one pipe on exhaust manifold running hot

[sklatt]

I have a 1972 02 with a new metric mechanic m10 engine and a weber 38. The header pipe on the number 2 cylinder is running at 480 degrees while all the others are at 310 +- 10. The head seems to have a diffusely consistent temp of 170 to 190. How is this happening?  No vacuum leaks. No exhaust leak. Valves all set correctly. Compression all 120.  Electric Pertronix timing.  Impedance on spark plug wires the same with minor differences relative to the length of the wire.  

[John76]

How are you measuring the header pipes?

If using an infrared thermometer, you have to be really careful to measure "like" surfaces to compare temps. It takes a bit of scientific technique to get the absolute (true) temp, but a relative reading can be achieved if you measure uniformly heated surfaces with the same type of texture and surface coating at exactly the same distance and angle with the appropriate wavelength sensor.

Most IR temp. devices used by mechanics have fairly large F.O.V (field of view) that is not represented by a small laser pointer.

These are fine for reading large, similar objects at close distances such as tires, radiators, rubber hoses, etc.

However, huge errors can result when measuring small or shiny metal objects (chrome, brass, aluminum, etc.) with a typical "hobby" IR thermometer.

[TobyB]

Not nearly hot enough- I usually see about 1600f when I'm just a bit rich.

 

When they hit 1900, then you'll want to jet up or air corrector down.

 

t

 

[John76]

1 hour ago, TobyB said:

I usually see about 1600f when I'm just a bit rich.

Toby,

How are you measuring this???

IR or thermocouple??  If thermocouple ... how??

[TobyB]

Aircraft Spruce aviation- style thermocouples about 6" from the ports.

 

t

Westach 2-1/4 Square Dual EGT 700-1700F 2Da2 Non TSO | Aircraft Spruce

WWW.AIRCRAFTSPRUCE.COM

---

Westach 2-1/4 Square Dual EGT 700-1700F 2Da2 Non TSO Two instruments in one size case - perfect for the home builder. All pressure instruments come complete with pressure senders only (matched to instrument). On all other instruments order

HOLY CRAP!  I don't think I paid much over $100 for a PAIR of these.    I remember all- in with 

thermocouples was something like 2 and a half...

[ray_]

8 hours ago, TobyB said:

Aircraft Spruce aviation- style thermocouples about 6" from the ports.

 

t

Westach 2-1/4 Square Dual EGT 700-1700F 2Da2 Non TSO | Aircraft Spruce

WWW.AIRCRAFTSPRUCE.COM

---

Westach 2-1/4 Square Dual EGT 700-1700F 2Da2 Non TSO Two instruments in one size case - perfect for the home builder. All pressure instruments come complete with pressure senders only (matched to instrument). On all other instruments order

HOLY CRAP!  I don't think I paid much over $100 for a PAIR of these.    I remember all- in with 

thermocouples was something like 2 and a half...

Reviews there are considerably less than stellar, too! 

[sklatt]

Thanks for the replies.  I am using an IR gun held as close as I can, 2 in away abouts.  The pipe in question has burnt off the ceramic coating and is a dull color while the others are still shiny. Could that account for the difference?

['76mintgrün'02]

Yes.

 

Edit-- although, the fact that it is burnt and the others are not, might imply that that one got hotter at some point.

Edited January 9 by '76mintgrün'02

[John76]

24 minutes ago, sklatt said:

Could that account for the difference?

Absolutely, positively!

The dull surface emits a greater amount of infrared @ 8-14 microns than does a shiny surface at the exact same temperature.

This is called "Emissivity" and is dependent on the material, surface, temperature and wavelength among other things.

Result is the temp of the dull surface will be much higher, and closer to the true temp than the shiny surface. 

Solution: Put a dab of flat black hi-temp (BBQ) paint on your shiny pipes, large enough to fill the FOV of your IR sensor. Then measure the paint dabs. 

 

 

[Preyupy]

Carefully put a mark on the front pulley for TDC cyl 2&3 (180 degrees from the factory mark)  Then with a timing light make sure each individual cylinder is at the same advance.  I have had a couple of Pertronix units that have had missing or misplaced magnets in the ring.  There is 1 magnet for each cylinder so even if cylinder 1 is timed correctly there is a chance that one or more of the others might be off.  

 

Also 310-450degF at idle isn't very warm and the important number is what is it running under load?  Running Exhaust Temperature Gauges (EGT) used to be a common way of checking fuel mixture before accurate Wide Band O2 sensors became available.  You don't want to see much more than 1250-1300 degrees F under load but you can easily see 1600 degF on a light cruise or decel.  Usually an engine running at a reasonable A/F ratio at idle will be in the 600-700 range.

['76mintgrün'02]

I stuck some BBs on the magnetic ring once, to get a look at magnet placement; but I did not try to take any measurements to see if they were offset by 90 degrees.  I'm not sure that'd be accurate enough anyway.

 

One thought I just had (which might be a dumb one) was that you could check the timing on cylinder number one (using your innova 5568 variable advance timing light) and then remove the magnetic ring, rotate it 90 degrees and reinstall.  Then, recheck timing and repeat three more times.  If the magnets are accurately placed inside the ring, you should get the same advance reading all four times.  In theory....

[popovm]

3 hours ago, '76mintgrün'02 said:

Yes.

 

Edit-- although, the fact that it is burnt and the others are not, might imply that that one got hotter at some point.

If we are considering all possibilities, I am wondering if the finish or prep on that one could have been worse than the other 4 and burned off prematurely. John's clever BBQ paint test could verify or rule out the temp differences and seems to be a good place to start. 

[tzei]

Or you’ve got one good spark plug. Idle good?

[Son of Marty]

I'm not sure if air-cooled VW's dist rotate in the same direction or not but I do know that many of their caps were made with the post for the cylinder by the oil cooler was retarded by a few degrees so it would run cooler to make the oil cooler more efficient. I know the caps will fit either one. It's way out there but it might have some bearing here. 

[jimk]

4 hours ago, Son of Marty said:

I'm not sure if air-cooled VW's dist rotate in the same direction or not but I do know that many of their caps were made with the post for the cylinder by the oil cooler was retarded by a few degrees so it would run cooler to make the oil cooler more efficient. I know the caps will fit either one. It's way out there but it might have some bearing here. 

How does a slight change is post location affect when the points open and the spark happens?  The points lobe would have to be in a non-symmetrical layout to change the spark timing on that cylinder.