High altitude and combat engine

OK - got the point that Combat engine prolly more sensitive to altitude changes.
Given that... would just changing to larger main jet when going to lower altitude be useful?
Or would I need to go deeper and change needle position / pilot jet too?
My Amal Premiers are stock except for 190 mains right now.

Not looking for peak performance, just don't want major problem to result.

Our kissing cousin Harley has vendors supplying a type of ThunderJet 3rd external fuel circuit that enrichens for extra power when carb can't compensate well other wise.
http://www.zippersperformance.com/produ ... carbs.html

ThunderJet™ is a jet-able, externally mounted third fuel circuit that improves the performance of 2-circuit carbs such as the S&S Super. Unmodified, these carbs typically have a low speed, or intermediate, circuit that supplies fuel from idle to approximately 2500 rpm, at which point the carbs’ main jet circuit becomes active, delivering more fuel to the engine. These two circuits must then supply fuel for the rest of the rpm range. The problem is: the remaining rpm range is too wide (typically 2500-6500 rpm) for only 2 circuits to handle efficiently. The tuner generally encounters problems jetting the carb to give good, crisp mid-range response and still have strong topend power. A compromise is the result. Back the main jet down, carburetion in the mid-range is good but top-end is lacking. Increase the main, top-end improves but now the mid-range is rich; flat spots or hesitation is encountered. The answer? ThunderJet™!

My Norton lives at around 7400 feet with normal rides down to around 6500 and up to 9500+ One thing that helps a bit performance-wise at altitude is advancing the spark a few degrees. However, if the bike normally lives at sea level, I wouldn't do a thing to it unless you are planning to stay at higher altitudes for an extended cruising period. IOW, if you were going to be at those altitudes for several weeks, I would make some adjustments. But if you are just going up/down mountain passes on a day's ride, I wouldn't bother.

mike996 said:

My Norton lives at around 7400 feet with normal rides down to around 6500 and up to 9500+ One thing that helps a bit performance-wise at altitude is advancing the spark a few degrees. However, if the bike normally lives at sea level, I wouldn't do a thing to it unless you are planning to stay at higher altitudes for an extended cruising period. IOW, if you were going to be at those altitudes for several weeks, I would make some adjustments. But if you are just going up/down mountain passes on a day's ride, I wouldn't bother.

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Exactly. I have no plan to make any adjustment for high altitude as I'm only there a couple of days a year. It's the first time riding a full combat spec at this location. Have had a '73 750 up there many times in the 80s and '74 850 JPN as well. They loose some power, but not as noticeable as the combat last week.