MPG- Twin Premiers vs single 34 Mikuni

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The SUs would if taken up high attitudes be adjusted to a weaker setting by a perfectionist and who would do the same again making mixture richer coming back down again. Older aircraft have a mixture control setting in the cockpit .
So the SU's don't compensate for altitude (at least to some degree) on their own?
 
So the SU's don't compensate for altitude (at least to some degree) on their own?
Maybe, or maybe not, but going UP every 1,000 feet the engine would benefit from a slight adjustment of a flat or two to compensate the jet for the lower air up there . . . . and the same again on the way down or it WILL be too weak!!!!¬
 
Re the air-valve carb, the ratio of the slide opening to the amount of fuel being supplied stays essentially the same at altitude as it would at sea level. At higher altitudes, the slide simply does not open as far as it would at sea level with the same throttle setting. Since the amount of fuel per movement of the slide is determined by the jet/needle, if the carb is jetted for sea level, it's still going to be too rich at altitude without an appropriate change in jetting/needle, etc.
 
Re the air-valve carb, the ratio of the slide opening to the amount of fuel being supplied stays essentially the same at altitude as it would at sea level. At higher altitudes, the slide simply does not open as far as it would at sea level with the same throttle setting. Since the amount of fuel per movement of the slide is determined by the jet/needle, if the carb is jetted for sea level, it's still going to be too rich at altitude without an appropriate change in jetting/needle, etc.

Not sure I should bow to your superior knowledge- why? The carb jets are set for either at sea level, or for that ***** ft attitude, for the higher or lower attitude it is adjusted to suit, The SU would not be set up to perform at say 10,500 feet if it was set at sea level ! or are you trying to tell me it would?
 
Thought I read somewhere that Norton was considering using SU carbs at one point (towards the end).

If I could find an SU conversion kit for my commando I wouldn't hesitate to give it a shot. Heavier oil in the chamber would deal with vibration, although I'd imagine space for such a carb being an issue...

Currently have a vm34, started first kick last weekend after sitting for 2 months, still runs a bit rich...
 
Not sure I should bow to your superior knowledge- why? The carb jets are set for either at sea level, or for that ***** ft attitude, for the higher or lower attitude it is adjusted to suit, The SU would not be set up to perform at say 10,500 feet if it was set at sea level ! or are you trying to tell me it would?

NO, just the opposite, I'm saying it would NOT perform properly at altitude if jetted for sea level. I'm sorry for my explanation being unclear. It made sense in my head when I wrote because I know what I mean but it didn't come out properly in print. :(

The main thing is that the carbs have to be re-jetted (leaner) at altitude, regardless of the type of carb.
 
Thought I read somewhere that Norton was considering using SU carbs at one point (towards the end).

If I could find an SU conversion kit for my commando I wouldn't hesitate to give it a shot. Heavier oil in the chamber would deal with vibration, although I'd imagine space for such a carb being an issue...

Currently have a vm34, started first kick last weekend after sitting for 2 months, still runs a bit rich...
Thought I read somewhere that Norton was considering using SU carbs at one point (towards the end).

If I could find an SU conversion kit for my commando I wouldn't hesitate to give it a shot. Heavier oil in the chamber would deal with vibration, although I'd imagine space for such a carb being an issue...

Currently have a vm34, started first kick last weekend after sitting for 2 months, still runs a bit rich...
Yep if you Google "Norton 76" you will see the commando with mag wheels,SU carb etc
Cheers
 
Yep if you Google "Norton 76" you will see the commando with mag wheels,SU carb etc
Cheers

I see... Don't know how I feel about that bike... But that's a conversation for another thread! Thanks for the tip
 
In the UK where i live just above sea level the opportunity for varying the altitude is limited to about 1220 ft on the highest motorway or about 2199 ft on a mountain pass in Scotland.
Taking my SU equipped MK3 up the classic mountain passes in Switzerland and Austria didn't affect performance as far as i could tell. Maybe i was enjoying the views to much to notice!
Economy wise 80 MPG (imperial gallons) and at least 240 miles range before anywhere near reserve.
And she will still top the ton (not at 80 mpg though!)
 
Generally, most carb-equipped engines were considered to operate normally with standard jetting from sea level up to 3000 feet. Above that for long-term use, not just a trip that included climbing to high altitude and back down, jetting changes were usually recommended.

I had to drop two jet sizes, from 260s to 240s when I moved my 850 from sea level on Long Island NY to 7400 feet in Mexico City. Friends here with SU-carbed bikes have them jetted similarly "lean" compared to sea level.
 
Oxygen levels are less with less air pressure, but cold air is denser. If you go higher on a hot day . . . ?
 
I had thought the purpose of the vacuum activated opening on the SU was to accomodate changes in air pressure. Low air pressure and the intake suction wouldn't pull up the needle (or whatever) so much, thus restricting the fuel flow to compensate for thinner air. Here's another technique that worked as a stop gap measure when visiting high altitudes. A little above Denver my old Buick was running poorly. You could tell by the exhaust it was way too rich. A guy at a garage told me if you were just visiting for a short timme you didn't have to re-jet. Just advance the spark (turn the distributer until you heard a little detonation pinging). It worked. We drove right up to a visitors building at the top of some 12,000 ft mountain -- having to pass a muscle car of the day that was revving the hell out of it about the 8,000 ft leval and lunging foreward about two yards. Over and over. I wondered whether he was going to rip out his transmission first, or give up and turn around. Got to the top and went back down and never saw the guy again. Maybe he ripped out the transmission -- and then backed her around to drift all the way down. I do want to look at the SU equipped factory Norton though.
 
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