In April of 2019 I did a post titled “Cylinder Head Porting & Flow Test”.  In that report, I tested various port modifications using a home-built flow bench.  The results were encouraging.  I was able to realize as much as a 17.5% increase in intake flow, and a whopping 43.1% improvement in exhaust flow.
 
Some of the port modifications were cheap & easy to do, and others required special tooling and/or expensive parts.  One of the exhaust port mods would also require that we find a suitable filler material for the operating environment.  As of today, I have not found any sort of filler that I feel is suitable for the exhaust port.

I decided that rather than implementing all the changes at once, I would first incorporate all the stuff that was cheap & easy to do, things that most any forum member with basic mechanical skills could accomplish.  So, I came up with what I call the “Stage II Head”.
 
I break down the mods into three categories:

Stage I: A basic port clean-up.  Smoothing out the surfaces, eliminating any steps or ridges, but keeping port geometry & size stock.

Stage II:  A basic port clean-up along with improvement of the short-side radii, back-cutting the valves, raising the intake port floor, and enlargement of port cross sections in key areas.

Stage III:  All of Stage II plus installation of slightly larger intake valves, increasing the seat diameter and angle in the bowl area, and, if possible, eliminating the stepped area in the exhaust port by filling it in.

I accomplished the Stage II mods on my spare head, installed it on my motorcycle, and spent a long time testing and grooming the head.  This post provides my observations on the Stage II head.

I developed a tool to monitor improvements moving forward, but the baseline starts with the stage II head.  I have no concrete way to compare the Stage II head to the stock head.  All I will be able to share with you is my seat-o-da-pants evaluation along with vids of the performance running through the first three gears.

The baseline for the Stage II head was established by shooting video of the tachometer while accelerating WOT in second gear.  I make six pulls from about 2500 rpm through 7500 rpm.  Then I review the vids with a stopwatch, timing from 4K to 7K.  So, six pulls, each pull timed three times, for a total of 18 timing events.  I calculate the mean and the average.  If the average is too far off from the mean, I do the timing over until I am satisfied that the data is valid.  I like this method because it eliminates driving errors.  There’s no launch from a dead stop, no shifting, no reaction time.  You go WOT well below the timing point and hold WOT until well after the timing point.  If you do it in the same place under similar climate conditions, it should be a reliable tool.
 
The baseline for the Stage II head is 3.62 seconds.  That time was recorded with the Stage II head, a stock carburetor (rejetted), stock muffler (modified), Mac 1.79” ID header, DR650 cam, three-inch flywheel, stock airbox (modified), K&N air filter, stock ignition, stock ignition timing, stock compression ratio, stock gearing, stock rear tire, stock weight, 92 octane pump gas, Mobil 1 20W-50 V-Twin synthetic oil.

How does it run?  Excellent!  The engine pulls clean and strong all the way up to 7500 rpm.  It has great low-end pulling power, hits the pipe pretty good at 4000.  It’s a lot of fun to ride with this set up.  It starts right up, idles smooth, has no hint of detonation, and no bad manners whatsoever.

I installed the head at 3001 miles on the odometer.  The odometer now reads 4614 miles.  All seems well.  I inspected the epoxy at 100 miles, and it looked fine.  I inspected it again at 930 miles and, other than discoloration from fuel, it still looked fine.  The last inspection was 1400 miles and it’s holding up good.  It looks like a solid mod.  

Although the Stage II head runs well with the stock carburetor, it yearns for more air.  So, I installed a larger S&S carburetor and brought the 4K to 7K time down to 3.45 seconds.  That’s a .17 second improvement.  It really likes the bigger carb.  But I assure you, it runs darned good with the stock carb.

To give you an idea of how well the head flows, the stock carb needs a 162.5 main jet to keep up.  That was checked with a narrow band A/F meter, diligent spark plug inspections, and numerous timed runs to verify best jetting.  Here is a breakdown of the final setup for a stock carburetor.

MJ: 162.5
PJ: 52.5
AJ: 230
Jet Needle: 5C39 (stock) with .060” washer
Needle Jet:  766M X-7 (stock)
Float level: 25.5mm
Mixture screw: 1.5 turns
Slide: 1 additional vacuum port (#36 drill)

I ultimately put a small compound taper on the needle, but it really doesn’t need it.  It was just icing on the cake.  If I didn’t have the A/F meter I doubt I would have messed with the needle.

One item I forgot to mention was the compression test.  

Back when I installed the DR cam (like around 1000 miles on the odometer) I checked compression before and after the install.  It was 155 psi.

Then, just before I tore it down to do the Stage II head, I checked compression and it was down to 145 psi.  I was bummed that I lost 10 psi.

Then, after I installed the Stage II head, I checked compression again and it had increased to 165 psi.  I was gratified.  Assumed that the increase was a result of better head flow.

Today, I started prepping for the next modification, and naturally I started by checking the compression.  It's now a solid 175 psi.  I have to assume that the increase is a result of the fixed venturi carb.  With the CV carb the vacuum operated slide obstructs air flow during cranking.  Not the case with the old-school carb.

Anybody have any experience with carburetor impact on compression readings?

The engine is out of the bike and ready for surgery.