Ok, here's the whole situation. I bought a remanufactured 351M for my truck from Autozone about two months ago. I put it in, it fired right up, then died on me after about 20 minutes. I had Autozone check it out, turns out it had no compression.
They basically sent me another engine to put in, which I did. Taking the first one apart again, I found a bunch of sludge in the oil pan, looks like the rings came apart instead of seating correctly.

Then in putting the new engine in, I noticed the timing chain (which is supposed to be new) had slack in it whereas the first engine I got from them had a tight timing chain.

When I turned the key, it fired right up, but backfired through the carb even with the timing set and it didn't want to idle.

How much of a big deal is the slack in the timing chain? I still haven't given them the first engine back yet, so I'm thinking of switching the timing chains first since the other one was tight and was only run for about 20 minutes, 10 at idle and the other 10 getting about 10 miles on the engine before it died.

I haven't had time to check the spark plug wires again, but they were new last time and only have the same 20 minutes on them, and I checked twice that they were in the right firing order.

Since the engine was remanufactured, the valves should all be new or at least re-ground. The valvetrain was adjusted already when I got it and I didn't touch anything on the top end assuming it was set up correctly.

The other engine was really running nice before the rings gave out so I've made no carburator adjustments and kept used the same 14 degrees BTC on the timing as I used before.

Besides this timing chain thing, I don't see any differences between the two engines that might be causing the roughness and backfiring.

 

My engine did the same thing. I took it to my uncle and I think he did something with the timing of the distributor. Hope this helps.

Bob

 

I saw that somebody with a 400 was having trouble with timing and that the stock setting was too much. Should I try something small like 12 instead of 14, or go to like 10 or 8 BTC? Or, I'll just see if any of it works.

 

When I timed my 351-M it backfired out of the carb too, I realized that I had set the timing to the top dead center of the wrong stroke. It is supposed to be the compression stroke, you probaly already know this but maybe it can help.

 

Hi Sam, As long as there's not too much play in the timing chain, I wouldn't worry about it. It's possible that when they rebuilt this engine, they line-bored the engine. Line-boring does the same thing to the main bearing caps as boring the cylinders does to an engine. This is done to ensure the main caps are square to the cylinders of the engine. For instance, a .010/.010 crankshaft means the mains and rods are 10 thousandths under-sized. This is usually the cause of the extra play in a new timing chain in a rebuilt engine.
Then again, your distributor could be 180 degrees out, but your problem has got to be in the timing somewhere.


-Troy

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Sam I have rebuilt a few 351's play in the chain is not uncommen on fords. Timming is every thing with these engines. A improperly groung intake valve will cause this problem or the centrifical advance in the distributor could be bad. I had one bored over 40 it locked up after about a 10 ml drive when first built and that was with an hour of brack in, I let it cool down, restarted and drove it at hiwy speeds for an hour and now it runs great. Hope some of this helps.

Dan

 

I agree with all the other responces but one thing for sure is that Auto Zone should replace the engine. If this is the second one as you said. Do this verify TDC and check for timing first. If that is not right, then correct it. Carbs can be bad and have an internal jet problem, but most of the time it does not cause backfiring. I also work at Auto Zone part time. Email me at rpoole@cvnc.net and I'll give you a hand with this situation. The customer comes first, sounds like to me That auto zone store is not doing a good job.
Russell

 

This sounds likes the classic case of tracking inside of the distributor cap. Backfiring usually means that the plug is firing at the wrong time, like when the intake valve is open. Be sure to pull the cap and inspect carefully. Been there, done this one too.

 

Well, this is now the third engine I've changed and I made a big rookie mistake. And paid for it too. I finally gave up trying to fix this myself and took it to a local reputable shop. For $269.55 they told me I had:
a) swapped the 3 and 4 spark plug wires (doh!)\
b) the #2 cyl intake rocker backed off and they fixed it
c) I had too many vacuum lines going everywhere (same as it was when I got it) so they took what I didn't need off
d) the (brand new) plugs were fouled so they put a new set on

Needless to say, lesson learned. I swear I checked those plug wires at least 10 times.