Rich or lean - air:fuel ratios

needing said:

IF, as AMAL suggest, lowering float level creates a leaner mixture across the slide opening range

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The question, of course, is DOES it create a leaner mix across the range.
If the leaning out isn't uniform, you are in deep doo doo with your jetting.
Always jetting from a known datum point seems to be carburettor wisdom...

Could be why Amal recommend dropping the needle a notch, or even 2, and reducing main jet size for altitude.
No high roads in Oz anyway, why ye going down this road to nowhere anyway ?

I'm getting too this party a little late because I've been on a 10day road trip without a computer.
My thoughts on why main jet size reduction in Amals is all that's needed for altitude. Obviously the air is thinner, less oxygen to help with combustion and also there is, I would think a weaker vacuum signal at the carb mouth. As mentioned earlier the Amal does not have a lot of fine tuning parts available other than main jets. Needles are specific to carbs not machines and though an 850 uses a specific needle for it's application, it can be said the carb is specific (spray tube cutaway) too. By the way Tridents also have specific carbs and needles with unique spray tubes.
OK, getting back to that weaker signal. At part throttle when running mostly on pilot air, needle and to a certain degree slide cut, less air will be drawn in at altitude than with same throttle opening at sea level and the engine vacuum will be less too. Less venturi effect would draw less fuel through the needle jet and around the needle itself somewhat self regulating for altitude. At WOT, and at high RPM, even though the air is still lacking oxygen, a lot of it is being pulled by the engine. Here is where reduced main jet size comes into play. Venturi effect is strong enough too suck too much fuel through too large a jet for the amount of O2 available. So mixture would be too rich now so a smaller main is the fix for maintaining correct AFM.
Raising and lowering the fuel level will richen and weaken the carburation but is not the way routinely vary carb tuning. Think of it as a room lit with a single 60w bulb that is the perfect amount of light for the room. You could use a 100w bulb and wire in a dimmer to get to that same sweet spot of light but after all that you'd still find the 60w float level to be the best.

Well I've tried all the recommended Amal float level settings: .08, .10, 2mm, etc but I now I have a new one to try: 60 watts. Eureka, that may be the key!
Jaydee

#6 Mikuni needles work well in 34mm Amal carbs and give a better range of tapers. How often do you really run using the main jets - that is throttle wide open ? So if you are slightly rich on the mains - does it really matter ? The main part of the throttle range that you normally use it from 1/4 to 3/4 throttle, and that part of the carburation, especially near 3/4 throttle is critical. As long as the motor actually runs on the main jet at full throttle, you will always be safe. So when you are setting the mains and doing plug chops, make sure that fitting a larger main jet actually richens the mixture at full throttle (i.e. the tip of the needle in the needle jet is not obstructing the main jet).
My feeling is that air temperature might have as big an effect as variations in air pressure. You richen for cold air, and lean off for low air pressure. - Needle height , as you normally always run slightly rich on the mains anyway !
If you are riding your bike, and it is coughing and spitting as you move up and down through the gears, it is usually too lean on the needles - raise them one notch before you do damage. If you are tuning your mid-range you should try to get the motor to cough then raise the needles one notch. You can only do this by riding the bike - not in the garage. You can raise the needles by the side of the road, so don't stuff around with float levels.

Hi Biscuit.
Re your: "...Think of it as a room lit with a single 60w bulb that is the perfect amount of light for the room. You could use a 100w bulb and wire in a dimmer to get to that same sweet spot of light but after all that you'd still find the 60w float level to be the best..."

Working through your rationale but I got stuck at the above analogy.
Carbies have varying throttle openings while electricity is on/off and maintained to close tolerances (around the 240VAC in Australia). If electron flow were to equate to air molecule flow then a dimmer switch (jetting) is useful to correct electricity (air density) fluctuations. As the electricity supply varied, you would need to turn the dimmer up or down to compensate, so too for matching jetting to air supply at each throttle opening.
We have a 'Mount Lofty' here but it is actually at the top of a suburban street. May be I will have to approach the uni about using their bariatric chamber... <-
Ta.

