Specifications

Post 8: Tuning for Modifications
There are several jetting guidelines out there already, most notably the tips on the
MikesXS site and the "Minton Mods" article. In this section, I'll attempt to expand on
those, plug any holes they've left, and correct any inaccuracies I've found.
Some important notes up front. Every one of these 650s seems to respond differently to
mods and the jetting changes required for them. You must base your jetting on what
was stock for your year carb set, not the year of the bike. Many of these bikes have had
different year carb sets swapped onto them. The jetting specs were changed many
times over the model run. Also check the jetting you have as it may have already been
fooled with. CV carbs are very easy to over-jet. The slide lifts by vacuum and only as
much as the engine demands or needs. For this reason, you can have way too big of a
main and the bike will still seem to run halfway decent. The slide isn't lifting fully so that
large main isn't flowing at its max rate.
Now for some critique of the old standards. The "Minton Mods" deals with a '78 Special
so his jetting guidelines really only apply to the '78-'79 carb sets. Also, he makes no
mention of changing pilot jets, which these bikes usually need, especially if you drop
your needle all the way to its lowest position like he did. I guess he didn't mind that huge
flat spot he must have created just off idle .
The MikesXS Tips for the BS38s seem to mirror Joe Minton's as far as main jet size
goes. A 140 to 145 main is usually fine for a '78-'79 carb set which came with a 135 as
stock but would probably be too rich for the earlier carb sets. For instance, the '76-'77
carb sets had a 122.5 main as stock. I doubt you'd get a minorly modded '76-'77 model
to run right with 140 to 145 mains. That's an increase of 7 to 9 sizes which is a big jump
for any bike. Mike's recommendations for the BS34s seem pretty good and I would
follow them to start. Once again, you may not need as big a main as he suggests (only
testing will tell) so don't feel you're doing something wrong if you can't run that large of a
size.
OK, let's get started, shall we? First, you must realize that the 3 main circuits of the carb
- the slow speed or idle (mix screw and pilot jet), midrange (slide, slide needle, and
needle jet), and the high speed or main (main jet) - all overlap the circuit next to them
and changes made to one will slightly effect the other. In the case of the midrange,
changes will effect both the upper part of the slow speed circuit and the lower part of the
main circuit. You are going to have to buy a few jet sizes and will end up with extras
when you're done, there's no way around this. Maybe they'll work in the next 650 you
get, though.
Here's the simple bare bones routine which I'll expand upon below. Increase your main
jet size until you incur break-up or stumbling in the upper midrange. Lower the needle to
compensate. The lower (leaner) needle setting will lean the upper low speed circuit as
well, many times creating a flat spot just off idle, and probably require a larger pilot jet.