How does one check the idle rpm on the S40?

If you've got a watch or clock that shows seconds you can get a pretty good idea by counting the exhaust notes per second. Do it three or four times and see what the average is.  For each exhaust note you hear at idle, two revolutions have occurred.  You should be able to hear 9-10 blips per second.  More than 10 will be too hard to count but you'll already know by the sound it's too fast.  5 to 6 exhaust notes per second is easy to count and you'll know for certain it's way too low.  For practice, place your hand on a flat surface and try tapping your index finger as fast as you can while counting the number per second.  Most people can do it up to about 6-7 times per second.  Your exhaust note needs to be a little faster than you can tap your finger.  Here is the math for counting the exhaust blips:

6 blips per second x 2 revolutions per second X 60 = 720 rpms.

7 blips per second x 2 revolutions per second X 60 = 840 rpms.

8 blips per second x 2 revolutions per second X 60 = 960 rpms.

9 blips per second x 2 revolutions per second X 60 = 1,080 rpms.

10 blips per second x 2 revolutions per second X 60 = 1,200 rpms.

If someone has a good tach and can record and post audio files, they should record the sound of rpms at 800, 900, 1000, 1100 and 1200 rpms for reference.

That's some great info.  I've never seen that post yet.  Man, I've got to go get the stuff to convert video to digital and post.  My exhaust doesn't really sound like any of those.  It's maybe a blend blend between the Raask and the classic HD sound.  Before the new exhaust nobody ever mistook my bike for a sportster.  Now it happens about every third time I ride.  I guess cause of the sound.  The usual first words out of their mouth is, "Where's the oil tank".  

Anyhow, great info on that thread.  It needs to be in the Tech Section TOC. Maybe it already is and I just missed it.

There have been times, maybe weeks between carb adjustments, where I'd find myself blipping the throttle during decel just at the point where engine rpm reaches idle speed.  I guess it sort of became a minor habit as it would sometimes drop a bit too low for a sec and return to normal idle.  To a point I think that is normal for a carbeurated engine but obviously stalling or stumbling is not.   The Turbinator seemed to improve this, meaning the engine decells (sp?) down to 1100 and smoothly stopped there versus dropping to 800 for a sec and then back up to 1100.  I guess the Turbinator reduces turbulence which in turn provides for a better vacuum effect at low rpm.  That improved vacuum effect means more air in the cylinder and more fuel pulled up through the jets resulting in decel stopping at 1100 and not dropping like it did.  I do know that my mixture screw sweet spot is different with the turbinator in than with the same tube/opening minus the turbinator.  If it sounds confusing, well, I'm still learning about it myself.  Mine was tougher to adjust because of the free flowing exhaust.  The exhaust back pressure plays some role in how fast an engine decelerates.  The back pressure acts sort of like an air cushion.  The slower the return to idle, the greater the resistance in the exhaust.  Listen to the muffler sound files in the Tech Section.  It sounds like the different recordings have slightly different rates of return to idle when the throttle it let go.  Mine drops faster in comparison and therefore tougher to adjust for.  The turbinator seemed to fix what I wasn't quite able to get just right through carb adjustments.  

Edit: Oops.  Turbulator, not Turbinator.

Wolf wrote on 03/23/09 at 23:31:03:


Oops.  Turbulator actually.  Verslagen named it that. Cool name IMO.  


A Turbinator is the Arabic version of:

A ball park on the safe side way - is to watch the healight - if it dims any at idle - you're  too low -