Again, acceleration tests showed a significant improvement.

Second Gear 4K to 7K: 3.59 seconds            Improvement:       0.11 seconds

Third Gear 4K to 6.5K: 6.76 seconds            Improvement:       0.80 seconds

Fifth Gear 3.5K to 5K:       6.15 seconds            Improvement:       0.76 seconds

Fifth Gear 3.5K to 5.5K: 9.61 seconds      Improvement:       2.03 seconds
     
It still maxes out at 5800 rpm in fifth gear, but it gets there a lot faster.  The low restriction muffler is helping a lot.  The 5th gear pass simulation is excellent.  The sound is sweet.

Fuel mileage went up significantly.  I averaged 65 mpg over several tanks of gas.  Since the jetting is richer, I have to assume that the improved fuel economy is the result of reduced pumping loss (i.e less back pressure).

It’s a simple matter to pop in the K&N.  It fits perfect.  One screw and your good to go.

Since this part of the project is focused on the exhaust system, we might as well look at the header pipe.

The stock header pipe is double wall.  The inside diameter is small for a 40 cubic inch engine.  On the left is a 1.3” ID stock header pipe compared to a 1.79” ID Mac header pipe on the right.

IMO, the stock header is too small while the Mac header is too big.  The perfect size should be about 1.62” ID.  How much difference do these header pipes make?  Let’s find out.  We know that the current setup with a stock header pipe runs really good.  If we install the Mac header with the LCGP muffler we can compare the two and see which one runs better.

Off with the stock header and on with the Mac.  The switcheroo took about 30 minutes.  No more special kinked adapter for this installation.  The muffler just slips right onto that big pipe.

This completes the exhaust system portion of the project.  I may revisit the exhaust system as the project moves forward.  Let’s take a look at the results so far.
 
We have improved the overall performance of the motorcycle.  Carburetion is better.  Throttle response has been improved.  Fuel economy is up by 8 mpg.  Afterfire and backfiring have been eliminated.  Audible noise has increased a modest amount, but the quality of the exhaust note is more pleasing to the ear.  All-out 3rd gear acceleration has improved by 3.95 seconds, and 5th gear passing acceleration has improved by 3.75 seconds (13.24 seconds if you run it all the way up to 5500 rpm).  It really is like a different motorcycle.

I think that’s pretty good for such a small investment in time and money.  Most of us could accomplish these modifications for under $200 bucks.
 
Next, I want to move on to the carburetor.  The stock carburetor’s fuel delivery is grossly restricted by the stock needle.  Even with the compound taper, it’s tough to maintain a safe air/fuel ratio.  As I mentioned in Part 2, simple measurements and calculations show that the stock needle limits the maximum main jet to about a #155.  Jets larger than #155 don’t have much effect on the air/fuel ratio at higher rpm.  If I want to amp up the air flow through the engine, I will need a better carburetor, or at least figure out a better way to set up the stock carb. It’s tough when there isn’t a full selection of readily available parts for tuning.

At this point, I have done 105 WOT acceleration pulls to dial in the different combinations.  The old girl is holdin up good.  No leaks.  No ugly noises.  Not using any oil.  Zero oil accumulating in the airbox.  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