Hi needing, Ha, ha, yes I guess the light bulb analogy wasn't the best. What I was attempting to do was compare correct fuel level to the 60w lightbulb lit room. And the dimmer to the fine tuning of the light in that room. Reading it back I realize I pretty much said the opposite. You actually explained it better. Sorry all.

There are mountains about 6000 feet high in our great dividing range which runs from Queensland to Victoria about 1500 miles. Which comes first and has the most effect on jetting - cold air or low air pressure ?

Cold air is denser so provides more air for the same metered fuel = leaner.
Higher altitude makes air less dense for the same metered fuel = richer.
If going higher and the air getting colder cancels out needing to change jetting is yet another question.
Ta.

needing said:

Cold air is denser so provides more air for the same metered fuel = leaner.
Higher altitude makes air less dense for the same metered fuel = richer.
If going higher and the air getting colder cancels out needing to change jetting is yet another question.
Ta.

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And don't forget humidity.

acotrel said:

There are mountains about 6000 feet high in our great dividing range which runs from Queensland to Victoria about 1500 miles. Which comes first and has the most effect on jetting - cold air or low air pressure ?

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But there are only 2 points in that entire length where the road gets even remotely that high.
Near Thredbo and near Mt Hotham.
I've done the road near Thredbo, carburettors didn't even blink.
(not a Norton however, YMMV). (Carry fuel if you are on a Roadster)
I didn't have any trouble breathing either...

Not my pic, obviously.
https://www.flickr.com/photos/legoblock/6000938463

I think that in Australia, air temperature has much more effect than height above sea level. As Rohan said there are probably only two places where we get up fairly high, the rest is usually below 2000 feet. In fact our Murray river falls about one foot per mile over its whole length which is about 700 miles. It's very noticeable that if your bike is jetted on the rich side, on a very cold morning it often performs much better.

Seem like if need be, as you start to sputter up to higher elevations or down to lower ones for that matter, you simply pull over, drop the plug, release one jet with the little T wrench add the desired one, do the other side and head on up to or down from the pass. I would say maybe 2 minutes side, tops? No big whoop, you'll catch up to the computer bikes in a few miles.

Also, turn the air screw in or out a 1/4 or 1/2 turn. These tool less air screws are readily available requiring no screw driver or even pulling over to adjust, just keep your eye on the road.


Thank you for letting me interject, let us now continue with the complicated bullshit.

pete.v said:

Seem like if need be, as you start to sputter up to higher elevations or down to lower ones for that matter, you simply pull over, drop the plug, release one jet with the little T wrench add the desired one, do the other side and head on up to or down from the pass. I would say maybe 2 minutes side?

Also, turn the air screw in or out a 1/4 or 1/2 turn. These tool less air screws are readily available requiring no screw driver or even pulling over to adjust, just keep your eye on the road.

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Then the sealing washer on the plug crumbles as you retighten roadside, gas drips, the bike backfires and flames erupt. Your nether regions are burned to a crisp, there will be no male heir to the throne and your family dynasty comes to an end.
I say better to drop the needles.

worntorn said:

Then the sealing washer on the plug crumbles as you retighten roadside, gas drips, the bike backfires and flames erupt. Your nether regions are burned to a crisp, there will be no male heir to the throne and your family dynasty comes to an end.
I say better to drop the needles.

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Faucet grease.

pete.v said:

Seem like if need be, as you start to sputter up to higher elevations or down to lower ones for that matter, you simply pull over, drop the plug, release one jet with the little T wrench add the desired one, do the other side and head on up to or down from the pass. I would say maybe 2 minutes side, tops? No big whoop, you'll catch up to the computer bikes in a few miles.

Also, turn the air screw in or out a 1/4 or 1/2 turn. These tool less air screws are readily available requiring no screw driver or even pulling over to adjust, just keep your eye on the road.

Thank you for letting me interject, let us now continue with the complicated bullshit.

Click to expand...

Not for nothing my friend, but that's the kind of thinking that helped squash the British bike (and car) industry in the first place. Always having to f with something was expected and if you were not willing to do so you just did not deserve a Triumph, Norton, BSA, etc. So the public happily bought Honda 750's and happily motored up and down mountains with nary a hitch.

