Specifications

Post 9: Tips and Troubleshooting
Once you get close on your jetting, changes aren't going to have a giant effect. The
effects will be more subtle and a quick test run may not show all the flaws. You're going
to have to run the bike that way for awhile and put some miles on it, maybe as much as
50 to 100. During this time, run it hard and gentle, fast and slow. Basically, run it under
all different conditions and see if any little
glitches show up.
Final Main Jet Selection
Due to the nature of the CV carb and the way it can mask over not-quite-right jetting,
you will probably find a series of consecutive main jet sizes (maybe 2 or 3) that all seem
to work. They'll all pull clean through the upper RPMs but the larger sizes will probably
cause more upper midrange break-up. Even if you can clean the midrange up through
needle tuning, your main may still be too large. Remember, we're looking for the size
that runs clean AND pulls the hardest. Run some tests with the size you've chosen.
From about 3.5 to 4K, grab a big handful of throttle and run her up to near redline. At
about 5K or so, you should feel a surge as the main starts kicking in. From there, the
motor should pretty much just zip right up to redline. Note how hard it pulls and how fast
the RPMs rise. Now try the next size smaller main and run the same test. If it revs faster
and pulls harder, your original choice was too large. If it runs slower, doesn't pull as
hard or rev as fast, then your first choice may have been right. To insure it is, now try
the size above it. This is one of the first rules of jetting, when you think you're right, try
the size above and below to make sure. Follow this rule and, eventually you will find the
right main jet size.
The other "tell" is gas mileage. After it's all said and done, you may end up losing a mile
or 3 per gallon with your mods and larger jets. If you drop 5 or 10 MPG, that's a sign
you've over-jetted. Most of these bikes get about 50 MPG on the highway, the '80-on
models with the leaner BS34s can push 55 MPG. Now this is a tankful burnt mostly on
the open road, not around town. Local around town riding can give you low to mid 40s.
Take that into account when checking your mileage.
Needles and Needle Jets
We're kind of stuck when it comes to these as there are no "other" sizes available
except what came in the various carb sets during the production run. The 38s use a
totally different style of needle jet than the 34s so no swapping is possible there. The
34s all came with the same needle jet and needle. Now that MikesXS is carrying the
slightly richer Canadian versions of these, that is an option. The 38s used 3 different
needle jet sizes over the years. These were the leanest Z-2 in '78-'79, the richest Z-8 in
'76-'77, and an in bewteen size, the Z-6 in '74-'75. If you wanted to lean the midrange so
you could run bigger mains in the '74-'77 carbs, then you could try the Z-2 jet from the
'78-'79 carb sets. You could also try the Z-6 in the '76-'77 carbs for a slightly leaner
midrange.