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How sensitive is jetting to altitude??

  • Thread starter Thread starter Darin Jordan
  • Start date Start date
D

Darin Jordan

Guest
My long ride this past week took me over several mountain passes (Snoqualmie 3500', Blewett 4500', Chinook 6500'), as well as through temperature swings from 45F up to 96F...

I have the 165 mains that Dynojet recommends, and I suspected that they might be a tad rich based on driving around town here, but figured that it would really show itself in the mountains, so I took my jetting kit with me.

Sure enough, over Blewett Pass, I could barely manage 75mph when running up the last couple of miles to the summit.

When I got to Wenatchee (my brother's house to spend the night), I rejetting to 160s. Felt way better around town, and felt really good on the flats runing over to Idaho on Hwy 2 and I-90.

Coming back on I-82, as I approached Yakima and the temps started getting warm, I noticed that the stumbling was back when I'd crack her open just past the 3/4 throttle point. Sure enough, as I went over Chinook Pass, I really noticed the stumble at these throttle positions.

Once I got back down to Enumclaw (about 500' above sea-level), the bike ran strong again, with only a SLIGHT hint of the stumble, right around the 7,000 RPM mark at 3/4+ throttle.

SO, my question is this... HOW sensitive to altitude should these bikes be, and is it possible to jet to have it work well down here, and up there both? Seems like I might be compromising my "down here" performance to make it work well up there, and vice versa.

OR, is this a needle position thing, seeing as it seems to run strong RIGHT up to that 3/4 throttle point, even at altitude??

Interesting note... at the summit of Chinook pass... the idle dropped by about 300 RPM and it almost wouldn't start again... I suspect this is farily normal??
 
Hi,

If your carbs are setup properly they can cover a wide range of altitudes. I have stock everything on my bike. I sync'd the carbs and set the idle mixture screws before I left for the 6th Annual Western States GSR Rally at renobruce's place. I live at about 1200'. During the weekend I traveled over several summits from 4500', 6000', 7000', and Tioga pass at over 10,000'. My bike ran fine with just a little loss of "oomph" at 10,000'. I had to make no jetting changes.

It could be that the Dynojet kits are a little more sensitive to altitude.


Thank you for your indulgence,

BassCliff
 
I just got done with a 1500-mile trip on ranging from 5,800ft. to over 10,000ft. and then back to around 750ft., and not one hint of problems.

...though like Cliff, my bike's not modified/jetted...
 
If your bike has CV carbs with the stock airbox, it compensates for elevation changes really well. Mine all run fine at 14,000 ft.
A little down on power, it may idle too slow, but you can ride it normally.

If you have VM carbs, you will be struggling, starting at about 4,000, and it gets worse as you go up.
 
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