Hi guys, here's what I have: Stock bottom end 6.0, 241 heads, new ls7 lifters, new trays, stock rocker arms, custom comp cam. New Crank and cam gear, and new timing chain. I have stock pushrods (I know I need different ones with the cam-more on that in a second) ls6 intake, base tune by Mike Noris motor sports.
So, after starting with the new cam, ran great for 1:30 and I shut it off. Went to start it up after 30 minutes or so, and it started backfiring through the exhaust. Never did it in startup or while running, all the pushrods were straight, all valves checked out (applied air into the cylinders to check for leakage through intake or exhaust valves).
Here's the NEW cam specs:
.601-IN/ .605 Exh. lift
283-In/287 @.050
113+4 Lsa

Here is the old cam specs for which the base tune was done, I talked to Mike and he said that the tune would be fine for startup, but may need more idle speed.
.588/.588
228/228
108 Lsa
Sorry for the long post guys! Thanks

 

I think you got your cam specs wrong..

 

Old cam:

 

New cam:

 

I mean your new cam. 283/287 @.050 is a monster of a cam.

 

You've got a 234/238 duration cam. Have you measured for proper push rod length? You could be holding some of the valves open possibly.

 

Yeah I def know I need new pushrods, but what's got me is the fact that I started it up and ran it and it was all good. It's when I restarted it later it started the backfiring.

 

throwing any codes?

 

You did rotate the engine manually and checked all the valve spring compression and travel limits.. Right?? And made sure the push rods were not getting compressed?

 

Yeah I rotated it several times prior to putting the valve covers back on. Going to pull the front cover to check that out, then go from there.

 

may u have weak valve spring, if its weak under WOT will bent pushrods

 

It's got brand new tooley dual springs,and never got above 1500 rpms

 

How much coil gap on the springs at full lift? should be a spec in the instructions from the spring maker
, usually you should see at least a 1/16 inch , if the coils stack you will break stuff...

 

Thanks for all the help guys, pulled the front cover and I think my brand new cloyes chain in stretched. Seems like a lot more play/slop in it than before.

 

That was going to be my next guess, that the timing chain hopped a tooth or something. Kinda rare to see that in my experience. Check compression after new chain to be safe!