Temperature on exhaust manifold

[mopetta]

I got one of these fancy IR-Temp gauges, so I checked the temperature at the exhaust manifold (bunch of bananas).

1 - 190
2/2 - 190
4 - 95 

After I checked the compression, all cylinders between 80 - 90. 

Any Idea why exhaust on cylinder 4 is only 95 degree Celsius ?

thanks

[John Mason]

Could it be that that the exhaust ports on1 and 4 are on the extremities of the engine so therefore the heat escapes easier. No. 1 exhaust port has the hot water outlet on the head above it thus keeping ar the same temperature as nos. 2 and 3.
Well that’s my two penny worth for what it’s worth.

John Mason.

[Ian McGowan]

I seems to me that the likeliest explanation is that No. 4 cylinder is not burning all the fuel - possibly due to it receiving an intermittent spark?? I would begin by checking appearance of plug (compared to the others) check distributor cap for tracking from no. 4 lead, check condition of lead 4, and check for spark constant presence at no. 4 plug.

[stuartu]

I use an IR thermometer myself from time to time and, for what it's worth, my first reaction aligns with Ian's.

Regards,

Stuart

[John Cornforth]

Hi

Before you get too carried away, remember that the acceptance angle of a typical IR thermometer is about 5 degrees, i.e. a "spot size" about an inch in diameter when held 12 inches away.  So you need to be close enough for the measuring area to only be seeing the manifold and not partly overlapping something cooler.  I found this out when trying to measure the temperature of 15 mm diameter central heating pipes.

[mopetta]

I checked the temperature twice, on the road and back at home. Both times the same difference, so it is probably not a mistake

[Colin Wilks]

Did you check the temperature at the confluence of your four bananas? Conduction throughout the manifold would tend to equalize the temperature of the four branches over time, so for number four to be that much colder its exhaust gases would need to be actively cooling it down (ie < 95°C). 
My guess is it's either a reading error or you're running on three cylinders.
If you put your hand over the exhaust tail pipe are you getting unburnt fuel coming through? An unburnt mixture would cool the pipe down.