I have never owned nor ridden an air-cooled bike with a carb that did NOT require manual changes to idle speed screw between cold start and warmup.  As the engine and intake and carb come up to operating temperature A/F ratio decreases (becomes richer).

If your A/F ratio was spot on when first off choke it would be rich after warmup.

If your A/F ratio was spot on after warmup it would be lean when first off choke.

Very generally speaking carbs are lean when first off choke because we set the idle mixture screw AFTER the engine is fully warmed up.  As the engine and intake and carb come up to operating temperature A/F ratio decreases (becomes richer) and idle speed increases.

Jetting for an air-cooled engine is ALWAYS a compromise as we have no control over ambient air conditions.  For every 9 degrees F we need to adjust A/F ratio buy 1% and this is not possible with such an archaic fuel delivery system.  Humidity can also play a big part in this equation as high humidity displaces available oxygen and increases A/F ratio.

In the Old Days autos with carbs used CCS (Controlled Combustion System) that kept incoming air at a static temperature in an effort to control this issue.

When I first start an air-cooled bike with a carb I give the idle speed screw somewhere between one half and one full turn in.  As the engine and intake and carb come up to operating temperature I turn the idle speed screw back out to suit target idle speed.

Obviously bikes with EFI do not suffer from this issue.


Have you adjusted your isle mixture screw for best/fast idle after warmup?

I'm surprised at how many people are willing to misadjust their carbs!!!  LOL

I'm also surprised how overanalyzed some of this stuff gets on this forum!!   Especially for a ~30 hp (claimed) lowish-performance motorcycle.

Your idle should be adjusted once the bike is warmed up.  I think the factory calls for  ~1100 rpm.  (It's not supposed to sound like a douchebaged out HD.)  The choke, or enrichment circuit, is there to aid the "low cold idle" until the bike warms up.  It doesn't mean your bike will idle perfectly while it warms up.  It might actually idle a little low even with the choke out.  The best way to fix that is to ride it.  

I suppose you can do whatever the hell you want, but I set mine and leave it alone.  It might "high hot idle" slightly on the hottest of hot days.  I don't sit in traffic on my way home from work and I shut my bike off if I get stuck at a railroad crossing, so I don't mess with it.

I'd argue that most people have no business even attempting to adjust their idle.  Suzuki did a fine job of that at the factory.  The guy I got my bike from set the idle at ~500 rpm. Probably thought it sounded cool.  

Idle war!!  

Gary_in_NJ wrote on 07/09/19 at 07:16:06:


Semantics my friend.  The Owner's Manual calls it a "choke" and any reader knows what we mean when we write "choke" just as any reader knows what we mean when we write "idle mixture screw" as we know it can be an air screw or a fuel screw.

I would also like to think any reader knows what we mean when we write TEV, Air-Cut, Coasting Enricher, etc.  They all serve the exact same purpose...

How about slow jet versus pilot jet?

Remember, black smoke is a sign of being rich.  If you get that black smoke twisting the throttle from idle, then I would agree that it is rich at idle which is flooding it out.

Now when you introduce the non-stock filter situation that adds another variable.  If your filter is too small, that limits the oxygen getting into the engine and hence you have an overly rich situation.  And it could also limit the vacuum as you stated.

Basically, I have no real answer for you other than you seem to be on the right track.  Maybe try removing the ryca filter temporarily and seeing if the additional air flow makes it run better. If so, that tells you something.