HP loss vs altitude:
HP Loss = (elevation x 0.03 x horsepower @ sea level) /1000
Approx 3% per 1000 ft

Dyno run data showed 34 HP at 135' above sea level (Hope)
Power loss at 5300' approx (our max alt ride) = -5 hp (28 hp)
CO

Interesting. Did you re-jet the carb for the different altitudes?

No, I jet it for up high because that's where most of our riding is done anyways.
Then it might be a bit lean at the bottom.
Run Avgas and you're golden.
CO

High octane fuel won't compensate for an improper air/fuel ratio. I will prevent detonation, yes. But altitude changes require adjustment to A/F ratio because of air density. EFI can do that automatically with sensors but with a carburetor you have to adjust manually. If you're jetted for 5000 ft it's not going to be the ideal jet for sea level.

Yup -- and I won't argue with anything you just said.
The issue is that years ago when I first got these goddamn things I kept blowing up engines.
I melted at least 9 pistons before I figured it out.
The cure all was Avgas. It allowed you to be miles off on jetting and still not hurt the Engine. It had a side benefit of the Engine running cool. I could probably run pump gas on two of my machines but I'm just tired of rebuilding engines.
The other issue is that we start at almost sea level but almost instantly, about 2 or 3 miles, are up high. The roads around here are steep. Straight up in many places. You're over 4000' most of the time.
What I should do is mix the Av and pump to get max benefit out of the fuel. I don't do that because it's a pain in the azz and the airport is about a mile from my house. I just go straight to the pump and fill my jerry cans. Just plug in my credit card and go. Nobody bothers me. It's a small town airfield and it's a free for all.
CO

I get ya. I guess there's no easy way around your situation with those drastic changes in elevation. So do you tune for the high altitude, sea level, or somewhere in between? If at some point you're lean, can you just pull the choke out a bit to compensate? Do you run EGT gauges on any of them?

That sounds like some awesome terrain btw -- and definitely not the kind of place where you want to blow an Engine and have to tow. We don't have anything like that like here in Pennsylvania. I'd love to check out where you ride on Google Earth.

So are your engines worked over? 34hp at the wheels is more than stock, right? What do you think stock WHP is?

1) I jet for the top. The reason is that most of our riding here is up high.

2) Do you run EGT gauges on any of them? ---- I am NOT a fan of gauges. I have several reasons. 1)There is no issue with putting a EGT gauge on a machine for tuning but after that it's a no fn good. I used to race formula cars and GT3 cars in SCCA. There was NO fn time to look at gauges. All you are going to do is crash your car or buggy. Keep your head up and both hands on the wheel unless you are shifting. 2) By the time you do notice the exhaust temp go crazy it's to late. Unless of course you are staring at it all the time. Then reason 1 above comes into play. 3) I had put a EGT on my brothers machine when I first bought it. Could not keep this Engine together. My brother said he was staring right at the gauge when it melted the piston and he said the gauge never went over 1200'F. It was a brand new gauge and sensor in correct position in pipe. Obviously things happen quickly. Found out this Engine was worked and we were running pump gas -- dummies.

3) 34hp at the wheels is more than stock, right? What do you think stock WHP is? --- That is the stock HP I got when I built my homemade dyno. I did a thread on it here: viewtopic.php?f=1&t=19665&hilit=dyno
The video of the clutch explosion I deleted from youtube but I can repost it for you if you want.

4) Google Earth Hope BC Canada.
Places we ride are Eureka mine, Caroline mine, Hunter creek, Sowaqua valley, Sourbie valley. Not sure if those names will come up on google earth but that's the names here.
Those are most of our rides because we know the fuel will last.
CO

Bah, I had a reply typed in and when I went to post it said I had to log in again and I lost it :P

I'll type it in again later when I have some time...