Fresh fuel , check pump , check carbs and pure some directly inside them
Spark - remove plug and test if it sparkles when staring the engine. IF no spark or low noise , then it is not strong enough . Also try with good baterry as low voltage might cause such problems.
Timing , if engine turns slowly or with difficulty although battery is new , then probably the distributor is off and give wrong timing causing problems (my current status with marelli). Open distributor cap , clean rotor and check pulse cable.
-If all above are ok , check compression ..... Not easy to happen but can occur if cylinder were dry from oil film and you tried to start it without usual preparations(like inserting small amount of oil inside cylinders and turning engine by hand with tool in order to lubricate. This technique is used ion engines stored for long time without prio maintainance....
Sprint 1.7 '85