Nortorious
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- Joined
- Mar 28, 2021
- Messages
- 126
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Hey folks, hope you're doing well and are healthy. I have a 73 850 Commando relatively new to me and am trying to sort out which jet size is best. I live at 5,400 ft above sea level and typically ride at 5,000-9,000 ft. above sea level.
I recently took the bike to a classic British bike expert in Denver and they swapped out my jets from 260 to 190 since the bike came here from southern California. Since this is a reputable shop that takes care of Nortons at altitude I figured they would know. But @MexicoMike commented that 190 may be too lean and it got me thinking..
I found the following reference in one of my books (see image):
"As a general rule, main jet size should be reduced approximately 5% for each 3,000 feet (900 meters) above sea level.
The bike would stall when warming up (it has no choke) and would even stall while idling with a hot engine. I'm pretty sure the main jet wouldn't cause that issue, so I adjusted the idle screw and air screw and it idles nicely now.
Honestly the bike pulls like a freight train and feels good riding. I'm new to Nortons though so I can't really say if these jets made it worse or better. I hadn't ridden it much at all before the change.
I'm running the dual Amals. The previous owner had bigger pistons installed, so I'm not sure if that comes into consideration. The airbox is original.
So does anyone have experience with altitude that knows the ideal jets? Or a suggested process to figure it out? I'm a novice when it comes to anything but cleaning carbs. I've read Bushman's carb tuning guide but have limited experience tuning, especially changing jets.
Thanks for the help!
I recently took the bike to a classic British bike expert in Denver and they swapped out my jets from 260 to 190 since the bike came here from southern California. Since this is a reputable shop that takes care of Nortons at altitude I figured they would know. But @MexicoMike commented that 190 may be too lean and it got me thinking..
I found the following reference in one of my books (see image):
"As a general rule, main jet size should be reduced approximately 5% for each 3,000 feet (900 meters) above sea level.
The bike would stall when warming up (it has no choke) and would even stall while idling with a hot engine. I'm pretty sure the main jet wouldn't cause that issue, so I adjusted the idle screw and air screw and it idles nicely now.
Honestly the bike pulls like a freight train and feels good riding. I'm new to Nortons though so I can't really say if these jets made it worse or better. I hadn't ridden it much at all before the change.
I'm running the dual Amals. The previous owner had bigger pistons installed, so I'm not sure if that comes into consideration. The airbox is original.
So does anyone have experience with altitude that knows the ideal jets? Or a suggested process to figure it out? I'm a novice when it comes to anything but cleaning carbs. I've read Bushman's carb tuning guide but have limited experience tuning, especially changing jets.
Thanks for the help!