What is a typical temperature for header tubes at idle? On my hot rod Coupe, it is 140 on the left bank and 450 on the right bank. Obviously one of them isn't right, and I'm thinking it is the left bank. Maybe my idle circuits are plugged so I'm not getting much combustion at idle? Either that or the right side idle circuit is plugged so that side's running really lean and thus hot?

It depends on which fuel and idle RPM.
I would try swapping the carbs from side to side and see if its them or maybe a vacuum leak on the intake
I can't remember now if you have Stromberg Carlsons, Holley carbs or the other brand that I can't think of right now Either way they are fixable, it is a matter of figuring out which part needs fixing, idle circuit, transition circuit, float level or????
Good luck, let us know what you find wrench: up

Your Skin Should Stick!

Sorry for the Friday Humor/

I see 450-500*F at idle typically on every tube, I check it about an inch from the flange with an IR gun. Do you have thermocouples, or was that an IR reading?

If that 150*F is real, those cylinders aren't contributing anything.

If I had a cylinder that was at 140 degrees idling, I would suspect that it's not firing.


Joe

Intake manifolds are a mess at idle with large cams and overlap. I'd rev it up to 2500 or so and see how they look.

It is gasoline, 1500 idle, big cam. 8 carbs on a crossram, each side has its own plenum with 4 carbs, 2 of which have idle circuits. I'll rev it up enuff to get it off the idles circuits and recheck.

HF IR temp gun.

Yep with a heat gun on the outside of the tube at idle I see 400 to 600 degrees depending on the engine. If you have a cylinder a couple hundred degrees less, there is a problem. First thing I would do is run a compression test to verify base engine condition. And read the plug while it is out.

140 is not firing, 500 degrees would be firing. Fast idle on the dyno is usually 900 degrees, 1300 to 1400 at WOT.