But to get back on the subject. I bought my MK3 from Harry's Motors in Denver. Stock mains were 230. The shop jetted it with 220's for the "Mile High" and it ran great. One summer I rode it back to New England with the same jetting and it ran fine. Now I live in New England and have jetted it with 240's, and it runs fine. Me-thinks we are brain f'ing this whole altitude and jet thing. Unless you 're racing and/or are on the mains most of the time, a couple steps either side of 'book' will be OK. :wink:

pete.v said:

Seem like if need be, as you start to sputter up to higher elevations or down to lower ones for that matter, you simply pull over, drop the plug, release one jet with the little T wrench add the desired one, do the other side and head on up to or down from the pass. I would say maybe 2 minutes side, tops? No big whoop, you'll catch up to the computer bikes in a few miles.

Click to expand...

For altitude riding, its not so much the mainjet that needs changing, its dropping the needles a notch or 2. (as Amal of old recommends).
Since most riding is done on part throttle, unless the hills are VERY steep....

Now of Page x of this discussion, and this is all it boils down to.

you know, on the relative importance of temperature and pressure on A/F ratios, what the jet itself really sees is air density-- density goes down when temperature goes up, and density goes up when pressure goes up. but the jet doesn't care whether the air density was changed by pressure or by temperature. a warm day at sea level may require exactly the same jetting as a cool day in the mountains, because the air density at both situations was the same, even though the temperature and pressure were not.

you can calculate relative air density using uncorrected barometric pressure and temperature, or you can measure it directly using a RAD meter:



the RAD meters are useful because if you record the relative air density whenever you finish correctly tuning your bike, you can re-tune it correctly at some other temperature and pressure by using those same settings, if the relative air density is the same.

Biscuit said:

pete.v said:

Seem like if need be, as you start to sputter up to higher elevations or down to lower ones for that matter, you simply pull over, drop the plug, release one jet with the little T wrench add the desired one, do the other side and head on up to or down from the pass. I would say maybe 2 minutes side, tops? No big whoop, you'll catch up to the computer bikes in a few miles.

Also, turn the air screw in or out a 1/4 or 1/2 turn. These tool less air screws are readily available requiring no screw driver or even pulling over to adjust, just keep your eye on the road.

Thank you for letting me interject, let us now continue with the complicated bullshit.

Click to expand...

Not for nothing my friend, but that's the kind of thinking that helped squash the British bike (and car) industry in the first place. Always having to f with something was expected and if you were not willing to do so you just did not deserve a Triumph, Norton, BSA, etc. So the public happily bought Honda 750's and happily motored up and down mountains with nary a hitch.

But to get back on the subject. I bought my MK3 from Harry's Motors in Denver. Stock mains were 230. The shop jetted it with 220's for the "Mile High" and it ran great. One summer I rode it back to New England with the same jetting and it ran fine. Now I live in New England and have jetted it with 240's, and it runs fine. Me-thinks we are brain f'ing this whole altitude and jet thing. Unless you 're racing and/or are on the mains most of the time, a couple steps either side of 'book' will be OK. :wink:

Click to expand...

Even when racing, the main jet doesn't matter much as long as it is not too lean, especially if you are using methanol fuel - petrol is a bit more critical because the jets are half the size, so a minor change has twice the effect. With petrol in a four stroke, you should still be able to use main jets two sizes larger than the optimum without the motor slowing much. The needle jet size and needle taper are critical.

Hi all.
My original question re AMAL main jet/altitude advice was mentioned by someone with actual experience riding at altitude:

- by christulin » Sun Jun 23, 2013 3:55 am
"...I have fiddled a lot with Amals on bikes being run at 6000' altitude (where I live) and higher. This causes standard setting to run too rich up here. The advice from the shop manual about changing main jets is useless. I lower the float height , sometimes lower the needle a notch and use a different slide...".
fuel-flow-through-32mm-amal-bowl-t16708.html#p206554
Ta.

If you reread the thread you will see there are a number of us who live and ride in high places. 80 percent of the Province I live in is covered in mountain ranges. You also have a response from a Colorado dweller, that state is pretty much all high elevation.

Glen