Quick tuning question

[pablohoney]

Hi There!
I just finished rebuilding my 1979 XS650 Special and put on straight pipes and pod filters, BS38 carbs and I just wanted to know the tuning protocol. Start it up and let it idle then check the plugs. If it is rich or lean what do I adjust? The pilot jet or Mixture screw. I don't have a tach so how do I adjust the screw? Also for the main jet etc. Thanks a bunch!

Taylor

 

[grizld1]

Taylor, you've been around too long not to know where the Carb Guide is--there's a link in the tech references. It's going to be a damsight less work for you to read it than it is to write the whole thing out again just for you. And forget about trying to tune by pulling the plugs and looking at them every 30 seconds, you'll just wear out the threads in the head. You have to tune by what the motor tells you to do.

 

[kopcicle]

Take a #2 phillips screwdriver, heat the tip with a torch until cherry red and plunge it into your eye. It will hurt less. Really. Bikes with CVs have airboxes. Bikes with slide needle carbs have pods. Even Mark Dobeck, former owner of Dynajet, cops to a loss of midrange using his kits.

Anything that makes a machine more difficult to service GOES!

And for the other eye heat a 3/4" drill bit chucked up in an air drill ...

I can get BS38's to almost work on the 650-700-750 with one serious caveat . They will pull hard with the throttle arbitrarily pinned OR they will mix fuel cleanly at part throttle cruise . Pick one I don't know how to give you both . Mark Dobeck and I went back and forth for most of a summer getting several of my customers bikes and my 998 (kaw) urban terror two wheeler running right . The customers are probably still pissed even though I neither recommended nor supplied this serious impediment to performance . In the end I knew neither was right (thanx for the flash runs Ron !) But the one that was the best and closest to right got me the most grief . That guy thinks I'm an idiot and in ten years hasn't wasted an opportunity to say so . Funny that the last two guys to touch it asked how I was able to get it so close . The one that isn't so good , that guy thinks I'm magic .

just a partial list

various attempts at
springs
needles
needle jets with and without discharge shields
pilots
mains
main air bleed
air horns within the air filters
closer to or further away from the head
adjustable cam sprockets
different headers ....

All because in one case the owner wanted to look like all the other cool kids
and the other thought it was a pain to get the carbs in and out with the stock aribox

coincidently both of these mental midgets went from 45-52 mpg to 30-31mpg and might have gained a couple of hp on top at the expense of having me spend untold hours filling a huge hole around 4k .

So I was asked recently if I would jet another for "pods" ( is it that hard to say individual air filters ? really?) good thing I kept my notes .

~kop

 

[pablohoney]

Grizld1:
Sorry man, I checked the tech section the before but missed it, found it now.

Kop:
I did the eye things but it didn't help, I will continue tuning.

Thanks

 

[kopcicle]

I'll be as specific as I dare as application is everything and in going too far carbs are easily ruined or at least need difficult repairs to be functional again .

The increased pressure differential with the airbox not installed tends to raise the slide earlier and expose more needle jet discharge than with the airbox . Because of differences in BS34-36-38 you will have to correctly identify the main air bleed in the carb entry bell and carefully ream and exhaustively test . This one modification will promote better metering of slightly leaner overall main needle/needle jet discharge . This also allows you to use a larger main jet with out sending the top end into overly rich breakup .

A deflector placed in front of the needle jet discharge aided in early transition from idle to main and allowed a generally smaller main jet . I used brass sheet silver soldered into a groove . It stood no more than 1/8" high and 3/16" wide when finished . I made the groove because I was afraid that just butt soldering/brazing might allow the piece to be ingested . I did this by hand with a dremel . The best deflector was a piece of fine screen soldered to the front of the needle jet . I didn't leave it there because of my fear that it too would be ingested . It seems that additional pressure drop as well as turbulence aided in the transition to midrange operation .

The velocity stacks I mentioned were 17 degrees and only extended 2 1/4" into the filter . I used about the largest tapered oval that will still carry the appropriate opening and lucked out as the close out part had an angled entry allowing a better fit to the chassis . The stacks were slit and pressed on to the carbs using the air filter retaining clamps to hold both the filter and stack in place . The bulk of the the volume was below the stack so the entry was cut at an angle and given approximately a 5/16" radius .

I did experiment with connecting not only the vacuum barbs on a twin but connecting the diaphragm covers with a section of hard 5/16" fuel line and fittings . It was something that we did on dual 4 barrel , vacuum secondary carb setups back in the day . I have no experimental or anecdotal opinion as to whether either helped or hindered .

hope it helps

~kop