Topic: Carb tuning and listening to the cycle

[eurban]

Hey all-  I'd be interested in hearing from those who have some experience with the sounds the cycle will make when it is running lean or rich but not just necessarily extreme examples.  Things like a deepening in the exhaust note,  or the sound becoming more "burblely" etc.  Basically this is in the interest of fine tuning the carbs settings (idle mix, needle position, main jet.) I do know about plug chops but that is not what i am asking about.  Thoughts?

[TwoTired]

Assuming we are talking about mechanically operated slide carbs,  I may have a couple tips for you.

Understand that the mixture requirements vary with the load on the bike at any given RPM.  The "labored" sounds from the engine vary not only with the load but with mixture attained at that load/RPM combo.

Without a sound reference, you can get an indication of what the mixture was at a given throttle setting when opening the throttle from that position.  Assuming non-accelerator pump equipped carbs there are three response characteristics.

1.  Cracking the throttle open achieves smooth quick pick up in engine speed.  Mixture is acceptable though not necessarily ideal.
2.  Cracking the throttle open gives a burbling pickup as though some cylinders are firing every other firing cycle.  Mixture is/was too rich.
3.  Cracking the throttle open causes a wheeze almost like the igintion was turned off momentarily, and then the engine catches and picks up.  Mixture is/was too lean.

Caveate:  Mechanical slide carbs without accel pumps can't take snap open throttle from low speed without the wheeze effect.  But, should respond briskly with 1/2 throttle opening changes.

This info should be helpfull if you happen to have the carbs types referenced.  Your query didn't specify.

Cheers,

[eurban]

Overall I'm interested in the sounds and behavior of the too lean or too rich conditions for cycles in general as I will probably end up messing with carb tuning on a variety bikes.  However the project at hand is my 78750K with the stock carbs (pumpers) and pipes mixed with a 836 engine, mild cam and pods (for looks mostly).  With the stock engine, and particularly with pods the #35 idle jets always seemed to make the bike very cold blooded and a bit weezy at small thottle openings so as an experiment I drilled a set of the pressed in pilots to the aproximate size of a #40 jet.  Thats what I'm currently using with good start, and idle characteristics as well as power at small throttle openings.  The motor also pulls very strongly with rapidly whacked open thottle and in general at large thottle openings.  The mixture seems to be off in the mid throttle openings particularly in hill climbing and.  Acceleration is less strong and the engine's song deepens to sort of a "wauuuuugh" sound.  The main jet needles are the adjustable type sourced from 77 carbs.  Plug chops done after a steady state in this zone don't really show too much amis but perhaps I'm not skilled at reading the subtleties.  I figure I should make the effort to pull the carbs and move the needle clips and it sounds like a should lower the clip on the needle to lean the mixture. Thoughts?

[8 Track]

When I set my carbs too lean (keeping it vauge here) my wife notices that the seat and pegs feel more buzzy at 55-60 mph.  I don't feel this sensation through the bars, but it sounds different, like a more hollow exhaust note.  Is that what you're wondering?

Michael

[TwoTired]

The mixture seems to be off in the mid throttle openings particularly in hill climbing and.  Acceleration is less strong and the engine's song deepens to sort of a "wauuuuugh" sound.  The main jet needles are the adjustable type sourced from 77 carbs.  Plug chops done after a steady state in this zone don't really show too much amis but perhaps I'm not skilled at reading the subtleties.  I figure I should make the effort to pull the carbs and move the needle clips and it sounds like a should lower the clip on the needle to lean the mixture. Thoughts?

Given you've changed the displacement, valve timing, and air flow restriction, I can't predict what setting you should have and may well be unique for your specific set up.
However, your description does seem to support moving the needles a notch.  As to which way, you could guess.  If cleaning the plugs, cruising at midrange constant throttle for several minutes followed by a plug chop and plug examination don't tell you what you want to know.  Then trial and error needle settings are an alternative approach to tuning.  You've got a 50/50 chance for getting it right the first try, eh.   I do hope you already have the main jets sorted, as those feed the needles.  If you get the needle working right and then change the mains, you may have to go back and readjust the needles.

Best of luck!   You'll soon be the expert to respond to the next query about carb tuning, just like Bob!  And, you'll amaze yourself at just how adept you'll get at putting carbs off/on your bike...just like Bob!

Cheers,