Head temperatures

[Aches n Pains]

Took a nice ride yesterday. When I got back to the garage I took a temperature gun and shot a temp for each cylinder right at the header mount.

#1 - 173 degrees
#2 - 250 degrees
#3 - 250 degrees
#4 - 175 degrees

Haven't pulled the plugs yet but just curious if that makes sense to you guys. Seems like the outside cylinders run cooler.

Thanks,

Doug

[natemoen]

The inner two are going to run hotter because they have less cooling area and are sandwiched between the other 2 cylinders.

Because of that fact yamaha attempted (but gave up) to fix that issue by increasing the main.jet size on the '80 model year to try and get it to run cooler in the middle 2.

[jayel]

I have checked several makes and models SR500, XS650, XS1100, 350 to 550 Honda's Harley sportsters and big twins and properly running air cooled bikes do seem to run in the mid 250's range, the best place to check is right next to the spark plug base (hard on in-line four) next best the exhaust port but not the header, your reading are entirely consistent with what I have seen

[MPittma100]

The inner two are going to run hotter because they have less cooling area and are sandwiched between the other 2 cylinders.

Because of that fact yamaha attempted (but gave up) to fix that issue by increasing the main.jet size on the '80 model year to try and get it to run cooler in the middle 2.

Leaner is hotter. Richer is cooler. But you have to go over-rich to get any engine cylinder temp down - especially an air cooled engine. They had the right idea, but should have gone ahead and made the outer two cylinders rich as well. Guess they had to work around the US emissions requirements. Then again, if you really want to bring the temp down, you could always run methanol LOL.

MP