Tachometer - Hard starting / bad idle?  By  lain (Read 3,731 times)

I remember not long ago I adjusted the needle in the carb to be a little higher (put the clip on the 2nd from bottom out of 5 slots on the top of the needle) to help with the issue where it was bogging on takeoffs before. It seemed to help a lot then, but it was also like 10 degrees then. Today it was 45, tomorrow it's going to be 50

So, Lain - I was thinking a but more about your scoot's condition and had a thought.

You set the carb up NOT havign a tachometer and thus not knowing what the RPMs really were.  And - when you DID get a tach the normal idle speed of about 2k seemed very low to your ear.  That implies you previously had the idle set quite a bit too high.

Now - some have said here that the idle jet dominates air/fuel at low engine speeds, and the main jet is "off" until 40mph or so.  That is flat out incorrect, really.  The main jet is never really 'off" - what it does is a function of the gap between the needle you wetre adjusting and the hole in the main jet that the needle slides up and down in.  When the needle is at it's lowest point (idle) it's almost completely blocking the et since the thickest part of the needle is sticking into the jet, almost completely filling the hole.  Little fuel can get by.  but the key here is "ALMOST".

As the engine speed increases the carb lifts the needle, resulting in a thinner part of the needle obstructing the jet's hole (because the needle is TAPERED).  With the bigger clearance between the needle and the hole in the jet more fuel can get by into the airstream (and does).  And at WOT the very thinnest part of the needle is into the jet's hole, resulting in the biggest clearance and the most fuel flowing into the airstream.

Now the upper part of the needle's taper is almost nothing - about 1/2 inch or 1 inch, at the top, is about the same diameter (the taper really starts AFTER this part).  But even there the main jet comes into play "a little".

So ~~~ maybe ~~~ and I'm guessing here .... when you set the carb up before the RPMs were high enough that you were getting a bit more contribution from the main jet than is normal.  Then, when you set the a/f screw it was correct - but ONLY WITH THE HIGHER RPMs AND THE MAIN JET SPITTING A BIT OF FUEL INTO THE AIRSTREAM.  then, when you reduced the idle to where it should be - almost eliminating any contribution from the main jet - your a/f mixture was not correct.

I don't know if I've made sense here, OR if what i said is actually correct in your situation - but it MIGHT explain a bit of what has been going on for you.  The simple solution, of course, would just be to set the carb up again (though that might - MIGHT - mean a different pilot jet).

Anyway - my 2 cents.