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The distributor must be installed with the #1 cylinder on TDC compression stroke( both valves closed). The rotor tip must line up with the arrow on the distributor housing with a #6 on it.
Is the distributor installed in this position??
These are not easy to install the distributor correctly the first time if you have not performed this operation on a 4.3 V6 engine vortec. There are two marks on the balancer and a V mark on the timing cover to indicate TDC. One mark will be at 1:00 and the other will be at 4:00. The upper mark has to line up with the V mark on the timing cover
Verify this is installed correctly , and let me know. Also verify plug wire installation. Numbers on cap should match wires from cylinders.
The mark at the 1;00 position is the TDC mark you need to use. Then the rotor should line up with the arrow on the distributor housing like the picture shows.
There is no adjustment that is possible for the distributor. If it is off only one tooth it will start but will set a trouble code for cam /crank correlation.
If it is off by more than one tooth it will not start, and will possibly kick back while starting.
If you used the lower mARK( 4;00) position to time this distributor, then this is your problem why it is not starting and kicking back.
Like I said earlier, this is not an easy procedure if you have not done it before.
1) verify the correct ark is used and the #1 cylinder is at TDC compression stroke.
( remove the sparkplug to verify piston is at TDC if you have to) ( Remove the left valve cover to verify both the intake and exhaust valves are closed on cylinder #1 , if you have to to verify compression stroke)
Then make sure the rotor is pointing at the arrow on the distributor housing.
NOTE: the #6 on the arrow is not for #6 cylinder. It is to identify the distributor housing is for a six cylinder engine.
The arrow is the correct position for #1 cylinder firing position.