Altitude Carb Tuning

Jerry Doe · Jul 17, 2014

Lately there have been some questions regarding which changes should be made for your carbs at certain altitudes. I have a spreadsheet you can download. It was meant for Mikuni carbs, but works on Amal too. You need Microsoft XLS.

This will probably help answer some questions:
http://accessnorton.com/support/AccessNorton-Altitude-Temp-Calculation.xls

Use at your own risk.

Cheers- Jerry

auldblue · Jul 17, 2014

Cheers BuB

That is second top on my bucket list , the first is getting marie Osmond pillion on my commando , if I get the jetting right maybe I could pick her up and head for the salt together.

I have read of problems Brits had at bonneville with jetting not as easy as you think!

J

mike996 · Jul 17, 2014

The Commando owners manual states the jet change needed based on altitude and it seems to work reasonably well.

My Commando is based in the Mexico high desert (San Miguel de Allende) and the altitude there is 6500 feet and we ride at alts up to 9000. I use a 240 main based on the owner's manual and plug-chop tests indicate it's perfect at the 6-7k altitude. The painful part of riding at that altitude is the 13+ HP loss

auldblue · Jul 17, 2014

Mike

What Hp do you have in mexico then?

J

mike996 · Jul 17, 2014

37!

(at the rear wheel)

auldblue · Jul 17, 2014

Mike

That's only 4hp less than I've got at sea level. Aw well ,that's me on a downer!

Cheers for that mike !
1up3D

The proof of the pudding is in the eating. There is nothing you can say to facts other than to agree.

J

1up3down · Jul 17, 2014

I live and ride at 6000-7000 feet altitude - with sea level stock jetting and needle position

plugs look just fine, cannot see any reason to go leaner

Nater_Potater · Jul 17, 2014

1up3down said:
I live and ride at 6000-7000 feet altitude - with sea level stock jetting and needle position

plugs look just fine, cannot see any reason to go leaner

You say the plugs look fine; if there's any color, with today's fuels, you're too rich. You may find that going leaner will actually help power output, and help prolong the life of the cylinder walls (less fuel washdown). You won't know until you try! I guess I'm just a sucker for tinkering/tweeking...

Nathan

worntorn · Jul 17, 2014

One has to be a bit careful with leaning out the mixture for perfect running at higher altitude. This is great if you stay at that altitude or above. Tune for 6,000 feet then bring the machine down to sea level on a tour and now it is running way to lean. Piston holing or seizure can and has resulted from this.
The converse, setting up for a near stoichiometric mix at sea level then riding with same jetting to high elevation, results in over rich running, maybe some 8 stroking at very high elevation, but no harm to the engine.

Glen

rvich · Jul 17, 2014

Nater_Potater said:
1up3down said:

I live and ride at 6000-7000 feet altitude - with sea level stock jetting and needle position

plugs look just fine, cannot see any reason to go leaner

Click to expand...

You say the plugs look fine; if there's any color, with today's fuels, you're too rich. You may find that going leaner will actually help power output, and help prolong the life of the cylinder walls (less fuel washdown). You won't know until you try! I guess I'm just a sucker for tinkering/tweeking...

Nathan

I thought the lack of color was mostly due to spark and fuel control that most modern engines use computers to achieve?

mike996 · Jul 18, 2014

1up3down said:
I live and ride at 6000-7000 feet altitude - with sea level stock jetting and needle position

plugs look just fine, cannot see any reason to go leaner

You will get more power at WOT with leaner jetting at that altitude than you will with stock jetting. The stock 260 mains produced 4HP less power at altitude on my bike than do the 240s. As noted, if the bike lives/operates normally at those altitudes, IMO you should make the jetting change.

Nater_Potater · Jul 18, 2014

rvich said:
Nater_Potater said:

1up3down said:

I live and ride at 6000-7000 feet altitude - with sea level stock jetting and needle position

plugs look just fine, cannot see any reason to go leaner

Click to expand...

You say the plugs look fine; if there's any color, with today's fuels, you're too rich. You may find that going leaner will actually help power output, and help prolong the life of the cylinder walls (less fuel washdown). You won't know until you try! I guess I'm just a sucker for tinkering/tweeking...

Nathan

Click to expand...

I thought the lack of color was mostly due to spark and fuel control that most modern engines use computers to achieve?

It was the tetraethyl lead that did the most for plug color. Current fuel mixtures require checking the color deep within the plug where the insulator meets the shell. The outer, visible part next to the electrode will be white under normal operating conditions.

Nater_Potater · Jul 18, 2014

worntorn said:
One has to be a bit careful with leaning out the mixture for perfect running at higher altitude. This is great if you stay at that altitude or above. Tune for 6,000 feet then bring the machine down to sea level on a tour and now it is running way to lean. Piston holing or seizure can and has resulted from this.
The converse, setting up for a near stoichiometric mix at sea level then riding with same jetting to high elevation, results in over rich running, maybe some 8 stroking at very high elevation, but no harm to the engine.

Glen

Glen, that's a very good point that bit a number of friends who ride snowmobiles. Some of them have adjustable main jets that they'd lean out when in the mountains, then seized their engines when heading back down.
As for no harm done with being too rich, it's still preferable to lean to the conditions for cylinder/ring life, but that only if the bike stays at that altitude. In that light, I'd rather have a bit higher ring wear than a stuck engine from being too lean on that odd trip to the coast.
Nathan

Mark · Jul 18, 2014

After reading this thread this morning and thinking about riding at altitude, I figured that I'd take a ride up into the mountains this afternoon:
(top of the pass 10839 feet = 3304 meters)

A single VM36 carb setup with a 240 main and I am able to pull grades at altitude without downshifting.

pete.v · Jul 19, 2014

If you have moved up from the flatlands then I might think a lowering of the needle by 1 notch and a 1/2 turn out with the airscrew should do it. No big whoop. If riding up to the pass and back down to the flatlands then maybe just a 1/2 turn out with the airscrew and back.

Altitude Carb Tuning

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