A combination of compression, fuel injector wear, fuel pump wear and assuming you're talking about engines with mechanical fuel pumps, possibly timing.


We have 3 old tractors with the same engine.   #1 starts really well in cold weather, #2 starts pretty good but not as good and #3 if you're not wearing shorts its gonna need a little shot of ether.

#1 has the least hours, #2 has the most hours and #3 is in the middle.   #3 has been a hard starting tractor forever.