misfire in 351C

[73stallion]

okay, i'm stumped. david has a '73 bronco with a 351 cleveland in it. the #7 cylinder has a misfire. i ran a compression check, all cylinders are between 170 and 185 lbs compression, #7 has 175. the cam lobes are not flat. it's getting spark (we put a spark plug on the fender, grounded it, it's sparking, but it looks weak). plug wires have been changed, and the plug has been changed 4 times. but still, the #7 cylinder is not firing.

i've had several people say that the 351c does NOT have the same firing order as a 351w, is this true? every website i've looked at says it's supposed to be 1-3-7-2-6-5-4-8. it has a MSD 6A box, blaster coil, and 8580 distributor.

i'm open to any ideas, i'm at a loss.?:?

 

[Skuzzlebutt]

The firing order is cast on top of the stock intake manifold. If it's not stock, maybe. If you've got any of it out of order there will be more than just #7 giving you grief. Also, being out of order does not affect the strength of the spark. Have measured the resistance (Ohms) of each plug wire?
What are the symptoms you're experiencing besides a weak looking spark?

 

[73stallion]

it really sounds like more than one misfire, but i couldn't find any other plugs that were fouled. #4 didn't look that great, but it was firing. thing that has me wondering is that #4 and #7 are directly across from each other on the cap, i'm not ruling out a bad distributor.

it's an aftermarket manifold, no firing order on it.

 

[DirtDonk]

i've had several people say that the 351c does NOT have the same firing order as a 351w, is this true? every website i've looked at says it's supposed to be 1-3-7-2-6-5-4-8. it has a MSD 6A box, blaster coil, and 8580 distributor.

Yep, same firing order as a 351 Windsor.

With that ignition, you should not get anything but real good healthy multiple sparks each time. Are you saying that the spark to #7 is weaker than the others? Or have you not checked the others yet? If they're not the same, then something is whacked between the distributor and the plug.
Either that or you just weren't getting it near a good ground, but that doesn't seem too likely.
If they're all weak, then you might not have enough voltage getting to the ignition. Check for a full 12 volts (if you haven't already) at all the appropriate points. No pun intended...

Paul

 

[bludorbronc]

Could be bad intake gasket or vaccum leak. Both will make engine run ruff.Once the plug on the power brake booster vaccum fitting blew out and truck imediately started runnng ruff. Siliconed a nickel in place and smoothed right out. Had gasket go bad on one cylinder and it effected cylinder following the intake runner across.I think it has something to do with engine cycles(#4 and #7 on same cycle).

 

[broncnaz]

I'd try changing the cap and rotor if you havent done so already. I know it was common to get a cross fire between the 7 and 8 wires back in the day but I would think new quality wires would fix that but just to be sure make sure you separate the wires. How is the #7 plug getting fouled gas or oil? could be a bad valve seal or the carb has a leak and is dripping into the #7.

 

[Airborne69]

sounds like your timing is wayy off. just fixed the same problem on a blazer today. borrow a timing light and get your timing down then if the problem persists dig in deeper.

 

[TexasBronco]

Ok this may sound like a stupid answer...but I have done this twice myself! ( you would think after the first time I wouldn't do it again)

Does it seem like it will almost run, and might even run or rumble for a bit then backfire? First off check to make sure the dist is not 180degress off.
Then second make sure you are going the correct direction on the cap as you hook up spark plugs. Counter Clockwise!! I know it seems more natural to go the other way....but trust me that little mistake will keep you stumped for hours.


And I am assumeing since you said the 8 and 4 are across from each other, you know that ford designates each cyclinder differently then Chevy or others?

 

[Pa PITT]

Was it running good... then not....I change the cap i've seem several that had one bad post in them ...

 

[Airborne69]

is your distributer tightened down? i might have rotated while the engine was running