Install a simple mudflap on the rear fender to help keep the dirt and water off the filter.  There’s never any crud in my induction system, so I think the mudflap and filter are doing a good job.

Once installed on the motorcycle, the cylindrical air filter looks professional (IMO).

Right off the bat I could tell the difference.  Now it would pull past 7000 in third gear (all the way to 7300).  After a few test & tunes, it was time to see how it did against the stopwatch.  I achieved best performance with a #35 pilot jet, #142 main jet, and the needle clip in the fourth groove (rich).

Second Gear 4K to 7K: 3.24 seconds            0.42 seconds faster than flat-panel

Third Gear 4K to 6.5K: 5.77 seconds            0.87 seconds faster than flat-panel

Third Gear 4K to 7K: 8.85 seconds

Fifth Gear 3.5K to 5K: 5.65 seconds            1.36 seconds faster than flat-panel

Fifth Gear 3.5K to 5.5K: 8.70 seconds      1.56 seconds faster than flat-panel

Time to reinstall the PWK and try it with the Mac header.  That was a simple operation, no surprises.  The bigger header along with the increased air flow worked great.  Jetting remained the same.  Pilot jet #35, main jet #142, slide needle clip fourth groove.

The stock engine with K&N 3x5 cylindrical air filter, PWK carb, Mac exhaust header, and LCGP high flow muffler ran:

Second Gear 4K to 7K: 3.13 seconds            0.11 seconds faster than stock header

Third Gear 4K to 6.5K: 5.58 seconds            0.19 seconds faster than stock header

Third Gear 4K to 7K: 8.20 seconds            0.65 seconds faster than stock header

Fifth Gear 3.5K to 5K: 5.48 seconds            0.17 seconds faster than stock header

Fifth Gear 3.5K to 5.5K: 8.29 seconds      0.41 seconds faster than stock header

It maxed out at 6200 rpm in fifth gear (200-rpm increase).  It pulled 7400 rpm in third gear (200-rpm increase).  Based on the acceleration times, it’s obvious that the larger carburetor combined with the larger header improves overall performance.  

This completes the carburetor portion of the project.  All it took was a little more air filter to get the budget mixer up to speed.  I think $28 bucks for the carburetor was $28 bucks well spent.  The jetting is dialed in.  That budget carb is so sweet I ordered a 36mm to play around with.  I’m curious how the two stack up.  These carbs might be an excellent solution for the folks who have a mixer that just spent the last two years with fuel in the tank and the petcock left on.  One of those beauties that look like the LaBrea Tar Pits.

Let’s take a look at the results using test data from the stock header setup (keeping the project simple &  cheap).  The setup includes a completely stock engine, PWK38 carb, K&N RD-0710 air filter, modified airbox, catch can, stock exhaust header, LCGP 2.5” resonator with baffle.

We have improved the overall performance of the motorcycle.  Carburetion is excellent but fuel economy is down by 9.5 mpg.  Backfiring has been eliminated.  Audible noise has increased a modest amount, but the quality of the exhaust note is more pleasing to the ear.  Compared to the box-stock motorcycle, third gear acceleration has improved by 4.71 seconds, and fifth gear passing acceleration has improved by 3.82 seconds (13.73 seconds if you run it all the way up to 5500 rpm).

I think that’s pretty good for such a small investment in time and money.  We haven’t done any major work; the engine hasn’t been opened up.  Most of us could accomplish these simple modifications for under $350 bucks.  The tests show that if you throw on a bigger header it will be a bit faster with no appreciable loss of grunt.

All the easy stuff is done.  We have picked all the low hanging fruit and kept a close eye on our bank account.    Now it’s time to start doing some open-heart surgery.  As I recall, the general consensus was camshaft next.  Part 5 will install a DR650 cam.

At this point, I have done 153 WOT acceleration pulls to dial in the different combinations.  There’s a little over 2000 miles on the engine since it was restored to box-stock configuration.  The old girl is holdin up good.  No leaks.  No ugly noises.  Not using any oil.  Zero oil accumulating in the airbox or catch-can.  It’s a solid motor.

I hope some of you find this project informative and can use the data I collect to help make decisions on your own project.  If you have suggestions or comments on my test methods, post a reply so we can discuss.   As mentioned earlier, if you have a particular component or modification that you would like to see included, let me know and we can collaborate.
 
Best regards, Mike          

I wanted to alert everyone about a problem I had with the 2" ABS elbow used to connect the air filter to the carb boot.   I've been testing a new engine and went to make an adjustment on the carb.  While removing the air filter I noticed a big crack in the ABS elbow.  The elbow was essentially crumbling in my hand.

This elbow is not under any stress, and it doesn't get hot.  I've been running it for a long time (since about December of 2019).  I guess the fuel vapors eventually break down the ABS plastic.  I picked up an ABS replacement just to keep my test program going and ordered a PVC 2" street elbow.  I have a sample PVC fitting soaking in gasoline to see how the PVC holds up.

Anyone else have any trouble with ABS?  Anyone else tried PVC?

If a piece of the PVC made it past the Venturi, it would be a bad day

Absolutely correct Gary.  A chunk of that ABS through the carb and it would have been a bad day.  Fortunately, all the pieces stayed in the elbow.  It fell apart when I went to remove it.

The PVC test specimen is still in the gas.  I haven't pulled it out but it looks OK.  Eegore, do you think PVC (white) will hold up better than the ABS?

Jdvt, thanks for the input.  Please let us know how your pipe holds up.  One thing I noticed on my elbow was a slight oily residue in the area where it failed.  I had been seeing that residue for several months but there was no crack that I know of.  It just seemed to all-of-a-sudden fail.