Practical Jetting

[Bill2001]

Now that I've got the basic jetting on the bike worked out, I need to work up a plan for extra jets to use with changes in altitude and even temperature when traveling with the bike.
In general, I would guess that the main jet would need to be changed. Would there necessarily be a jetting change needed for operation at temps of 100* or 30* when the jetting was done at 70* ?
Or for riding the bike at sea level, or at 5000' or 10000' altitude when it was jetted at 500' msl ?
In your practical experience, hoe many jets and jetting steps should I plan to get? Mikuni has a cardboard slide rule of jetting vs temp and alt which should be helpful.

[Hydraulic Jack]

I have never needed to rejet between sea level and 6,500 feet. Doesn't mean it can't or shouldn't be done for optimal running, but mine runs fine without changing anything. If you are race tuning, you may want jets one step above and below what you have. I wouldn't change the needle.

[zaz]

We have found that if it's jetted properly at sea level, you will need to go 2 sizes leaner on the main, 1 groove leaner on the needle, and adjust the air screw accordingly to run well at 7500 feet. That assumes you are using the same fuel and ratio.

[Bill2001]

Good info. If it's like anything else on 2-strokes the rejetting will be unique amd different for each engine.

[Hydraulic Jack]

Until the advent of fuel injection, cars ran from factory floor to the Colorado Rockies without changing carburation. Did they run rich at altitude? Yes. Did they routinely require rejetting? No. I still have such an automobile, and it still has factory jetting. Still runs good enough at any altitude short of pressure cabin altitudes. Depends on what you want your vehicle to do. It will run without rejetting. It won't make ultimate hp or achieve ultimate fuel economy without jet changes.

Consider: altitude, as seen by a carburetor, is not a distance above sea level as measured in feet. It is a density variable measured in barometric pressure. Pressure changes every day, often times changing several times each day. The differences in barometric pressure can change by several thousand feet day to day, even if you never go anywhere. Ask a pilot. Do we attempt to change jetting every day based on the weather? I don't think so unless we have a race bike and a lot of time on our hands.

Point being that if you want to change your jetting to suit your current location, do so, but don't obsess about it. You are only making a change that is good for that moment anyway.

[Bullfrog]

As a practical matter but focused on Hodakas with Mikuni carburetors, there is a loose "rule of thumb" regarding a "tipping point" of sorts regarding carb throat size and piston diameter. It goes something like this - as the carb gets larger in comparison to the piston diameter, then jetting sensitivity increases - and carbs which are half the size of the piston is the "tipping point".

Examples:
- 20mm and 22mm carbs on Hodie 100's and 125'x (50mm or 56mm piston dia.) - not very sensitive to atmospheric changes. (carb throat is LESS than half the size of the piston)
- 24 and 26mm carbs on Hodie 100's - starting to show some slightly increased sensitivity to atmospheric changes such as altitude, temperature and humidity (carb throat right at half the piston diameter)
- 28mm and 30mm carbs on Hodie 125's - about the same as 24's and 26's on 100's - some sensitivity, but not too bad
- 32mm, 34mm and 36mm carbs on 100's and 125's - GREATLY increased sensitivity to atmospheric changes. Tuners joke about tuning a 36mm on a 125 with comments like, "Yeah, I had the carb all dialed in - and then a guy stopped to chat and he spit on the ground next to the bike . . . and I had to start all over again." (humidity change)

In ALL these cases (all carb sizes on all Hodie engines) - if you are jetted properly at sea level and go UP to 7,500ft, the carburetion WILL be rich. The bike may run a bit ratty (small carb) or the bike may be un-rideable (carb considerably over 50% of piston diameter) - but the engine won't be damaged.

In ALL these cases (all carb sizes on all Hodie engines) - if you are jetted properly at 7,500ft and go DOWN to sea level - you'd better dang-well check the jetting before going out for a ride 'cuz it WILL be lean and engine damage could (will) result.

Ed

[viclioce]

All this talk about altitude has me confused. How much time are you really spending "Off the ground," and how much throttle are you using while in the air? Or maybe it's elevation to which you're referring? The difference between sea level & 7,000 feet is a difference in elevation, while altitude is how far you are above the current surface of the Earth. Just pointing out the difference.

I ride at elevations from 7,000 to 11,000 feet. I don't have to rejet for the 4,000 foot difference in elevation, but I did have to rejet the Wombat which was jetted for low elevation riding in California. But even if I jump the bike, I may go 10-12 feet in altitude before returning to the same approximate elevation! Victor

[Bullfrog]

Touche'
Ed

[Hydraulic Jack]

Point taken Vic, but for purposes of jetting a carburetor, altitude is the more correct frame of reference, not elevation, which I tried to explain previously. Elevation is the number of feet you find yourself at relative to sea level. Altitude is a variable that does not have a fixed relationship to elevation.

Assuming you live at, say, 7,500 feet above sea level, you will always find yourself at the same elevation every time you walk out the door and get on your bike. However, due to weather changes (barometric pressure and humidity levels) your effective elevation, expressed as density altitude, changes all the time. Altitude and elevation can be the same number if one (altitude) is corrected for current conditions.

So granted you are always riding around your neighborhood at the same elevation every day, but you are not riding around your neighborhood at the same altitude every day, whether your tires are in contact with the ground or not.

Regarding sensitivity relationships between carb bore and jetting needs, admittedly true, and also true for cars and trucks. But I might suggest that for those riding around with 36mm or larger carbs on a 125, you are not talking about your basic around town commuter bike, or average trail tuned Hodaka. Every case I can relate to where a really large carb is found on a really small bore bike, we are talking about a race bike, which I already suggested would more likely benefit from jetting for conditions. Perhaps the point buried in there is that more an engine is modified for performance, the more effort it requires to make it work. But generalities aside, my 125 with a reeded 32 Mikini has run without jetting changes at both 6,500 feet and 1,500 feet of elevation. Trail side changes included only a need to adjust the needle by leaning it out two notches to run at the lower elevation. Go figure. It should be the other way around, but barometric diffences were not measured at both locations and so can't be taken into account, and riding style was different in the two locations, with slower riding mandated at the lower elevation. My guess is that engine use (ride hard or cruise along) also has something to do with how well an engine runs at different density altitudes. We tend to jet toward an average rather than a strict target, and a carburetor is a flexible device. It is not a precision instrument.

[viclioce]

Jack, I agree with you in your statements. Just having fun with what was said.

I got this carb for my Wombat from a guy who used to frequent this forum. Talked to him on the phone several times and by email. Lots of great experience as a mechanic too! He sent me the carb I have now because the one I had was damaged & out of round. Bike runs great now and the carb he sent me doesn't stick like the old one would! He sure made me a happy camper and I am truly in his debt! Heard he also trued up my old carb & gave it to someone who needed one at Hodaka Days last year. He sure is a great guy! Victor