Good Morning America. I'm looking for advice on jetting my 1980 DT-175. I'm giving the bike to one of my kids, and he lives 3827 feet lower than I do. I'll change the jet before putting the bike on a pallet, and add some just in case I miss it a little.

Is it stock?  Has jetting been modified from factory specs?  Why not just give it to him with factory jetting?  ...I guess unless you live at 12,122 feet and he lives at 8,295 feet 

Otherwise you can use the Mikuni jetting calculations/charts which should be in the Mikuni tuning manual in the Tech Library...  pilot, main, and needle position may all need changing...  not just one jet.

 

You don't mention what either altitude is.
If the motorcycle is completely stock including jetting then you probably will not have to make a change.
If it was rejetted for your altitude then change it back to stock.
You have to richen thing up if you are jetted for a high altitude and go to a significantly lower altitude, such as your case.
Here is some info from an old book, "Motorcycle Tuning For Performance":

  

JayB, If I remember correctly, that book has a RAD chart inside the back cover and the instructions in the text on how to use it to figure out jetting adjustments? 

You remember correctly!  Here are the next 3 pages(16-19) and inside the back cover.  Since the site compresses the pictures, if these aren't readable I can send higher resolution pics to anyone. 



 

 

 

Thanks. Very informative. I appreciate it.

All this talk of percentages. I though the rule of thumb was change the jet one size for each 1,000' of elevation change, or each 10 degrees of ambient air temp.
Am I wrong? Not nearly as precise as the above, I guess, but that's what I thought I heard in Moab, Ouray, and other rides...

Just as a disclaimer, I have never rejetted for altitude or competitively,  So my opinion may be worthless!

  In this case I think it would be about half of the "one jet size per 1000 feet".  If you were running a Mikuni main jet of 270 at sea level, using your rule of thumb you would drop down to a main jet of 200 at 7000 feet.  That is about a 25 percent reduction in fuel delivery from the main jet.  According to the following pages you should reduce by about 9 percent to a 240 main jet and test from there.

  Sorry, I repeated the percentages!  I would always be more careful going leaner!

 

 
 

Sorry if the attachments are unreadable.  Here is a second try.And if you start at a 170 main jet, and your step sizes are 5, your "one size per 1000 feet" is not as far off.  I would still be careful going leaner.