Altitude Based Carb Adjustment

[JC85]

I'm planning o take a trip to see a friend in Colorado, at the end of the month. I haven't taken the old girl ('74 Eldo with Dellorto carbs) into high altitudes, before, so I'm wondering what to expect. What should I be on the lookout for, and what should I adjust, in order to keep her running well, as I get up into the mountains? Any tips or advice would be greatly appreciated.

[Testarossa]

What size carbs? What's your air filter system like?  If you're using a stock air filter you can drop your main and idle jets a size, but if you're running a looser air filter you are probably already running richer jets and may be able to go back to stock. Assuming you're happy with the way it's running now and have clean plugs.

We can make more specific recommendations if we know more about the carbs.

[oldbike54]

 It will start going a bit rich at about 4,500 feet of elevation , getting more so as you go higher . Unless you are going above 7,000 feet I wouldn't worry too much . You can drop the needle a notch to compensate if it starts loading up .

 Dusty

[lucian]

With stock jetting and airbox you'll likely have no issues.  Remember these things are farm tractors and tuned to +or- 3,000 ft of elevation. If in doubt a WOT has no preference
Long live FI  !!
 Hope you have a great trip!!

[JC85]

Thanks for the info, guys! To answer the question that was asked, the carbs are Dellorto 29 C, running stock jets, but the airbox has been swapped out for can filters.

[lucian]

Whats Denver elevation?  Maybe 4 to 5 thousand feet?  My guess is you and your  bike will be very happy!!

[alanp]

Whats Denver elevation?  Maybe 4 to 5 thousand feet?  My guess is you and your  bike will be very happy!!

Officially, Denver is 5280ft, the "Mile High City".  But Denver is not in the mountains, it is on the plains.  A few miles west and you are 7500 feet, a few miles more, 10,000.  You can't really ride in the mountains and stay below 8000 feet. 

[LongRanger]

Denver is a bit over 5000’, but Denver isn’t the mountains. Depending on where he rides, it’s possible he remains above 10,000’, and can go as high as 12,000’, I sold my carbureted SV650 years ago because the fueling couldn’t cope with those elevation changes during a single days’ ride.

[Testarossa]

So the question is whether you plan to ride west of Denver, over the passes. Top of the Interstate is 11,000 feet at the tunnel and Vail Pass. Top of Loveland Pass and some of the other paved two-lanes is 12,000 and change. Flat roads in the high valleys run 8000 and 9000. You'll likely run sputtery rich above about 7000. First adjustment as Dusty points out is to drop the needle a notch. The problem is running uphill at full throttle and not getting the power you need. If that's the case, you need to drop the main jet a size or two. If you're running a 45 main jet at sea level, go to 42 for the high passes. One of the nice things about the way Guzzis are laid out is you can change the mains without even dropping the float bowls. 

[moto]

... The problem is running uphill at full throttle and not getting the power you need. If that's the case, you need to drop the main jet a size or two....

Good advice, but don't expect a full restoration of power. The lower air pressure up there results in less oxygen in the cylinders and dramatically, proportionally less power even with the correct mixture. This is quite noticeable even with fuel injection, as I found riding a rented Indian Scout up to Cripple Creek, Colorado. (What a pig it became!)

The greater danger is running a high-altitude-adjusted engine at lower altitudes, where the too-lean mixture can cause overheating. So if you're going to change jets on the way up, you must not forget to change them back on the way down.

I personally wouldn't bother with rejetting for brief runs at high elevation unless I experienced missing due to plug fouling. Only then I would take smaller jets out of my pocket. In case you need to do this, make sure to put them in your pocket before you leave home, along with a spare set of spark plugs.

Moto

[Chuck in Indiana]

Pilots know that full throttle at 7500 feet equals 75% power on a naturally aspirated engine. It goes down rapidly as altitude increases.
Than said, I ran the Lario and MZ at 10000 feet with no jetting change and they were ok. It's not like you are at those altitudes forever..

[Ted N]

I ran a Cali 1100 at 10 to 12,000 for years with no problems except for a reducttion in power. I would not make changes just for a trip.
Ted

[Tkelly]

Rode my SP all over Colorado and it never missed a beat with original set up.

[LowRyter]

Independence Pass 12100 ft.  It's "mostly" down hill from there.   My EV with the Mrs did fine





Cottonwood is a little higher. 

[LongRanger]

I’m impressed. That Independence Pass sign usually has throngs of tourists hanging all over it, trying to get a photo op. You did well. Cottonwood was recently paved on its west side. Fun road!

[Moparnut72]

I worked in a bike shop in Denver in the late 60's. The only bike I had to rejet for a guy coming from back east was an H1 Kawi triple. It waskind of OK in town but he was heading to Cali so to get over the mountains I leanedit out a bit. Warned him to rejet once he got over the hills or he would damage his engine. I showed him how to do it.
kk