High altitude and combat engine

Last weekend was the NCNOC 4 day weekend in the Sierras at the Dardanelle Resort on Hwy 108. It's an annual gathering to ride high altitude passes and view the meteor shower. This year the combat engine seemed to run even lower on power than I recall previously. At close to 10,000 ft these carbureted engines loose considerable power. I'm wondering since I changed the camshaft to an original combat (previous owner had a standard cam in the combat engine) would the cam profile with a lot of overlap be more prone to power loss as it needs compression to work properly. Any thoughts? Purpose of posting this is my paranoia about the engine not running to it's potential although it runs very well at local sea level plus a few hundred feet.

Absolutely, The bigger the cam the bigger the power lose at altitude.

You would definitely be happier with a stock or webcam 12 grind for most street fun. Jim

Perhaps I am not understanding the question.
Wouldn't changing the jetting/needle position for occasional high-altitude excursions be more practical than changing the cam?
I have a Combat, live at 5K, and the bike pulls strongly up to redline with 190 mains in the Amal carbs.

Although the solution of carb adjustment is probably across board, the point is is that the radical cam is simply more suseptable.

I might think that a simple 1/2 turn out of the air screw(amal) and a notch lower of the needle should do for temporary events.

tomspro said:

Perhaps I am not understanding the question.
Wouldn't changing the jetting/needle position for occasional high-altitude excursions be more practical than changing the cam?
I have a Combat, live at 5K, and the bike pulls strongly up to redline with 190 mains in the Amal carbs.

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Two separate issues... contributing. You mention air/fuel mixture, barometric pressure changes the carbs mixing ratio. An EASY fix for a weekend in the mountains.

The cams alter the ability for the now lessened barometric pressure to (try to) fill the cylinder with fresh mixture.

tomspro said:

Perhaps I am not understanding the question.
Wouldn't changing the jetting/needle position for occasional high-altitude excursions be more practical than changing the cam?
I have a Combat, live at 5K, and the bike pulls strongly up to redline with 190 mains in the Amal carbs.

Click to expand...

A hot cammed engine can certainly be tuned to work at a high altitude but a hot cammed engine is going to be affected more by a change in altitude.
Jetting and timing will help if you are doing big altitude changes in you trip..if you don't mind roadside tuning. Making up for the loss of cylinder pressure is not so easy.
A mild cammed engine can certainly be usable over a much wider range of elevation than a hot cammed engine. I also live near 5000 feet and any direction I go will see serious altitude changes. Jim

comnoz said:

Absolutely, The bigger the cam the bigger the power lose at altitude.

You would definitely be happier with a stock or webcam 12 grind for most street fun. Jim

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That ain't gonna happen Just got the combat cam in there after trying others over the last three years. The combat engine is happiest with a combat cam.

Appreciate confirmation that the "hotter cam" is more affected by the altitude. Makes me less concerned about figuring out what's wrong with the engine. I'm losing perspective with riding the 1200 Sportster that never is at a loss for power.

illf8ed said:

comnoz said:

Absolutely, The bigger the cam the bigger the power lose at altitude.

You would definitely be happier with a stock or webcam 12 grind for most street fun. Jim

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That ain't gonna happen Just got the combat cam in there after trying others over the last three years. The combat engine is happiest with a combat cam.

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Yep, if you got a Combat head on it you will be best with the bigger cam. You will have to live with the fussiness for the high rpm rush. Jim

The combat head is probably more the reason for the altitude sickness than the cam.

Well the club only goes up there once a year. Next month's ride is mostly sea level.

Crazy Combat sez... Waaaaah ! Never done Altitude. Blasted about to L. Scugog n' back yesteday. Waaaah !

Torontonian said:

Crazy Combat sez... Waaaaah ! Never done Altitude. Blasted about to L. Scugog n' back yesteday. Waaaah !

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En Anglais si vous plai

The more over lap a cam profile the more it likes higher compression ratio for starts and idle, off idle response and low rpm non bogging but once rpm raises over mid range the bigger cam should have advantage over milder cam as can pack in more mixture. Of course all engines must lean out to match lower oxygen pressure the higher ya go till just stalls out from lack of ignition. Can search up charts for amounts to lean per 1000's ft operated in. The gravity fed low pressure fuel injector self compensate for this and therefore most popular in air craft and snowmobiles in old days.

Gosh this Commando stuff takes about a doctorate of study to cover its scope then about another decade of trial and error to put into practice. Ludwig's advance ignition for rides above the clouds is a forehead smack insight for me, though more for my low compression poor breathing over cam'd lugging use 500 to 2200 ft. Wonder which feels the attitude sickness more, the old clunkers we ride once tuned for it or the moderns digital compensated wonders?

Altitude Compensation

For our first example let's say we find a new riding area WAY up in the mountains. Our jetting is dialed in for our usual riding area which ranges from sea level to 1500 feet. Our NEW riding area starts at 4000 feet and goes up from there. Going to a higher elevation will require will require a jetting change but which way? Like our fuel density, air density can also change. Higher elevations have less air density then lower ones. At high elevations our engines are getting less air, so they need less fuel to maintain the proper air/fuel ratio. Generally you would go down one main jet size for every 1750 to 2000 feet of elevation you go up (info for Mikuni carbs). If you normally run a 160 main jet at sea level you would drop down to a 140 at 4000 feet. Something else goes down as you go up in elevation is horsepower. You can figure on losing about 3% or your power for every 1000 feet you go up. At 4000 feet your power will be down about 12%-even though you rejetted! For our second example let's say we are still at our new 4000-feet elevation riding area and a storm comes in. We head back to camp and ride it out overnight. The next day there's a foot of snow on the ground the skies are clear and it's COLD! Aside from getting the campfire going and making some coffee you should be thinking about jetting again! Cold air is dense air and dense air requires bigger jets. If the 140 jet ran good the day before you will need a bigger jet to run properly today. If the temperature is 50 degrees colder than it was the day before you can actually go back to your sea level jetting, a 160 main jet! If you don't rejet you can kiss your assets goodbye when you rebuild the seized engine. Air temperature makes that much difference!

