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Questions about jetting and altitude

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Without getting into the nitty gritty details of which jet to install, I need some general or conceptual feedback on the effects of altitude, etc., on carburetor jetting.

My 2008 DR650 is bone stock.

Most of the reading I have done indicates the DR650 comes out of the box jetted lean and that it should be re-jetted in the interest of performance and ... for lack of a better term, "engine comfort" -- running cooler, etc. Then recently I've read where others said the DR is forgiving, just ride and don't sweat the jetting. Hold those thoughts.

Next, where I live we are at 300 ft above sea level. However, this summer I will spend some time out west and will have a chance to ride the Continental Divide Trail at altitudes ranging from 3,500 to 11,000 feet.

This brings me to my first question: "How do I determine if the bike really is jetted lean?" (And maybe the answer to that question is not really vital to know).

Next, about the matter of altitude -- and assuming the bike really is too lean out of the box, "If the bike is already lean, and air density decreases as altitude increases, won't riding at higher altitudes lean it even more?"

I am outside of my personal reservoir of mechanical knowledge but I think that answer is yes, so it brings me to the last question.

What changes do I need to make to (1) make the engine more "comfortable" at my usual altitude, while (2) prepping for the anticipated trip this summer?

If it is necessary to re-jet for "engine comfort" where I live that's fine. Except for this one trip out west, the vast majority of the riding I do for as long as I own this bike will be within 500 feet (of altitude) of where I live.

However, unless my health fails, the economy completely collapses, or a war breaks out on our soil I'll be able to make this trip. So what I would like to do is make jetting changes prior to departure and not have to do carb work on the trail. (I will if I have to, but would prefer not to...)

On a scale of 1 to 10 with 1 being clueless and 10 being expert, I have about a 1.125 understanding of jetting, so if my assumptions in asking the questions are incorrect please steer me in the correct direction while getting at the engine comfort/altitude/rich-lean correlations.

Thanks in advance,

Rick

PS: A 1.125 knowledge level means I know there are jets in the carburetor, and that they impact how the bike performs... :banana:

Edited by basketcase
clerity, speelin' and grammer

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I don't claim to have all the answers either but your theory about the bike getting leaner as you go up in altitude raised a flag so I decided to do some checking. It actually turns out that as you go up in altitude your mixture gets richer (more gas vs. air) due to the lack of pressure. More info can be found at http://www.motorcyclecarbs.com/carbs101.pdf

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I don't claim to have all the answers either but your theory about the bike getting leaner as you go up in altitude raised a flag so I decided to do some checking. It actually turns out that as you go up in altitude your mixture gets richer (more gas vs. air) due to the lack of pressure. More info can be found at http://www.motorcyclecarbs.com/carbs101.pdf

Roger that, and thanks.

Now that I read your post it seems a bit clearer.

If I understand your post, jetting changes the volume of raw petrol delivered for the fuel mix, which means that if the bike is jetted for a dense air supply (i.e., at sea level), the fuel supply would have to be "leaned back" to match the thinner air at altitude.

Do I have the correct take?

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Yup, you've got it. You want to jet leaner for altitude...and richer as you get closer to sealevel where the air is more dense. Richening your main jet will greatly reduce engine temps and the bike will run much cooler. Main jet is responsible for "1/4 to wide open throttle" Your pilot circuit or slow jet is responsible for closed to 1/8 throttle. In between the two is where your jet needle and slide come into play...there is a plethora of jetting info on here and elsewhere on the "net"...waste a day or 10 and read up. I have a strong understanding of jetting and still know nothing it seems:smirk:

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Thanks to both of you for the feedback.

Also, OKVet, that short tutorial with pictures was a godsend. :thumbsup:

The fuel screw on the DR650 is on the front of the carb, meaning it regulates fuel (cf, carryover paragraph at the top of page 2, left column).

So if my at idle stumble is a too rich condition, it means the screws needs to go in just a bit to lean the mix.

Apparently, even 1/8-th of a turn makes a difference so I'll be tinkering and will come back with a report.

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As an update, I installed the Dynajet needle at the fourth notch. What would be the impact of resetting the needle to the third notch?

Next, I am not yet satisfied with the thing at idle. Reading up on it I am thinking it is idling just slightly rich.

I have fiddled with the fuel screw and at present it is out from seated about 3/4's of a turn. Per the Dynajet instructions that is lean, but when I turn it further out (richer) it seems to develop a "rich idle bog." If I leave it as is it has a stumble at low speed.

What suggestions do you all have for getting the idle business correct? Is the adjustment "butt-hair width" sensitive to incremental turns of the screw?

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Update #2.

As I asked above -- and to answer my own question, yes even a hair of a turn can make a difference.

Yesterday I finally got a chance to do a long dirt road ride with a half-dozen stops where I could listen to the idle and gauge throttle response.

Fuel screw wise the "sweet spot" for this bike is between 1-1/2 and 1-1/4 turns out. After reading some commentary by mx_rob I am going to leave it on the lean side (1-1/4) and call it set.

Thanks again to all for the feedback. :thumbsup:

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