Hi guys.

One thing that as been puzzling me is the reason for running a larger jet size in the rear cylinder. I will be jetting shortly and have been giving it some thought.

Common reason seems to be that the rear cylinder runs hotter. But if it is because of it being hotter it would need a smaller jet not a larger one. Higher temp, lower air density and a smaller jet, just like being at altitude with lower air density. If it was COOLER it would use a larger jet.

This is my understanding of jetting. Have I forgotten something?

Does anyone know what is happening with this cylinder?

One thought of mine is that it is due to the fact that the cylinders have different length intake trumpets, and therefore the airspeed will differ between cylinders.

What do you think guys?

 

It's because of less cooling in the rear cylinder.

You're making a leap of logic. Cooler air is more dense and can accept more fuel.

But hotter engines require a somewhat richer mixture to avoid detonation.

 

Thanks jschmidt.

I was thinking about intake temperature, not cylinder temp. Makes sense now you have pointed it out. Thank you.

I new there had to be a good reason!

Is this cooling difference due to airflow over the moving vehicle, or due to something in the cooling system?

The reason I ask is that when it comes to dyno tuning, if it is due to airflow, it might not show up in a static enverinment. Will you notice this on the dyno, or is it only seat of the pants tuning that it will show up?

 

It's purely because it is blocked by the front one. All v-twins experience this to some degree.

It's just proper jetting. It would be a rare person who could ascribe a fueling error to a particular cylinder. It shouldn't notably change a dyno run, particularly if there is a fan.

 

Righto, thanks. I will remeber that when I get it back together and it's time for a tune.

Cheers,
Shayne