KDXRIDER.NET

I'm going to enter my first race next week. We'll be in Monticello, Utah which is at 7,000 feet. The high temp for that day might be 85F which is a density altitude of about 10,000 feet.

My modded KDX (carb, head, VF3, PC2 or FMF pipe) is currently jetted for Phoenix in July (1,200 feet and 115 F). 148 main, CEK-3, 35 pilot. Air screw variable. Maxima 40:1 with 91-Octane (that's common in Phx).

I rode the bike (jetted fatter) a few weeks ago up in Flagstaff under similar temps as what we'd expect in Utah next week. At the time, it was 150/CEK-3/38. It ran okay, but likely not optimal, natch. No spoo to speak of (I'm aware of the "don't jet for spoo" axiom).

I don't have any leaner jets, so if I were to take advantage of the Saturday practice (Sunday race), I'd have to know what jets to get. Any ideas?

Thanks,

Rick

Rick, I'm guessing you're hoping that someone at those conditions will give you some feedback and that you've already considered some of the handy-dandy charts out there that help give us a sense of the magnitude of change . . .

but in case you haven't check out the "Jetting Correction Table" about 3/4 down the page in this link for some ideas:

http://www.dirtrider.net/justkdx/carbtuning.html

Your Flagstaff jetting sounds like it would've felt fat. For comparison purposes, I'm at <1,000 ft and 85-90F and running a 148MJ and a chop shows things are dandy. Everything else: 38PJ, CEK-3, 93-octane w/ a 32:1 premix. I'm RB'd, too; 'got 607 reeds and the rev-profile FMF, though. O2 concentration would thin out more significantly with the altitude than it would increase with the temp drop, so it would seem that you would need to lean things up a little moving from the Phoenix conditions to the Utah conditions. A 145 for a main perhaps. The PJ already sounds lean but I suppose that's the DFIII (?)

I do have a 145 MJ. That should help. Since I typically do slow single track, I've found that the 35 PJ works pretty well when the temps get around 115.

When I use a 38 in this heat, I find that I can't use the choke to start it. When I began using the 35, I had to pull the choke out for the first kick of the day.

I only use this jetting (148/35) for June and July. After that, the rains come and cool things down to about 105F.

Rick

I can't begin to imagine riding slow single tracks when it's 115F . . .

seriously - how do you get a break from the heat? Surely you don't venture far from your vehicle . . . I suppose not having a Camelbak would be downright dangerous.

We complain when it's much above 90F !! 'Course it IS more humid than in Phoenix and that raises the misery index.

But 115, sheesh . . .

I guess Utah will be heaven, good luck in that race BTW

Ya know Rick....I hate you. I was looking forward to that race for so long and then found out I couldn't go! Half off-road and half MX sounds like tons of fun, alot like those WORCS races.

It's really too bad you guys couldn't come up here again due to forest closures. Monticello is very similar to up here both elevation and temp wise. Oh BTW, the way Phoenicians ride when it's 115F is drive 2 hours north and ride in Flagstaff. :)

Whoa sometimes I don't even ride if it's gonna be above 90! Have fun in the AZ heat bleak

Comparitively it's not even that hot where I live, Northern Arizona in the mountains is much cooler. A normal high here is in the 80s. Last week though was hot, a record breaking 97 up here and IIRC it was over 120 in some places in the valley.

The race on Sunday is going to be hot as well, I wouldn't doubt it if it reaches into the upper 90s in the cinders. There is some hope of rain, which would be awesome but fills the whoops up with water and makes the riding like jumping into puddle after puddle for 60 miles straight.

Jeb wrote:I can't begin to imagine riding slow single tracks when it's 115F . . .

Uh, it's a "dry heat" ??

Like a convection oven.

I can't imagine riding in 100% humidity. We're usually around 10% humidity ("relative" in every sense of the term). The only evidence of sweating is a salt ring around your sleeves. We are never without a full 100oz Camelback, even in the winter.

To be honest, the only time I ride when it's above 110F is in the early evening as the sun is going down and we're at the track (cooler of ice water nearby). We usually try to be RTR by sun-up (5 to 5:30am), but even then, the temps are easily above 85F and sometimes around 95F. By the time we get done riding at 7:30 or 8, the temps can climb above 100F.

It ain't easy jetting for Phoenix.

Rick