Carburetor Jetting Technical Article Sponsor
http://www.4strokes.com/tech/howtojet.asp
Our final example will deal with something often overlooked. We are still up in the hills enjoying our NEW riding area when we notice the old fuel supply getting shorter. No biggie; there's a little store/gas station just down the road. A short trip a few bucks change hands and we are ready to go again. Out on the trail the bikes are running funny, sometimes "pinging" and running HOT. What happened?! When we changed jets to compensate for altitude and temperature we were still using SEA LEVEL gasoline. Gasoline sold at higher elevations have a different blend of additives to compensate for the altitude. Generally high elevation gasoline is less dense to compensate for less available air going into the engine and to aid starting. The lighter specific gravity of the high elevation fuel actually "leaned out" our mixture! One to two sizes bigger main jet will get us back into the hunt. If you ride in vastly different areas try to bring enough or your normal fuel along to last the entire ride. It will save you hassles and gray hair in the long run!

There's also this little gizmo of the Thunder Jet variations

http://www.thunderproducts.com/thunder_powerjet.htm

Would you trust your Commando engine to fly this? How'd they get this Triumph to run so well?
http://www.vortechonline.com/choppy/

Steve, I saw that 650 Triumph powered helo on display at Boston's World of Wheels about 1982. Everyone them commented on the unreliable reputation the engine had. I was amazed at the loooong v-belt drive arrangement for the tail rotor :shock:

As for Cdo being magical.. just another example of EVERY normally aspirated recip engine's lessened output as altitude increases.

illf8ed said:

Last weekend was the NCNOC 4 day weekend in the Sierras at the Dardanelle Resort on Hwy 108. It's an annual gathering to ride high altitude passes and view the meteor shower. This year the combat engine seemed to run even lower on power than I recall previously. At close to 10,000 ft these carbureted engines loose considerable power. I'm wondering since I changed the camshaft to an original combat (previous owner had a standard cam in the combat engine) would the cam profile with a lot of overlap be more prone to power loss as it needs compression to work properly. Any thoughts? Purpose of posting this is my paranoia about the engine not running to it's potential although it runs very well at local sea level plus a few hundred feet.

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Now that your home and all things normal does the bike run OK ???.... Paranoia and toooo much information can maybe be a problem...
So where are the photo's of the "Dardanelle Resort on Hwy 108"...

Just a moot point, if you are going to weaken your carb mixture as you go higher and higher, don’t forget as you come back down to sea level you will have to richen the mixture at the same stages or you might possibly hole/eat a piston :!: :shock: :roll:

Holy smokes hobot, If I were ever to get stuck riding with a guy going by THUNDERPRODUCTS guidelines I'd either shoot him, myself, or more than likely just run away. Sounds like on anything but a flat road and a perfectly stable day, he'd be pulling over every 20 mins. or so to screw with his carburation. "Oh Oh, hill ahead, gotta change my jets. Oh Oh, now we're going down into a valley, gotta change my jets. Getting a little chilly mate, don't you think? Guess I'll put on a sweater and change my jets."
" Par-a-noia runs deep, into your life it will creep "- Buffalo Springfield.

Biscuit I don't completely understand how the ThunderJet automatic altitude compensating gizmo works but they been offering them for couple decades at least so seems the snowmobile runners like em well enough to stay on the market. Seem like carb would be set for highest altitude leaness then the ThunderJet adds richness below that height. Flying is always leap of fate but especially so with a hi tuned Brit Iron engine which is known to lower its time to over haul, in shop or in flight... Heilicopters do have some safety factor with auto rotation landing under no power but it many not be a very soft one. Would imply 650 Triumph owners have more faith than 750 Norton owners. Carbs can freeze up and clog in cold humid air yet carb heat richens the mixture so there are complex rules of thumb to apply in light aircraft to avoid power failures.

Biscuit said:

Holy smokes hobot, If I were ever to get stuck riding with a guy going by THUNDERPRODUCTS guidelines I'd either shoot him, myself, or more than likely just run away. Sounds like on anything but a flat road and a perfectly stable day, he'd be pulling over every 20 mins. or so to screw with his carburation. "Oh Oh, hill ahead, gotta change my jets. Oh Oh, now we're going down into a valley, gotta change my jets. Getting a little chilly mate, don't you think? Guess I'll put on a sweater and change my jets."
" Par-a-noia runs deep, into your life it will creep "- Buffalo Springfield.

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That product was designed for two-stroke snowmobiles, much more sensitive to atmospheric pressure change, playing in the mountains and valleys, -20F in the morning, +30F in the daytime. It makes a TON of sense. A great tool for a tuner, and also before OEM's had HAC, DPM, and finally fuel injection.

Never seen one on a bike.