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Which is it?

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

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On inline four cylinder motors the inside cylinders have larger main jets than the outside cylinders. I have heard two theories on why this is necessary. One is that the two inside cylinders run hotter because of less cooling fin area, and the richer jets would cool those cylinders better. The other theory is something to do with the H-pipe changing the exhaust flow characteristics of the two inner cylinders. Does anyone know for sure which one is correct? Or is it for some other reason?
 
I'm not sure about about all displacements but the 550's have larger center main jets. I just want to find out why.
 
I think it's because the middle cylinders run a little hotter due to less air cooling. Interesting thing though, the larger mains can only help with cooling at 3/4 to full throttle. The typical rider is usually operating on the needle circuit with an overlap from the pilot circuit. Maybe the makers are only concerned when the bike is being ridden hard? Not all models use larger mains in the middle carbs.
Another way to richen the middle cylinders is to allow a bit more vacuum when doing your carb synch. You can do your synch at around 3,000-3,500 rpm's and set the middle carbs vacuum a bit higher. You'll spend more time riding at these rpm's.
Another option is to adjust your mixture screws a little richer.
My bike's mains are all the same and I synch my carbs straight across. As far as I can see, no problems related to hotter running middle cylinders.
 
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