Jetting for pipes

[SXlover]

Not to call anyone out here or anything because I respect anyones opinions about any topic. But me and another guy are kind of going back and forth on what pilots I should run. And I'm not saying he is wrong either just need more opinions to solve this.

Now if you go HIGHER up in elevation(sled needs less fuel)...Yamaha jetting charts show to go to a SMALLER NUMBERED MAIN, but a BIGGER NUMBERED PILOT.

Opposite holds true if you go DOWN in elevation(sled needs more fuel)...Yamaha jetting charts show to go to a BIGGER NUMBERED MAIN, but a SMALLER NUMBERED PILOT.

So my question for everyone one is this. I'm stock jetted(VIPER MTN) at 156.3 mains and 45 pilots, and I'm gonna put on Speedwerx pipes.

Now that means I need more fuel(same as going down in elevation). Now I follow starting with a bigger numbered main such as a 162.5, but what I don't follow is that I've been told to start with a 47.5 pilot(Isn't this a leaner pilot than a 45?), shouldnt I start with a 42.5?(which would be required if I go further down in elevation or want the sled to be richer?)

 

[Mysledblows]

bigger pilots flow more fuel. a 47.5 is richer than a 45. I can't tell you why it is that you need more fuel in the pilot circuit when at altitude, but it has something to do with air pressure and such.

You need to figure out a baseline altitude and temp - doesn't matter if you do it for home or the hills - and then adjust the jetting according to the location.

 

[SXlover]

bigger pilots flow more fuel. a 47.5 is richer than a 45. I can't tell you why it is that you need more fuel in the pilot circuit when at altitude, but it has something to do with air pressure and such.

You need to figure out a baseline altitude and temp - doesn't matter if you do it for home or the hills - and then adjust the jetting according to the location.

Hmm that's an interesting concept, never knew that. But yea I understand all the jetting that comes with having pipes, like have a baseline like you mention. I was just lost on the pilots.

 

[Mysledblows]

As an example here at home I run 45's and when we go out west I go to 55's (I think that's the spread. Notes are at home and I'm not.)

 

[SXlover]

Yea I followed that when I go up in elevation I need a bigger pilot, I just didnt get the concept that the pilots where richer when you went higher, to me it made sense that a bigger numbered pilot would be leaner, just because that thought of being higher = less fuel. So I assumed that to be richer with the pilots I'd have to go to a lower number, guess I was wrong.

 

[harness racer]

.

Now that means I need more fuel(same as going down in elevation). Now I follow starting with a bigger numbered main such as a 162.5, but what I don't follow is that I've been told to start with a 47.5 pilot(Isn't this a leaner pilot than a 45?), shouldnt I start with a 42.5?(which would be required if I go further down in elevation or want the sled to be richer?)[/QUOTE]


47.5 is not a leaner pilot than 45 , smaller the # leaner it is

 

[SXlover]

I was assuming a higher pilot # meant it was leaner, so thats why they used higher pilots at high elevation because you don't need as much fuel up there.

It's just kind of weird that you want it richer in the pilot circuit and leaner in the main circuit at high elevation.

But I guess that higher # pilots are infact richer and needed at elevation because of what Ryan B pointed out, the air pressure or whatever.

I understand it now.

 

[9801srx]

I was assuming a higher pilot # meant it was leaner, so thats why they used higher pilots at high elevation because you don't need as much fuel up there.

It's just kind of weird that you want it richer in the pilot circuit and leaner in the main circuit at high elevation.

But I guess that higher # pilots are infact richer and needed at elevation because of what Ryan B pointed out, the air pressure or whatever.

I understand it now.

what ryan b pointed out is correct . the reason is that the pilot circuit relys on engine vacuum and air preasure in the float bowls to flow fuel , air preasure decreases as you go up so if the engine vacuum is the same but the air preasure is lower you need a larger pilot jet to get the same fuel flow. the main circuit is less affected by air preasure but is really affected by air density ,the more you go up the air is less dense so you use a smaller main jet to get the same mixture. hope this helps.

 

[SXlover]

Yea that definitely makes sense. That's kind of the way I was thinking about it when I pondered on it for a little while. I knew there had to be some reason. Thanks a bunch.