Carb Comparison – Installation & Performance

This is a comparison test of four different carburetors.  The contenders are the stock Mikuni BS40, the Mikuni VM38, the Keihin PWK38, and the S&S Super-E.

The six categories for the comparison are installation, tuning, drivability, acceleration, fuel economy and maintenance.  I assigned a number from 1 to 4 for each category, 1 being lousy and 4 representing superb.  A perfect score is 24 and the worst possible score is 6.

Installation: Ease of installation.  Is it plug & play, or does it require extensive fabrication/modification?

Tuning: Is it close out of the box or does it require modifications and/or hours of test & tune to get it dialed in?  Are tuning parts (jets, needles, etc.) readily available?

Drivability: Throttle response, smoothness, power delivery, idle, starting, afterfire

Acceleration:  2nd gear time from 4K to 7K

Fuel Economy: How many miles per gallon can you ride under normal riding conditions?

Maintenance:  How hard is it to work on?  Is it easy to change jets?  Is it easy to remove, replace, adjust?

All four carburetors were installed on my modified 2016 LS650.  The modifications include ported and flow tested cylinder head, Web 340b camshaft, RD valve springs & titanium retainers, three-inch flywheel, Mac 1.79” head pipe with modified muffler, hi-flow airbox with K&N filter, 10:1 compression ratio (Wiseco 94mm piston), ignition timing retarded 2°.
 
Each carburetor was tested with the same engine setup, no changes were made to ignition or cam timing, intake or exhaust length, etc.  Gearing and tires were stock for all testing.  In other words, it was a level playing field.  Install the carburetor, dial in the jetting, test & evaluate.

Jetting was dialed in using an AEM Wide Band Air Fuel Ratio Meter, timed acceleration, and spark plug readings.  Air/Fuel ratios of 12.0:1 to 12.5:1 consistently correlated with the fastest acceleration.

Air flow measurements are for reference only, they weren’t used as a test criterion.

Mikuni BS40 - Stock Carburetor

The Mikuni BS40 is a constant velocity carburetor.  It utilizes a transient enrichment valve (TEV) to help mitigate afterfire (the big KABOOM when you decelerate).  There is no accelerator pump.  The BS40 has the smallest venturi, so it flows the least volume of air at WOT.  It flowed 159 cfm @ 15” H2O.

Installation 4: It’s already installed, no cable issues, fuel hose is correct, no special manifold or air filter.
 
Tuning 2: Tuning the stock carburetor for a high output engine was not as easy as I thought it would be. It ran well at idle and part throttle but had a nasty tendency to go dangerously lean at high speed (around ¾ throttle). At WOT, it would return to a safe air/fuel ratio.  It needed a compound needle.

Aside from main jets and pilot jets, there just isn’t much available for tuning the BS40.  Different size needle jets and jet needles are unobtanium.  The 5C16 needle for the early model LS650 incorporates three grooves for the clip instead of a single groove and provides a little more flexibility.   I eventually dialed it in by sanding a second taper at the end of the needle.  The compound taper on the needle combined with a 205 main jet got things just right.

Final jetting for this engine setup was: Main Jet #205, Pilot Jet #52.5, Needle Jet #X-7, Jet-Needle #5C16 with custom compound taper (clip in mid groove, no spacer or washer), Pilot Air Jet “1” #45, Pilot Air Jet “2” #230, Main Air Bleed stock (fixed .024”), Float Level 28mm, Idle Mix Screw 1.5 turns, Slide one extra air hole .107” diameter (#37 drill).

Drivability 4:  It is an excellent carburetor for a daily driver.  Starts easy, almost always on the first crank.  Quick warmup.  Great throttle response.  No hesitation.  Super-smooth cruising.  Occasional afterfire on deceleration.

Acceleration 1:  It was the slowest in the acceleration category with a 4K to 7K time of 3.30 seconds.  
 
Fuel Economy 2: Fuel economy, 56.6 mpg.  It’s a compromise.  To get the stock carb to deliver enough fuel at WOT I had to sacrifice some mid-range economy.

Maintenance 2:  Maintenance is not the easiest due to the size and complexity of the carb.  Compared to the other three carbs, it’s harder to remove, harder to clean, harder to adjust, and easier to screw up.

Total score for the Stocker is 15.

On the left is the late model 5C39 needle.  On the right, you see the early model 5C16 needle. The 5C16 is a genuine Suzuki part that I purchased from Babbitts Suzuki Parts House.  It’s for the early model (1985-95) LS650 (part number 13383-24B40).  The 5C16 is a nice addition to your tuning capabilities, but not a panacea.

This shows the 3rd hole in the slide.  It’s .107” diameter (#37).  IMO, it helps a bit with response but can also aggravate afterfire.  With the extra hole, the slide lifts faster, but it drops faster too.

See those little screws in there?  They can be tricky.  A magnetized screwdriver is a must.  If you install the retainer plate upside down it partially obstructs the air holes.  It’s also easy to pinch the rubber diaphragm or install it in the wrong orientation.  You must be careful.

There is no plug for access to the main jet.  It would be a nice feature, especially if you are running the stock battery box.

The large #230 air jet “2” (lower left in picture) is easy to mix up with the main jet.  The #230 belongs up here in the top of the carb, not in the float bowl.

More TEV parts equals more complexity, but I think it’s a good feature.

This should give you a feel for how much additional room you have with a Sportster battery installed.