Motorcycle Repair: 73 scrambler, keyster carb kit, scrambler 350

Question
I am working on a 73 Scrambler 350.  Problem being, it initially fires right up and runs great!  However after running for a while and warming up it seems to choke out and die and does not want to run.  Even with the air cleaners removed it does not want to run.  I have checked the timing with a light and have cleaned the carbs.  I have tried to synch the carbs after it warms up like the specs say but once it warms up it just plain runs terrible and seems to have no power.  Initial start up--plenty of power and runs great however.  Any ideas of where I need to go?

Thank you!

Answer
Jared, if the bike fires up w/o choke, then you already have a problem. Normally, these bikes are cold blooded and need full choke to start and about 3/4 to run for the first few minutes. Unless you live in FLA during the summer, choke is pretty much always needed to coax them awake. 1973 was the last year for 350s and there were a number of carburetors and settings for them through the years. If your bike has the original set, then the specs should be 26 mm float level, #68 or #70 primary jets, #100/ 105 main jets, #35 idle jets and mixture screws out about 3/4 to 1 1/8 turns. If you overhauled the carbs with a KEYSTER carb kit, there may be issues with the calibration of the kit. The carbs have numbers stamped on the carb bodies. Yours should probably be stamped 3D, 722A, 726A or 728A. If the carbs do not have these numbers, they may be the wrong set for your bike.

Early on there were some reports that the condensers were not properly grounded to the chassis due to some paint left underneath the mounting points. Check the condenser for a nice tight fit and all leads where they connect to the coils/points. If the points are nice and clean, gapped to about .012"-.014", set to open at the F and LF marks, then do check the mechanical spark advancer behind the point plate. If it is sticking or frozen up the ignition timing will not be correct and the bike will run horribly.

Watch the points with the cover off and the bike running. IF there is a lot of arcing on one set, they are either dirty, the condenser lead connection is loose or the condenser is failing.
You could also have spark plug cap failures.. resistance values should be about 5K ohm. Make sure the leads going into the caps are clean and tightly fitted.

Bill Silver