Motorcycle Repair: 1977 DT100, starting, dies

Question
hello i have a 1977 Yamaha dt100 and it is not firing it has a alright spark and good compression the carb has also been fully cleaned and fuel lines are not blocked but i cant think of anything else if you have any ideas on anything else i could try that would be a great help

Matthew Humphreys

Answer
Hi Matthew,
It's difficult to pin it down with just a bit of information. Try a new spark plug if you haven't yet, (you probably have already). Many factors can affect starting; ignition timing, air/fuel delivery and exhaust restriction are possibilities that are fairly easy to check. (If the engine has been apart; top end repair, transmission, alternator/ignition/wiring work the problem may be related to previous repairs). If possible contact the previous owner, ask them about any repairs.
Remove the exhaust system and see if this affects your starting; don't run it except for a few minutes and at low RPM (idle) with exhaust system off, as it can be hard on your ears and the engine otherwise.
1. Ensure the timing mark on the flywheel lines up properly with the mark on the crankcase, these will be shown in the repair manual.
2. Ensure the stator/field coils, pick up coil/trigger and mounting plate are secured and at the correct alignment on the crankcase. The condition of the wiring is important also, I trust you have already inspected and corrected any issues you have observed.
If you are certain the issue is not fuel related, the compression is at least 100 psi (correct value {psi} will be stated in manual) it will start if the ignition and port timing are correct and you have good seals. (This is assuming the crankcase seals are good).
All these things have to be reasonably close to run:
Correct mixture of air/fuel.
The air filter/exhaust breathing freely.
Crankcase seals not leaking, good compression, port timing and spark).
Best wishes,
:) Terry.