How much slack should a brand new timing chain have? I was under the impression that a new chain has absolutely no slack at all but I just put one on a 351M and its a little loose. Reason I ask is after starting the engine it backfired a couple times even after I set the timing, so with a loose timing chain the timing would be erratic hence the backfire. If I'm wrong, any other ideas what causes backfire's (through the carb)

Thanks much

 

Sam,
A new chain usually has no slack. A loose chain is not likely to cause backfire thru the carb. More likely to be crossfire due to bad ignition wires or wires are too close together causing induced current. Also might be a bad intake valve or one or more valves with less than zero lash. If all the ignition wires were off might also check the firing order and ensure the wires are on the right plugs. Good Luck

 

If the block has ever had the mains align-honed, the crank will have been relocated closer to the cam, thus creating the need for a shorter chain.

 

I have had troubles with new timing chains having too much slack as well as tight and loose spots as the engine is rotated. If you buy a new timing chain, install it and check it carefully before you close everything up. The last Sealed Power timing chain I bought had enough slack after 10,000 miles to need replaced. The culprit seems to be the use of powered metal sprockets (upper) and poor quality control. I bought an Edelbrock timing chain set and was well satisfied with it.

 

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.

 

I have the same problem, I was replacing the front seal on my oil pan when I noticed the timing chain was loose, so loose that I could wiggle it at least an inch in either direction, I went and replaced it, and that seemed to help somewhat, but it is still pretty loose, does anybody have any ideas as to what I could do to fix this problem?

 

 

Sam, if you are using a double roller timing chain, a LITTLE slack in it is perfectly normal. O.E.M chains usually go in tight like a tiger but stretch fairly quickly. If you have less than a quarter inch of displacement don't worry. Also,after you fire it up when the oil has saturated the timing set, it will tighten it up a little. Also, double roller is good, TRUE ROLLER BEST!

 

Dude! this thread dates back to the Clinton administration.

 

I wonder if Sam realized in the past 12 years not to buy engines from autozone? He might still be working on the timing chain.

 

By now, Sam may need a new engine and this info will help...............

 

No, I bet he gave up and sold the truck for scrap and subsequently bought a Vespa.

Josh