First start on a fresh motor and the headers are glowing after only a couple minutes. I had to shut down right away for a fuel leak and again after about five minutes for a coolant leak. On the second shut off, I got a big backfire out the mufflers...too rich? The restart was called on account of rain, so it's back in the garage till next weekend.

Why are the headers glowing? Timing? Idle mixture? Both?

Is there anything I should do before I start it again to finish the break-in?

How many degrees of total timing do I want at 2500 RPM?

Holley 650 spreadbore...how do I set idle mixture?

It is either too lean and/or the timing is off. Get a timing light. The curve should be in between 2,500 and 3,500 for performance use. You want around 16 intial and 34 to 36 total. Check the idle mixture with a vacuum guage and check your float levels.

your timing is off possibly retarded the timing instead of advancing also the fuel mixture is probably way to lean

+1, glowing headers is from the fuel still burning as it exits the chamber and into the primary tube,, timing WAY retarded is the usual suspect

Retarded timing...turn the distributor CCW to advance?

Floats are not adjustable on this carb without removing the bowls. What am I looking for with the vacuum gauge? Most vacuum at a given RPM?

Couldn't the timming being real bad retarded cause this also?

Your timing is retarded. I had this happen to me when we dropped a 350 in my buddys 85 Z28. We forgot to unplug the wire up on the firewall and after we set the base timing the manifolds were glowing. the timing was way retarded. we went back and unpluged the wire up on the firewall and reset the base timing. The manifolds have not glowed since. You do not have to worry about the wire up on the firewall, its only for computer controlled engines.

for the 20 minute run in or cam break in,, leave the distrubitor hold down clamp just snug enough so you can rotate the distributor by hand with the engine running. as the engine fires and you bring it up to 2500rpm or so,, rotate the dist. counter clockwise/advancing,, till you hear it lean misfiring at a steady throttle of about 2500rpms,, then back it off slightly till it clears up.
with no load on the engine it's really at about 4.0:1 dynamic compression or less with the throttle blades mostly closed up. there's almost no chance for detonation with no load free revving it in neutral/park like this for the run in. unless you've got 13-14:1 compression on 87 octane or something wacky like that. it'll be fine to get you through the run in. it won't make the headers get red anyway.

then after the run in,, get out the timing light. unless you've got a buddy or 2 to get it timed for total timing right when you get it started,, the above will get you through it.

as said above,, it's the retarded late firing that burns in the cylinders and out the exaust that makes the headers red. it's not finishing the burn in the combustion chamber,, it's finishing the burn when the piston is already down the hole some,, actually more in the cylinder area,, and like said,, everything doesn't get completely burned till it's in the header tubes. hope it helps.

uh, maybe those are some new type headers which double as exhaust manifolds as well as trouble lights.....

I've seen shorty headers glow during an engine break in from just sitting there, no air flow over them to cool them . . .

That is why the fine print on ceramic coated headers states that the warranty is void if you use them to break in your new cam... Somebody was kind enough to warn me about that. I've kept my old headers for just that purpose.

I knew that when I bought my ceramic coated headers. Broke in the motor with other headers and now the Hookers are still nice and shiney...

Very helpful! Thanks everybody. I'll try again this weekend.