Motorcycle Repair: raptor starting problem, crank shaft, jumper cables

Question
"hello CHRIS

i have a problem with a 2001 yamaha raptor 660r i rebuild the the engine from the crank shaft up new push rod and bearing piston and rings
when the engine is cold i connect the battery to my van with jumper cables  and i crank the engine for about 15 minutes and some times it back fires out of the carbs into the airbox with and with out flames and when the exhust pipe on the bike heats up to an untouchable tempurture the bike will start and run perfect but if i let it cool down for about 3 hours same problem again i have cleaned and jetted the carbs changed the egnition coil checked the timing SOMBODY TOLD ME THE VALVES ARE TO TIGHT but i did not touch the valves or test the thoery DOES THAT MAKE SENSE FOR THE RESON IT EVENTULAY STARTS AND RUNS NICE
now i am all out of ideas . any help you have will be greatly appriciated
                     thanks in advance
                        chris kazalas
                  KAZALAS@AOL.COM


Answer
Hi Chris.
 Has the entire engine been tuned to specs in accordance with the factory repair manual.

 Since the engine has been rebuilt, then there are a lot of things to check.  Have you checked the synchro on the carbs?  A static check to see if they all pull together is the first step.  Then, once the engine is running, you can do a proper synchro.

Check the valve gap and timing.  They could be off.

You could have a leak in the new pistons.  perhaps the rings didn't seat correctly or the cylinders need to be rebored.

Check the seals of the manfolds to the engine as well as the connections on the manifolds to the carbs.  An air leak can really screw things up as well.

When you jetted the carbs, the mixture was changed.  What is the condition of the plugs after running the bike for a while?  If the plugs are fouled, then the mixture is too rich, if the plugs are white, the mixture is too lean.  ALL of the carbs must be adjusted to the exact same setting.  If you have even one carb is set so much as 1/8th of a turn away from the rest of the carbs, it will mess things up.  To make sure that the carbs are right, screw the mixture screws in to the stops (not tight, just until it contacts) and then back each one out 4 turns.  Ride the bike for about 10 minutes or so and check the plugs.  Adjust all the carbs 1/4 turn at at a time, clean and replace the plugs and ride the bike for a while again.  Recheck the plugs and adjust the carbs accordingly.  When it starts looking better, make your adjustments in 1/8th turn increments until the plugs come out looking normal.

Start with those and see what you find.

Good luck and ride safe.
FALCON