How do i adjust a S&S super B carb, cold start mostly
The Super B carb is not as forgiving of improper jetting as the Super E model carb due to the lack of an accelerator pump. The only adjustment you have on the carb is the idle mixture screw and idle stop screw. The idle stop screw simply adjusts the idle speed of the engine. The idle mixture screw should be adjusted somewhere between 3/4 and 1 1/4 turn out from the bottom. If it is out of this range, the carb is not jetted correctly.
Through experience over the years I've found that the Super B used on a Big Twin Harley should be jetted 29.5 on the slow jet with a 70 or 72 on the main jet. If used on an Ironhead Sportster, slow jet of 26 with either a 66 or 68 main jet.
The way to tune the carb slow speed circuit is to allow the engine to warm up and adjust the idle mixture screw one turn off the bottom. Then try to rev the engine. If the engine does not rev cleanly, try coming out a half turn on the idle mixture. If it seems to help, continue until it revs cleanly. If this is the case, you slow jetting is too lean, install a richer jet and repeat the above procedure. Once you get it tuned to where the engine revs cleanly with the idle mixture screw at one turn out, you low speed jetting is correct.
The high speed jetting is not quite as simple to set as this has to be done on the road. I usually find a long uphill run to test the jetting to keep top speeds reasonable. At about 55-60 miles per hour, roll the throttle on in high gear. Feel the way the engine pulls. If it pulls good, back off on the throttle about 1/8th turn. If it pulls better, it's just slightly lean. If when you roll the throttle on, the engine starts spitting back through the carb and doesn't run well, you are way lean. Either way move upwards on your jet size one step at the time and retest.
If the bike seems sluggish and blows black smoke when you do the "roll-on", you're too rich. Install leaner jet one step at a time.
Now, most people confuse a "mid range stumble" with a carb problem. All efforts to tune this stumble out done on the carb are futile. The problem is in the exhaust system. Most bikes do not like open drag pipes for street use. They will exhibit this mid range stumble. The remedy is to put a bolt with three nuts on it on the inside of the pipe or any other means to create just a bit of backpressure on the exhaust. This usually cures the problem or at least makes it easier to live with.
Full tuning procedures can be downloaded on the S & S site. Go to "Technical" page, then "Installation instructions" to download the full paperwork that came with your Super B.
I hope this helps and Good Luck.
Feb 03, 2010 |
1993 Harley Davidson FXR Super Glide