ah an easy motor, i love small black chevy's! its one of two things for sure, either your distributor is out of time or your valve timing is out. the 305 is not an interference motor so you dont need to worry about running the piston into a valve. depending on weather or not its a TBI or carb motor the ignition timing may vary, best bet is to start at about 2-5 degrees (this may be pre igniting on some motors but its a good starting point) and work your way out to no more than 17 degrees. unless you got a radical cam you should be able to time your vehicle somewhere in the middle there. if you suspect valve timing thats easy to check, with a sharpee or keal mark on the distributor base or intake manifold where the number one point is on the distributor, remove the cap and turn the motor over by hand(youll need to remove the spark plus to release compression) until your at TDC. at this point the rotor should be pointing at the mark you made. if its not then your cam is out of time and you need to replace your timing chain/gear and re time your valves. another way to test is pull the valve cover on the driver side bank and watch the valves move as a buddy turns the crank by hand for you. if the valve are still open or moving once you get to TDC then your out of time. hope this helps, good luck!
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