?  Should I try a fuel additive to clean out the system?  Should I check the spark plug?  Could it be an issue with the carb?  Basically, what's my next move?

Not sure if this is related but my bike requires choke for about a minute upon startup, even with air temps at ~85 deg and higher.  Without the choke, the engine idles very low (I already adjusted the idle screw) and dies.

Any thoughts or points into some direction are greatly appreciated.

Put some Sea Foam in the gas it will clean the carb and lube the carb slide.

I rode a stock unmolested bike when I was riding used ones looking for one to buy.  I noticed that this bike did not run real smooth at low throttle settings just off idle....the way you would ride down the street in your neighborhood when you want to be nice to the neighbors.  It rode much smoother under full throttle.

I believe the engine was running a bit lean and would have benefitted from a larger pilot jet and the modification to the slide to replace the white spacer with the three washers.

Go to the Technical Section, find the index, and look under Carb mods.

Having read the MSDS list of ingredients in Seafoam, I believe it is a completely worthless product. The carburetor slide is lubricated by gasoline, which is itself a very light oil. And gasoline is at least as good a solvent as anything in Seafoam, particularly if the gasoline includes ethanol. However, if you'd feel better putting an additive in there, go for it. At the very least it will do no harm.

The thing about Seafoam and other gas additives is that you put small quantities (an ounce of Seafoam per gallon of gas, less than 1%, for instance) in a tank of fresh gas, then you run it. The fresh gas, with its Federally mandated detergent package, flushes out the system, and usually that is about all that is needed. The additive gets the credit, even though it basically does nothing. It is the placebo effect writ large.

My throttle was really choppy when I bought my bike brand new a few months ago.. I believe it has to do with the carb.. I did the white spacer mod. I changed out the white spacer with the 3 small washers I got from the jet kit I ordered from LANCER... Right after that mod I could ride down town, 2nd gear 25mph and it was smooth!! No lagging and it wasn't choppy at all!! I also changed my muffler to a harley dyna muffler and got rid of that CANON they call stock muffler, re-jetted my carb with the jetset I got from LANCER and it runs and sounds amazing!!! I also changed my spark plug to a performance star plug. I hated this bike stock but after a few mods, I am really starting to love it!

(My opinion)

denistheruski wrote on 08/20/12 at 15:15:22:



Are you hearing any misfiring?

Nope, not lugging.  I feel some jerkiness through most of the RPM range, even at mid to higher revs.  Could this be a hint: even when I'm sitting in neutral, a minute or so after start-up, if I rev it to mid-RPMs, it's very sluggish to respond.  After more warm-up time, it's better in neutral but I still feel this "sluggishness" or jerkiness when I'm in gear.

I searched for some youtube videos of misfiring to hear what it sounds like.  Correct me if I'm wrong but it should sound like out of synch popping?  Anyway, I don't think I'm hearing that on my bike but I'll listen closely on my ride tomorrow.