exhaust manifold glowing red

[jg1977c20]

77 chevy c20 - 350 -   had the exhaust checked for restrictions -none - heat riser plate is removed - no cats


 but under load and steady cruise above 60mph - my passenger side manifold will glow red and it appears to be hottest between cyl 6 8 and 4 - all plugs look the same and normal - doesnt do this driving around the city at lower speeds  the driver side is not affected at all - any ideas where to start ?

[Captkaos]

What is your timing set to do you have any vacuum leaks?  Is it running lean?

[jg1977c20]

timing was set to 8 but put it back to 4 - its only 1 manifold that glows and i can feel some vibration,  when its red it looks like cyl's 4-6 &8 are doing it - #2 looks colder - if it was timing both manifolds would glow , is it possible for the carb to be lean on one side off idle ? when i step on it theres tons of power

[Captkaos]

It could be a lean condition on one side.  What has changed on the motor last, has it done this always?  It is possible the valves aren't seating.

[jg1977c20]

yeah its definitely looking like a lean condition - and it is only the one side - ironically the truck ran fine till last week when it spat out the heat riser flap right out the tail pipe - had my buddy cut the pipe open to confirm its gone .

been thinking about the valves -whats the best way without yanking the heads to kinda diagnose that more - i did an old school test and held a piece of paper near the tail pipe -it ddidnt suck the paper towards the pipe -

could i be experiencing valve float as it seems to glow when im at the higher end of the rpms ( 4:10's  doing 60 mph) or 100 k here in canada

just add - intake gaskets changed last year -checked for leaks and tightness today nothing found , checked the dist cap and shaft for play , pulled the valve cover off -no sunk valves - ran it open everything looked normal and no sounds of out of adjustment

[jg1977c20]

got to thinking - would a partially plugged jet cause this -?

[jg1977c20]

okay so heres an update - i did a vaccum test and the needle was steady like a healthy motor - revd it up needle dropped bounced back high then settled to normal .

changed plug wires since #8 was leaking - idles smooth , great power on acceleration no back fires /pops

tried spraying carb cleaner to find a leak , tried propane even though vaccum showed normal

yet the 1 manifold on the passenger side heats up and glows orange after 20 minutes on the highway yet when its orange the temp is only 60 deg higher than the non glowing manifold that reads about 700 f

the manifold only glows from cyl 4 back to 8   from 4 to 2 the manifold isnt glowing

checked valve adjustments on those cylinders - ok

any other ideas ?

[Engineer]

Check the color of your plugs. Reading plugs is a science. But in short, you want them to be tan. White is lean black is rich.

What engine do you have? What carb and intake are you running? What is your ignition timing?

If your exhaust manifolds have the AIR ports you could check to see if your exhaust has a restriction, or too much back pressure.

[jg1977c20]

Check the color of your plugs. Reading plugs is a science. But in short, you want them to be tan. White is lean black is rich.

What engine do you have? What carb and intake are you running? What is your ignition timing?

If your exhaust manifolds have the AIR ports you could check to see if your exhaust has a restriction, or too much back pressure.

77 c20   350 4bbl 4mv carb stock everything no emissions -pcv valve only

-   vac guage/tests show no faults
-   manifold will not glow at idle even if i tried ( only passenger side glows after 20 minutes of 60mph)
-   plugs are whitish tan - not blistered - but more white probably due to ethanol in fuel
-   no pops/backfires at intake or exhuast
-   pipe was cut to confirm heat riser flap is gone
-   the belly of the manifold glows from #4 exh port back to #8  but doesnt glow where it attaches to the head ( forward of #4 exh doesnt      glow ever )

starting to wonder if theres a hair line crack in the exh manifold that i cant see maybe ?

[philo_beddoe]

Glowing red is pretty hot. Cant see how a crack could cause that.  I know a guy that has the answer, i'll get back to you asap.

[philo_beddoe]

What engineer said makes sense, the heat may not be escaping, perhaps blocked port(s), restriction etc.

[Engineer]

760°F shouldn't make cast iron glow.

Interested to find out what is going on with this.

[Dr_Snooz]

If you raise the RPMs and keep them steady, what happens to the vacuum reading?

[jg1977c20]

If you raise the RPMs and keep them steady, what happens to the vacuum reading?

stays high normal -

[jg1977c20]

760°F shouldn't make cast iron glow.

Interested to find out what is going on with this.

i know right - it's got me baffled -   i will definitely update if this gremlin ever rears its ugly head