Exhaust header temperature

[David Isted]

Just after some advice please, am I right in saying that cylinders 2 and 3 will always run a bit hotter than 1 and 4 due to 2 and 3 not being able to shed heat as efficiently due to the design of the engine. My bike is a 1978 GS 750

[Brendan W]

Seems reasonable and you'd think the builders thought about that and figured out how to make that difference less than the point where it makes a difference.
Air cooled motors have relatively unstable operating temperatures anyway. At a guess that variability is more than the differences between inner and outer cylinders.
Have you measured something?
My 850 had completely blocked up air passages between the inner cylinders.
I recall a story about the big Kawasaki triple two strokes that the inner pot had a bigger main jet and it was effectively petrol cooled.
The VX800 for some reason has different jets fore and aft and it's water cooled.

[rphillips]

The Suzuki GS1150's had one size bigger mains on the # 2 and 3 cyl. than on # 1 and 4 I've always thought for a little extra cooling

[David Isted]

Thanks for the answers, at idle cylinders 2 and 3 run about 30 degrees Celsius hotter than 1 and 4.

[748cc]

While we're on the subject: Where should normal temperatures at idle be when measuring at the J tubes? What's considered dangerously hot/excessively lean? How cold is too cold?

[David Isted]

I’m getting readings of about 75 to 80 degrees Celsius on one and four at idle and about one ten to one twenty on two and three.

[Brendan W]

I'm thinking that the real question may be at what rate is heat leaving the cylinders through the walls and head.
The fact that the inners may have less area on the outside would not necessarily mean that the heat outflow is any less.
If you think about the hotter surface, it has to be dissipating more heat per unit area.
Ultimately the constraints are the conductivity of the aluminium and the transfer to the air and local environment by conduction and radiation?
Both these transfer mechanisms increase with surface temperature.

[rphillips]

Very well put, and exactly what I was thinking ^^^^.