Question about jetting rule of thumb

[hostilerob]

I have been out of it for so long that my memory isnt serving me well. Back in the day if we ran a 170 main in 70 degree summer, we may go up to 185 or 190 in 20 degree winter. I am currently running 182 main in a 330R tuned perfect for warm summer days and just want to know if anyone has a general rule of thumb for jet size increase for every 10 degree of temp drop.

So basically if a bike likes a 182 main in 70 degree weather, is 195 a safe bet for 20-30 degrees?

[El Camexican]

Here' my take on it.

Temperature, humidity, barometric pressure all play a role in what you truly need to be concerned with and that is "corrected altitude". remember those two words as they are all that matter when discussing this subject.

A mechanically sound normally aspirated, carbureted, gasoline engine runs best with an air to fuel ratio of between 12: and 14:1. However that ratio is typically with assumed oxygen content of 21% (sea level). If temps rise and you go up a mountain the volume of air your engine ingests in a cycle is the same, but the amount of air and therefore oxygen are greatly reduced, so it needs less fuel.

In short you need to establish your "base line jetting" (remember that too) and then you can adjust your jets to suit conditions like a pro, if you know the corrected altitude. Case in point being Super Gas and other drag racers. They can tow a car across America, unload, determine corrected altitude, spin a screw on the throttle stop one way or the other and usually run within a thousandth of a second on their first pass and a track they've never been to before.

Short of getting a portable weather station I seem to recall that you can call a local air strip and ask what the corrected altitude is, but I might be wrong.

So once you determine your base line jetting the rule of thumb is:

Air Screw - Set between 1 to 3 turns out. Can be used to adjust changes in corrected altitude 2,000' either way, If the engine runs better with less turns out go up a size on the pilot jet, if it runs better at more than 3 turns out go down a pilot size.

Pilot Jet - Go up or down a size for every 2,000' of corrected elevation change.

Needle Jet - Once set properly it should not have to be changed, but it can be if you find yourself way out of whack and without jets, or want to know which way to go without dropping the float bowl. Typically if you have the correct needle installed the clip should be in the center groove. If you find you're running better at either extreme of the 5 slots you likely need a different needle, or a jet change. The 2 intermediate rings are a great way to tell if you can improve on your pilot or main jet. Not to say you can't leave the clip in other locations besides the center one if it runs best there, but its an indicator that there is room for improvement.

Main Jet - Go up or down one size for ever 1,000' of corrected elevation change.

Sound like to much hassle? I agree, that's why its nice to know that once you get jetted close you can just use common sense when conditions change i.e. don't do a 3 mile top end river run at -40 on your summer jets (speaking from experience on that one :rolleyes:) your engine will run anywhere from a 10:1 up to 16:1 fuel to air ratio, it just won't run as well as it would around . Your 195 sounds plenty safe, but depending on what your air screw is set at you may want to bump the pilot one of two sizes as well. Everyone worries about the main, but most of a non-racing engine's time is spent on the pilot circuit.

[hostilerob]

Thanks for detailed reply. The reason I asked is because the 39mm carb forced into the intake boot is a royal pain in the rear to twist or remove for pilot jet changes. Main jet is easy. All I wanted was a basic rule of thumb. If same location, same elevation, just a temp drop of 60 degrees could be a rough jump in main jet from 185 to 200. I dont mind going safe and rich. I dont need that fine line, full race tune. Big bores rip even when they run rich.