318 runs real load and shakes like crazy

Has anything been done internally to what appears to be an old engine? EG is it possible the timing chain is out of time?

Do you know and understand number 1 is front driver's side, that the plug wires / firing order goes cw around the cap, and understand the firing order? VERY easy to get 5 and 7 switched.

I would run a compression check. Pull the valve covers and inspect pushrods and valves for movement, looseness, unwanted valve lash, etc

Since you can't seem to find the timing mark, I would make or get a piston stop and check the timing marks.

This is my old one, I still have. Made in the SEVENTIES

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Remove the no1 plug, the battery ground, and get a socket to fit the front balancer bolt. Removing all 8 plugs will make things easier. Gently rotate the engine over until it stops aka the piston against the stop. Make a temporary mark onto the balancer under TDC. Now rotate the engine backwards and repeat. You will have two temporary marks, and true TDC is halfway between.


COMPRESSION, FUEL, and SPARK

Compression means the engine has enough compression to run, that it is in good enough condition that you could expect it so, and implies the camshaft/ chain timing is OK

FUEL means FRESH fuel and just because you have a "renewed" carb does not mean it is gonna work properly

SPARK means a good hot spark and AT PROPER time.

Those three things are what it takes.