Landracing Forum
Tech Information => Technical Discussion => Topic started by: jim reed on March 31, 2015, 03:18:20 PM
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I am finally going to the AMA motorcycle speed trials at Bonneville now that I am retired and have the time. My question is where should I start with jetting. I will be running a TZ-350 Yamaha 2-stroke. I've owned and road raced the bike for 35 years so I am completely familiar with the machine in that setting. I did run it several times at Maxton with the ECTA and there my Daytona jetting worked fine. I realize that at 4200ft the bike should run rich so it would seem that I would need to lean out the jetting. Correct?
Jim Reed
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I'm coming to Bonneville for the first time. I will be running a TZ-350 Yamaha in the msp-350 gas class. I have raced the bike for35 years and ran it with ECTA at Maxton where my Daytona jetting worked fine. At 4200ft I presume I need to lean out the jetting correct? At the same time WOT for several miles builds a lot of heat so should I leave my Daytona jetting in it? I just want to get a ball park idea so I don't needlessly seize the engine.
Jim Reed
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Why post twice? You won't get any more help and it just clutters the board. :x :x :x
Pete
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Sorry my screw up. Thought I could post on 2 different places on the forum.
Jim Reed
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Jim,
Not knowing how you measure or track effective air density, I can't gear my response to your technique. As a starting point air density at Bonneville is ~ 16% lower that at sea level. So I would start with 10% less jet AREA. Then read plugs (you strike me as an "old school" type). If you have equipment to measure atmospheric conditions all the better. Hope this helps.
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If you have equipment to measure atmospheric conditions all the better. Hope this helps.
It varies wildly over the day - temp, humidity, barometric pressure, but the timing slip has that information on it. :wink:
If you can change out jets while you're in line, have one of your crew head over to the timing booth where they hand out the timing slips and ask a competitor what the adjusted altitude is, and then jet accordingly.
Numbers are tools. :cheers:
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Don't wait for a timing slip: Kestrel 4000 Pocket Weather Tracker
http://www.delawarechassisworks.com/?page_id=332
It can actually be fairly predictable some times! Temperature is the prime driver of density altitude - they are joined at the hip. It can range from 6000' to 8000' in a day. :-o Then barometric pressure can confound what you got the previous day or the next day. :-(
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Jim,
Not knowing how you measure or track effective air density, I can't gear my response to your technique. As a starting point air density at Bonneville is ~ 16% lower that at sea level. So I would start with 10% less jet AREA. Then read plugs (you strike me as an "old school" type). If you have equipment to measure atmospheric conditions all the better. Hope this helps.
x2
Jim,
Whether your tune at your normal density altitude is "tight" or "generous", when you go up in altitude you will typically be rich(er).
One of the things I advise carbureted guys to do is to calculate the difference in jet area, from one jet step to the next. These steps will probably NOT be uniform.
You will then need to calculate the density altitude for your regular conditions. Then figure that the D/A at B'ville varies, from say 5500 to 8000+
Calculate your % change and you need to change the jet area somewhat less, say 5% or so. Because of the varying conditions you will need to have a selection of jets in your tuning package. Change and tune in conservative steps. A rich running engine is much better than none. Checking your plugs after jet changes is a requirement to keep your engine intact.
:cheers:
Fordboy
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Check out this guys technical posts and several calculators.
You can plug in numbers and see the results of changes in temperatures, pressures, humidity etc. His calculators cover just about any approach to the topic you might be interested in regardless of the source of your numbers. Note carefully that density altitude DOES NOT directly correlate to fuel air mixtures, it was intended for aeronautical applications (ie will the plane get off the ground with this load), the actual oxygen content of the air can be significantly different depending on why the density altitude is what it is (ie humidity, or pressure or temperature). High humidity lowers oxygen content but raises density altitude due to water vapor being lighter than oxygen and nitrogen.
http://wahiduddin.net/calc/calc_da.htm
http://wahiduddin.net/calc/calc_hp.htm <--- especially useful for planning your tuning changes
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Setting the jetting on a dyno at home and adjusting the jetting for other places using a jetting slide rule or nomograph works for me. Mikuni makes the slide rule.
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Why post twice? You won't get any more help and it just clutters the board. :x :x :x
Pete
Sorry for being so crusty this morning. Must have got up on the wrong side of the bed. Good luck with your project. You will find lots of help on this board and the odd member who's a little out of sorts!!! :-D :-D :roll:
You actually did originally post in two different areas and one of the mods must have consolidated the two postings to the one area.
Pete
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"Tweren't me -- really. Welcome, Jim.
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SSS You been twerking again???
Just checking to see if Tweren't-ing is some sort of a related activity?? :-D
Rouse
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Johnnie, I probably wouldn't know a twerk from a jerk. I've heard the word enough times to use it as a goofy word, but don't watch TV so don't know what one looks like. And if someone is "twerking" at one of the bar/dance halls we visit -- well, again, I wouldn't know it. Maybe it's something unique to polka music -- and we go mostly to Blues bars. Yeah, I betcha that's it. :-o
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After all the paper work and your rookie orientation is done head on over to course 4. This is the same course your did your rookie orientation on. On the way over stop at the timing slip tent and get the info the guys are talking about. Jet accordingly- on the rich side. Make an easy pass and see what things look like. Make as many passes as you wish. When you and your machine are happy go to another course and get a record.
Good luck- Have fun!
Steve
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Thanks to everyone one their generous and informative replies. Yeah I'm "old school". My effort is not called "Ludite speed" for no reason. Will definitely try some of the computer based suggestions to enter the 21st century.
Regards, Jim Reed