- Joined
- Dec 28, 2008
- Messages
- 2,035
Whether your motorcycle has an SU (vacuum slide) or an Amal/Mikuni (cabled slide) they all get the same vacuum signal from the engine. At sea level the vacuum signal pulls in air that is more dense than is available at, say 5,000', given that other atmospheric conditions like temperature and humidity are similar. Motorcycles with intake and exhaust plumbing and matched cams that enhances cylinder packing will be more noticeably effected by changes in altitude.
So, you select your jetting package for the altitude that you spend the most time riding in. The higher you operate at the less fuel you can burn optimally; the vacuum signal will pull the same amount of fuel at seal leave as it will at 15.000' out of the fuel issuing orifices.
You install smaller jets for higher altitudes, you are still looking for the same optimum A/F ratio as at seal level, but when get to 10,000' your engine will produce less power, still running great.
Some carburetors have more "bandwidth" than others, some have greater adjustability, some are cheap, some are expensive. I spent some time on the NOC website and read about Mikunis, Amals and SU carbs. The big Mikuni complaint was that it had too many jetting combinations. The Amal's were heralded and cursed for being primitive. SU's were difficult to install without hitting the backbone.
Be the vacuum signal...
Best.
So, you select your jetting package for the altitude that you spend the most time riding in. The higher you operate at the less fuel you can burn optimally; the vacuum signal will pull the same amount of fuel at seal leave as it will at 15.000' out of the fuel issuing orifices.
You install smaller jets for higher altitudes, you are still looking for the same optimum A/F ratio as at seal level, but when get to 10,000' your engine will produce less power, still running great.
Some carburetors have more "bandwidth" than others, some have greater adjustability, some are cheap, some are expensive. I spent some time on the NOC website and read about Mikunis, Amals and SU carbs. The big Mikuni complaint was that it had too many jetting combinations. The Amal's were heralded and cursed for being primitive. SU's were difficult to install without hitting the backbone.
Be the vacuum signal...
Best.