how do pros break in a top end?

[Rich Rohrich]

Originally posted by TTRGuy
You spoiled me Rich, I'm not used to answers like that from you :)

It's the honest answer though :)

Originally posted by TTRGuy
Any idea what some of the pros's run?

No clue, but I'll see what Eric says. He's bound to have some insight.

Originally posted by TTRGuy
In general do you know of any normal MX'r or Enduro type rider/bike that would actually 'need' a ratio of less than 32:1? Assuming proper jetting etc.

Nope. 32:1 is a pretty good all purpose ratio. Probably why the OEMs tend to go with it.

 

[Jaybird]

Originally posted by Rich Rohrich
Keep in mind no one ever broke an engine by running too much oil in their fuel :)

Good point.

TTRGuy,
Most of the data I would use for jetting and pre-mix comes from others. I knew that you could gain compression from more oil (better seal), so I knew I wanted to go that direction. Also, I ride a 125 and it makes high R's quite often...that entered into my decision. I experimented from 40:1 to +/-14:1 ratios. I found that I could in fact see a static compression reading that was ever so slightly increasing as I enriched the mix with oil. Im this particular instance, the reading seemed to level off at about 24:1 ratio (mind you this is a kick the starter compression reading I'm going by here, and a raise in compression by maybe 1-3 lbs at best).
Ok, so now I've decided to go with 24:1 and jet accordingly. With the elevation and humidity and easily accesible choice of jets for my carb, I had trouble getting to "the edge" (or what a spodely tuner like moi would consider the edge) so, I thought about all the factors involved and made an uneducated deduction and changed to 26.5:1
That ratio puts me about where I want to be within the tuning window for my riding style and conditions. Not the "edge" completely, but fine for me at this stage.
I still get some spooge, but it is my lower cicuits that are the culprits there, them and the complete trash fuel I use.
If I spent more time in the woods, I would proably be a James Dean diciple and look for the perfect low circuit settings and change them as needed, but I spend most of my concerns on the upper end of the throttle.

 

[MrLuckey]

Damn, a complete thread that I think I completely follow and understand :) (SO FAR) I think Jaybirds post brings up another point I was going to ask in another thread and never did. I think someone mentioned (maybe Rich) at one time that he would adjust the oil/fuel ratio ever so slightly if conditions at the track were different than expected or maybe didn't have the jets he really needed (in between sizes maybe?). If you were spot on with the pilot or needle but maybe off a little with the main (or any combination) wouldn't that in effect throw off the jetting in the other circuits? As long as were on the subject..... Will a change in conditions (altitude for ex) require a proportional change in jetting for the different circuits across the board? If so how would you achieve this. It seems to me that there are a ton of main jet sizes VS pilots for example. I seem to change main jet sizes and needles or needle positions a lot more than pilots based on conditions.

 

[Jaybird]

When you are on the edge between spot-on and leaned out, it's very critical that you not go into the lean area while at full throttle. If you need to make a ratio adjustment for lack of correct main jet, the other circuits should not notice much of a difference. A little spooge never hurt anything, and you probably will never seize anything by having lean low circuits.

 

[MrLuckey]

Sounds good , what about the.... the.....uh can I maybe make up a word? the proportionality of the circuits? Here is another question. If the ratio was such that it was dangerous to the bike... would the bike run still run good or would it be obvious that it was too lean. I know some of my friends that used to race 600-700 hp outlaw cars always said that they ran the best right before they blew.

 

[Rich Rohrich]

Originally posted by TTRGuy
As long as were on the subject..... Will a change in conditions (altitude for ex) require a proportional change in jetting for the different circuits across the board? If so how would you achieve this. It seems to me that there are a ton of main jet sizes VS pilots for example. I seem to change main jet sizes and needles or needle positions a lot more than pilots based on conditions.

Fixed orifice jets (pilots, mains, and needle jets) tend to react consistently to external changes
Air fuel ratio is based on weight of fuel and air, but jets essentially meter by volume. So we really need to know the weight of the fuel flowing through a jet to understand all this. In simplest terms fuel weight is a function of the area of the jet multiplied by the value of the square root of the fuel head pressure multiplied by the density of the fuel.

It looks like this:
weight of fuel = jet area * ( SQR Root (head pressure * fuel density) )

As altitude and barometric pressure change so does the head pressure on the fuel. Problems arise from the fact that air density (weight of the air) is changing at the same time, and you get the extra fun of changes to the fuel's evaporation temperatures as the pressure/altitude changes. It's possible to calculate jet changes based on changes in air density, but it still takes tuning to get it spot on. Even Honda with all their brain power has to tune at the track :) Hence they tend to react in a predictable fashion.

Best power comes in the vicinity where detonation is most likely to occur so it can be dangerous territory. Sligthly rich of best power tends to have a minimal impact on power but power usually falls off fairly quickly once you got to the lean side of this point.
I have a bunch of old notes on ways to calculate jet numbers for Mikunis and Keihn carbs based on air changes. I'll try and put them together in some sort of reasonable order and hopefully make this more understandable. This subject deserves something better than a couple of generalized paragraphs like I just gave it. :D

 

[MrLuckey]

Originally posted by Rich Rohrich
In simplest terms

Huh? What? Thats you're simplest terms? Damn freak :p j/k

What I got out of that. I totally get that even the experts have to jet at the track but... To borrow a word I may have made up earlier. Proportioinality, if say the main jet needed to be 20% leaner, would the pilot and needle need to be 20% leaner also? Ya just had to go and ruin the only real techie thread I have understood all the way though :moon: Oh and speaking of 'fixed orifices' I see you finally got around to responding to ZIO! Dayum that dude is hilarious but he worries me at times :aj:

 

[Rich Rohrich]

Originally posted by TTRGuy
. Proportioinality, if say the main jet needed to be 20% leaner, would the pilot and needle need to be 20% leaner also?

Generally that's the case. You can change the make up of the fuel like adding or removing oxygen bearing components (MTBE etc) to make changes across the board to test it out.
So many other factors enter into the mix that it's tough to make blanket statements about carburation without looking like a twit. Fortunately I wear the twit suit well so I forge ahead. :confused:

 

[MrLuckey]

Aha, so it looks like I am pretty much out of questions then. Not sure if
thats a good thing or not but I can't think of anything semi-intelligent to ask.
That is unless you consider this intelligent... Can anyone help me out quick please, I think he's driving me insane :D
http://www.dirtrider.net/forums3/showthread.php?threadid=51358 bwaahaahaa

 

[Jaybird]

Don' stop now, TTRguy! You've got Rich on a good roll! :)