Timing Order Question

Yep it will buck sound like vacuum leaks, idle like junk, if it stays running.

Firing order for a stock mustang engine 1-3-7-2-6-5-4-8

4-8
3-7
2-6
1-5

Looking at the motor from the front of the car ^

On the Distributor, #1 is where it starts, going counterclockwise use the firing order above. I know there is another firing order more associated with the 351's but i don't have it memorized.

Good Luck

This doesn’t prove that the block is a HO block. Some trucks evidently use a HO firing order
with a low lift cam. However, it will definitely prove that a block can’t be HO because the firing order is wrong.

Remove the #1 & #3 spark plugs. Put your finger in #1 spark plug hole. Crank the engine over until you feel compression on #1 cylinder. Slowly turn the engine until the TDC mark and the timing pointer line up. Mark TDC on the balancer with chalk or paint. Put your finger in #3 spark plug hole and crank the engine 90 degrees. You should feel pressure trying to blow past your finger. If you do not feel pressure, repeat the process again. If you feel pressure, it is a HO engine.

No pressure the second time, remove spark plug #5. Put your finger in #1 spark plug hole. Crank the engine over until you feel compression on #1 cylinder. Put your finger in #5 spark plug hole and crank the engine 90 degrees. If you feel pressure now, the engine is not a HO model, no matter what it says on the engine.

Using a small carpenter or machinist square to mark the harmonic balancer off into 90 degree sections may be helpful here.

A 15/16 deep socket & breaker bar or ratchet may be used to turn the engine.

The HO firing order is 1-3-7-2-6-5-4-8.
Non HO firing order is 1-5-4-2-6-3-7-8

Do you know the history of the engine? Car?

Just to add to what jrichker said, the cam determines the firing order. If you follow his instructions and determine that it is the non HO order, this only proves you have a non HO cam.

Actually, if are currently using a firing order that doesn't match your cam, I'm surprised it drives at all. When I had the motor in the 67 rebuilt, the cam I used had the 351/HO firing order. The machine shop failed to tell me this. Long story short, the engine would only idle, very very rough at that. As soon I put it into drive, it would stall.

Same here. ^^ Then I swapped my cam and everything worked!

Hope all this information helps.

Something else to add:

If the computers injector firing order does not match the cam installed, then yes, you will have power/drivability problems.

The injectors are opening in the HO sequence: 1-3-7-2-6-5-4-8
But, the spark plugs are firing in the non HO sequence: 1-5-4-2-6-3-7-8