This is the second in a series of reports intended to document the results of progressive modifications to the LS650 engine.  Part 1 outlined the project, set the rules, and established the baseline performance values for a box-stock LS650.  Part 2 examines the airbox, air filter, and stock carburetor jetting.  The carburetor jetting is included because almost any significant alteration to the airbox requires a jetting adjustment.

An internal combustion engine needs air to produce power.  Most modifications intended to increase the power output of an engine are intended to get more air into and out of the cylinder.  High performance air filters, larger carburetors, velocity stacks, high performance camshafts, ported cylinder heads, headers, and free-flowing exhaust systems are all focused on moving air and combustion products through the engine.
 
Superchargers and turbo chargers have one function, cram more air into the cylinder.
       
Larger displacement engines generally make more power.  A bigger engine can induct more air on each intake stroke.  Hmmmm!  More displacement = more air.

It’s all about the air.  Additional air permits the introduction of additional fuel.  It’s an unbeatable combination.  If you can get more air into a cylinder, then you can add more fuel.
 
Seems to me the logical place to look for more air is where it first enters the powerplant, the airbox.

What I am about to describe worked good for me.  If you decide to try this stuff on your own, you assume responsibility for the outcome.  If you don’t have the skills, don’t do it.  If you don’t understand something, STOP and get help.  Get a manual.  Read up.  Comply with ALL the safety requirements outlined in the manual.  Make sure you know what you are doing before attempting any of this stuff.

Let’s get started.

If you remove this decorative side cover (I call it a “tin”),

and you have access to the air filter element.

If you want to see if your bike will run better, just leave that plastic airbox cover off.  Put the tin back in place.  Give it a whirl, but be careful, it’s gonna run lean.  Don’t go wide open throttle (WOT) in any gear higher than 3rd.  With the cover removed, my 3rd gear acceleration time dropped by a little over 1-second (10.48 to 9.35).  Don’t run your bike like this for anything more than a quick test.  It will be too lean. You can hurt it if you get carried away.

So, a simple test reveals that there’s some significant performance lurking in that airbox.  I contend that the only modification necessary at this point is to leave the plastic cover off, put the tin back on, and rejet the carburetor.

The tin provides adequate weather protection.  You can’t turn a hose on the thing, but under normal pleasure riding conditions everything works fine.   I’ve spent about 2 years riding around without the airbox cover.  No problem whatsoever.

The only parts needed are #150 main jet, a #55 pilot jet, and a suitable washer (.27” OD x .12” ID x .06” thick).   The washer is used to raise the slide needle.

Remove the float bowl.  I suggest you convert the float bowl screws from pan head to socket head.  These are M5 x 0.8 pitch x 15mm long stainless-steel socket head cap screws.

Change the main jet.  A #150 works good.

The pilot jet is optional, but I prefer the #55.  It’s a bit fat, but afterfire is greatly diminished. Overall response and cruise are better too.  Once the jets have been changed, reinstall the float bowl.

Carefully remove the spring.