351W Firing order and misfire - Ford Mustang Forum
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post #1 of 6 (permalink) Old 3 Weeks Ago Thread Starter
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351W Firing order and misfire

I bought a used 87 mustang with a carbureted 351w about a year ago, haven't been able to work on it for a while. I have finally been getting into it and am having a misfire issue with cylinder 1. I am not sure where the block came from but it is rebuilt with hypereutectic pistons, an unknown cam, decked gt40 heads and a Holley 670 Street avenger.

When i got the car the secondaries did not open, bad diaphragm so i rebuilt the carb and ever since rebuilding the carburetor i have been having issues getting it to run properly.

I checked the timing and its okay. I noticed that the seller had the firing order in this manner: 1-6-3-7-8-2-5-4.....I know that the firing order for the 351W should be: 1-3-7-2-6-5-4-8

When I changed the firing order to the correct one, the car didn't even start. I changed back to the initial firing order and it "fired" right up. But the misfire was still there. I pulled the wires one at a time and replaced them after I noticed if there was a change.

Using the 1-6-3-7-8-2-5-4 firing order the #1 plug wire is pulled the car idles fine. Any suggestions?

If I do have to check the distributor position what is the procedure to check if the distributor is 180 degrees off?

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post #2 of 6 (permalink) Old 3 Weeks Ago
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Here's the simple solution: First, figure out which plug is #1. Make sure you're following the Ford pattern, which is 1234 down the passenger side, and then 5678 down the driver's side. The bug rotates counterclockwise. So, figure out where the bug is pointing when you are at TDC on the compression stroke. (pulling the #1 plug will show you, as it will 'blow' when the valves are closed and piston is coming up.)


Once you're at TDC, figure out what post the bug is pointing at. That's your #1 plug wire. Go around the cap CCW with your plug wires, in 13726548 order, and you should be golden. The actual direction the bug is pointing does not really matter too much. A lot of people try to make it point at #1 cylinder, but that really doesn't matter too much. Making it so that you've got plenty of room to adjust your dizzy side to side (with the vac can pointing to the front/driver's side of the car) and getting the timing dead on is most important.

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Since it is an "unknown" cam it is reasonable to assume that it might have been ground with the 221-260-289-302W firing order. Remember that there is a lot more of the smaller engines out there that the "big" small block 351. So many cam grinders use the more common firing order.

Take the plugs out and find TDC of the #1 cylinder on the compression stroke. This is done by turning the crankshaft clockwise by hand and putting your thumb over the spark plug hole. If you don't know how to do this then look it up.

Mark the crankshaft damper at that position. With the 351W factory firing order the #3 cylinder should be in the middle of its compression stroke. If the #5 is in it's compression stroke then your cam has the more common firing order, and of course you should use that for your ignition timing.

Why did Ford set the 351W up with a different firing order than its sister blocks? I don't know the answer, probably a minor vibration issue. But in your case, it's not important because the bigger motor will run with a 302 cam just fine, as long as you set up the ignition to fire the plugs in that order.
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post #4 of 6 (permalink) Old 2 Weeks Ago
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Quote:
Originally Posted by andrew0228 View Post
I bought a used 87 mustang with a carbureted 351w about a year ago, haven't been able to work on it for a while. I have finally been getting into it and am having a misfire issue with cylinder 1. I am not sure where the block came from but it is rebuilt with hypereutectic pistons, an unknown cam, decked gt40 heads and a Holley 670 Street avenger.

When i got the car the secondaries did not open, bad diaphragm so i rebuilt the carb and ever since rebuilding the carburetor i have been having issues getting it to run properly.

I checked the timing and its okay. I noticed that the seller had the firing order in this manner: 1-6-3-7-8-2-5-4.....I know that the firing order for the 351W should be: 1-3-7-2-6-5-4-8

When I changed the firing order to the correct one, the car didn't even start. I changed back to the initial firing order and it "fired" right up. But the misfire was still there. I pulled the wires one at a time and replaced them after I noticed if there was a change.

Using the 1-6-3-7-8-2-5-4 firing order the #1 plug wire is pulled the car idles fine. Any suggestions?

If I do have to check the distributor position what is the procedure to check if the distributor is 180 degrees off?
This is where I would start, identify the block to know exactly what you have and do go by what the seller say's he sold you.

Q. How do I tell the difference between a 302, a 351W, a 351C, and a 351M?
A. By using combinations of the following facts, you should be able to positively identify what engine you have.
The number of valve cover bolts per head
o FE = 5
o 351W = 6
o 351C = 8
o 351M = 8
o 335 series big blocks = 7
The thermostat housing location
o 351W has it on the intake manifold
o 351C has it on the block
Fuel pump mounting style
o 351W has bolts on each side of the fuel pump
o 351C has bolts on the top and bottom of the fuel pump
Water pump boss
o 302 water pump boss goes right up to the bottom of the head
o 351W water pump boss stops below the top of the block " about 1 below the bottom of the head
Firing order
o 302 firing order is 1-5-4-2-6-3-7-8
o 351W and late 302 H.O. firing order is 1-3-7-2-6-5-4-8
Dipstick location
o 351C has the dipstick going into the block
o 351M has the dipstick going into the oil pan
351W eighth digit in the engine ID number is generally an H


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post #5 of 6 (permalink) Old 2 Weeks Ago
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You say with the firing order as you have it now, it fires right up with a miss on #1. If you had the firing order wrong it would be a lot worse than one miss-fire cylinder. I would think you have the firing order right, with some other ignition problem. It could be a cracked dist. cap, bad spark plug wire, bad spark plug, etc., but the firing order would be the last thing I would suspect with your symptoms.
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post #6 of 6 (permalink) Old 2 Weeks Ago
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Quote:
Originally Posted by Hwyman View Post
You say with the firing order as you have it now, it fires right up with a miss on #1. If you had the firing order wrong it would be a lot worse than one miss-fire cylinder. I would think you have the firing order right, with some other ignition problem. It could be a cracked dist. cap, bad spark plug wire, bad spark plug, etc., but the firing order would be the last thing I would suspect with your symptoms.
As the number one plug is the one most often used as a test source. the wire maybe bad the connector on the plug end maybe loose, the electrode on the input side the plug may be loose. A host of issue with number one are possible.

also the above mentioned firing order and cams. the True 69 351W engine had a modified firing order to take the load of the front bearing under continuous low rpm usage (police cars sitting) many upgraded cars 9351w blocks and what not use the more common and more plentiful 289/302 cam shaft and firing orders. Go through a cam catalog the 351w has a handful of choices while the 389/302 can b e had in literally hundreds of grinds
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