For the new owner of a barn find classic motorcycle, there is a great temptation to put fresh gas in and attempt to start it. Occasionally this approach works, but the possible damage that can be done to a rare classic is not worth the risk. In particular, if the history of the bike is not known, the new owner must realize the bike was stored for a good reason which can include internal engine damage.
If the bike was bought with the intention of restoring it, there is no point in trying to start it too soon; part of a complete restoration will include a complete engine rebuild.
However, if the new owner feels he would like to see if the engine will start, there are some basic checks that will ensure no damage is done to the internals of an engine during the attempt.
Access to certain parts of the engine will be necessary during these checks; therefore, a good quality tool kit is essential.
First, the intake system must be inspected as animals often nest in motorcycle air boxes and even inside inlet tracts. The same applies to the exhaust system; however, the inlet will, of course, allow debris to be drawn into the engine as it is rotated, therefore it is more important to clean it out.
To rotate the engine, neither the electric start nor the kick starter should be used. Both of these items will spin the crankshaft over too quickly, and if there is a sticking valve, for instance, a piston could also be damaged. The safest method of rotating the crankshaft is via the flywheel.
A wrench applied to the flywheel (after removing a spark plug) can be used and will give the mechanic a chance to feel for any resistance (other than compression). Any undue resistance indicates internal problems and these must be investigated.
Having successfully rotated the engine by hand, the engine oil should be changed, or at least checked, before the engine is spun over either via the electric starter or the kick starter. At this point it is good practice to do a compression test.
(Note: Although manufacturers recommend conducting compression tests with a warm engine, it is often impracticable. Additionally, the difference between a warm engine's compression and a cold one is negligible).
If all of these checks prove the engine to be in relatively good order, the carburetors will almost certainly need to be thoroughly cleaned as will the fuel tank and feed pipes.
If the new owner merely wants to hear the engine run for a few seconds, a shot of WD40 into the inlet tracts will often be sufficient to fire it up briefly. However, this method does create a fire hazard if too much fluid is injected. As with all mechanical work on a classic, workshop safety must be paramount.