Jet size to use at altitude?

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!

From http://amalcarb.co.uk/downloadfiles/amal/Mk1_Hints_and_Tips.pdf

You could do worse than ask Matt Rambo (@cNw) as most of his "running in" is done at altitude and at least some of the bikes will have had twin Amals.
Cheers

Show us your plugs

gortnipper said:

Show us your plugs

Click to expand...

That only really helps if you know what the (constant) throttle opening was at the "chop".
Otherwise will be a muddle of mainly idle circuit.

I think you should check with Matt or run it with a fuel mixture analyser before you use it too hard. Especially if hot.

That seems lean to me. I used 190 jets on my 500 race bike at 1500 ft. Mk 1 concentrics. Based on dyno runs with mixture analysis.

From the Riders Handbook for 1970 BSA Victor Special. Concentric Mk 1.
They are intended for altitudes of 0-3,000 feet. . . .At altitudes of 3,000-6,000 feet a reduction in main jet size by 5% is usually necessary and for every 3,000 feet increase over 6,000 feet a further 4% reduction is needed.
So same as said before by gortnipper.

Do you know if the 260 was actually correct when running in California?

The ideal jet for your bike is the one it runs best on, which will vary from bike to bike and exactly what is in your build.

If they just changed the jet without any testing at all that isn't helpful, they are making a lot of assumptions, but maybe they did test, do you know? If the stalling you refer to was after they had done work, it sort of sounds like they didn't do much of a job on idle mixture settings! Or was that before?

If you aren't running high rpm and using wide open throttle, the main jet isn't going to affect you much, but if that is the case they didn't make a change that would be effective at the throttle settings and rpm you do use. Or did they drop the needle too?

Simple test, get the bike pulling hard at high rpm in a high gear on a wide open throttle, throttle back a touch, if it accelerates at that point, it is too lean.

BTW, we may need to better understand 'bigger pistons installed'.....

Do you mean it went from 77mm (828 standard) to 77.5mm (first overbore 850)

Or is it 81mm and a '920'?

There is a world of difference between these two.

I am assuming you meant first or second overbore, even third overbore (860), these would make little noticeable difference in your main jet choice...

45cc per cylinder might?

If you are unsure, go up a size. Feel better? Worse? Are you going to hold this thing WFO for more than a few seconds?

I would think you would also need to adjust the needles as well to cover the 1/4-1/2 throttle range, correct?

Thank you all for the advice! I gave the shop in Denver a call to confirm that the swap to 190s was accurate and not a typo on the invoice. He confirmed that based on their experience here in Colorado that 190s, or even 180s, are best. Just to confirm that it's good for my bike they offered to take a look at the plugs and take it for a ride to confirm. Very nice guy and clearly knowledgeable.

As I mentioned, it feels great at speed. My only question is if it could be even better since I don't have a point of comparison on Nortons. Once I dive into it with the guys at the shop I'll post the results here for reference.

@SteveA , good question about the 260s in Cali. I cannot be sure if they were correct there. The previous owner kept the bike as art and a collectible and didn't run it, so it was the owner before that who had the 260s and they were likely used in Austin, TX and New Mexico. On your other note, here are the customizations on the bike that I thought may need to be considered with the main jets:
Hepolite pistons +.020", stock compression 8.5:1 and Hastings rings
New big end bearings with STD rods
JS Motorsports Stage 1 Camshaft w/ bronze lifter inserts, radiuses BSA lifters and shorter pushrods for performance

I did some work on the the stalling at idle.. I adjusted the idle and air screw a bit and it's doing better. It needs to be revved for a few minutes after starting, but now it doesn't stall once it's warm. Before I was stalling at stop lights which is a blast to kick start in traffic .

@gortnipper somehow when you say "show us your plugs" it sounds dirty. But I'll get some pics for you, you perv.

Thanks again all for the replies. I'll follow up after some more research.

PS good idea asking Matt Rambow @cNw about his experience with altitude, especially since his shop is at about 7,000 ft elevation but I'm going to do some more work with the guys in Denver before bugging him since they have been working on my bike and are helpful.

SteveA said:

Simple test, get the bike pulling hard at high rpm in a high gear on a wide open throttle, throttle back a touch, if it accelerates at that point, it is too lean.

Click to expand...

Good advice. I would add that the the results are more dramatic if you are in 3rd gear open the throttle to wide open, when the engine gets the bit-in-its-teeth and starts pulling hard, then abruptly close the throttle, what SteveA said is called a lean surge, by quickly shutting the throttle down will be eminently more noticeable. I also agree that the main jet doesn't have a full time job, but I also feel that having the correct main jet is important for the times that you use WOT in any gear for the sheer thrill of unleashing your Norton. BTW, if the abrupt shut down the throttle and the engine goes directly into engine braking mode the mains are either just right or are rich; easy enough to go down a size and when you do get the lean surge go back up to the last size where no lean surge was detected.

You mentioned that you ride between 5K and 9K feet above sea level. If you run this gauntlet regularly then I suggest that you jet for the average altitude, 7K in your case, and be a bit on the rich side.

Amals typically come with 106 needle jets; try a 105s. Amal offers many different needles, try lowering the needle by one groove. Unless you have a set of Premiers, where you can change the idle jets, you can lean the idle mixture by opening the air screw and lowering the slides to get your idle RPM back.

Additionally you can advance the ignition timing. The reason you jet leaner for higher altitudes is that there is less oxygen available, so when you arrive at the correct jetting for the altitude you ride at the mixture is weaker (than at seal level, but not incorrect, just weaker) and it is harder to get a good flame kernel birthed. By advancing the timing you will get a weaker kernel started sooner so it will be available to do its job when the timing is at 28 degrees. I suggest that you start at 30 degrees, but not exceed 32 degrees. Do the jetting and plug tests after making any timing adjustments.

Best.

I really can't imagine that 190 jets are providing the best WOT performance at that altitude. My totally stock-engined 850 with a clean (new) ham can air filter and straight-through Peashooters needed 240s when it lived at 7400 feet. FWIW, when I tried some long, unfiltered, unscreened long velocity stacks, the jetting had to be increased back to 260's and was still showing a bit lean - I didn't have any larger jets to try. I really liked the improvement with the stacks but couldn't deal with the total lack of air filtering.

Further - I assume that you/the mech ensured that the air filter is clean. My bike was feeling sluggish at one point and acted too rich. I thought the carbs/floats/float needle had worn/changed adjustment but it was simply the air filter being dirty. I didn't have much mileage on that filter so I didn't initially think about it being the issue but it was.

Do you have the straight-through Peashooters or the mid-70's version that had the silenced end cap that we used to drill out?