So here is the skinny on my issue. Recently my bike started to pop a little more than usual when coming off the throttle. Whether I eased off or flat out let go from wide open. Now if I'm at half throttle or more and have to decelerate quickly (let go completely or throttle down at a quick pace) the darn girl shut off. Happens 2nd - 5th gear

Any ideas as to what the heck is going on?

She sounds like she is idling high enough. Not sure what the RPM's should be around though or how I can decide since there is no tach....yet  

Does the pet normally go out within the 1st 2 years of the bike?

The popping indicates a lean condition. Fuel starvation from a failing petcock certainly would cause this especially with the reduced vacuum that a fully closed throttle would produce. An obstruction in the fuel line is a potential problem as well.

Make sure that the carb boot between the carb to the head is not compromised or loose. Less likely are a dirty air filter (it would really have to be clogged) or a failing ECU.

Without a tach,.. you sorta' don't... but it should purr smoothly, not lope or stumble like a Harley...

Here's a test for the petcock...
This, is a link...-> Test your Petcock,.. don't just turn it to Prime..  ...

A fully closed throttle increases vacuum.  The vacuum tap is on the intake side of the butterfly.

I've never had your problem so I'm afraid I can't help with it.  

Check the petcock...it could be leaking into the vacuum hose.  If there's any gas in that hose, you've got a bad one.

Steve H wrote on 02/05/15 at 13:15:38:


It could be dying from being too rich...or too lean.

When you close the throttle, the needle drops down into the jet and closes off fuel flow....and the only fuel that goes to the engine is through the Pilot and Idle fuel circuit, and whatever may be added by the TEV valve.  If your Pilot jet (the smallest orifice in the carb) has any kind of deposit, varnish, corrosion or junk in it....the mixture may have gone so lean when you close the throttle that the engine can stop running.

Also when you close the throttle - the vacuum on the engine side of the butterfly does increase.  If the fuel pet thingy vacuum line is on the engine side of the butterfly...and if the diaphragm is going bad and allowing fuel down this path...the engine could also stall from being too rich.  The sound of this would most likely be noticeable, as the engine will "burble" rather than the noise most commonly associated with the banging and backfiring of a lean mixture.

Don't know if this helps. When I first got the 06 S40.
Used to, 'race', with a buddy on a HD.
(Within Speed Limit Of Course)
From Stop light to Stoplight.   ALWAYS beat him.
(However on the open road, I would get to 60, then he would, blow MY doors off)
When I suddenly backed off throttle, at the light, most of the time the motor would kill.

Now it doesn't do that, (at least last fall)

The only thing that changed, was, the PO, used cheep Ethanol gas, I use, Non-Oxy gas, and after a couple of tank fulls, that, 'motor stopping, after sudden throttle down and breaking', stopped.

When it did that, the only thing I did, before, was raise the idle to about 1050.  But it didn't stop, until I had run 2-3 tank fulls of good gas through.  Raptor and tweaking the carb, did after the, 'fast idle down motor stopping', stopped.

So I am thinking, running the Non-Oxy gas, 'cleaned' the carb.

Dave wrote:
Quote:


If that's the case - and I've got no reason to believe its not - then I wonder if running non E gas might reduce the tendency of a completely stock S40 to backfire on roll off and shut down, since those characteristics seem to be blamed in part on the excessively lean factory jetting

I'm going to experiment as soon as the weather here warms up enough to ride. I've just located a station near me that sells 91 octane non-E gas. It's a little more expensive than the 87 octane E-10 that I've been using but not by a lot.

Except for turning the mixture screw out about a half turn from where it was when I removed the plug, the carb on my '06 hasn't been touched. Stock exhaust. The bike runs fine but it does pop if I snap the throttle shut abruptly ( I usually try to roll of slowly) and it afterfires a bit when I shut it down most of the time.

Now that I can get the Non E gas without searching for it I'm curious to see what difference it makes. I'll run it this coming summer and see what happens.