Whats going on Boys, I just joined the forum just in time to develop a problem with my DS!!! Just my luck, anyhow I got some wierd issues with the DS going on...


I always thought that the DS runs rich at high elevation(3000-4500ft). But was told by my dealers mechanic that I was in fact wrong the DS will actually run lean at high elevation.

So I drove up early morning to Forest Hill and the DS('05 bone stock still under warranty) ran fine(as it has before). After and hour of good hard riding the first little miss-fire happened(power cut out briefly) then again and again getting more frequent with occasional backfire and popping, at its worst it was like riding in a car with someone just learning how to drive a manual transmition jerking back and forward with power cutting in and out horribly... like it wasnt getting any gas. I tried removing the air box to lean it out a bit, because I always thought you ran rich at elevation, but had the same results. I checked the plugs and they seemed decent to a little too dry.

So i packed up and headed down the hill to my dealer, and had them hot lap it around their parking lot and it ran fine. They were quick to blame it on elevation, but I have ran fine at the same elevation before. This is when the mechanic told me that my thinking was wrong and the DS actually runs LEAN at high elevation(which was news to me, but by the looks of my plugs seemed to make sense)


So I ask how does temperature affect performance at high elevation that is the only variable that could have changed, but even then it never got too hot out there... I AM SO CONFUSED!!!!

 

The higher you go in elevation the less dense the air becomes. Your still supplying the same amount of fuel, therefore your mixture will become more rich. Any mechanic worth a beans should know that without hesitating. Regardless, I don't think your problem is related to jetting.

It sounds more like you may have a bad rectifier. THat little metal thing with fins that sits up under the airbox. Have the dealer run a voltage check on your charging system. I seem to recall others with bad rectifiers experiencing similar problems as yours.

Oh and welcome to the forum!

 

What octane fuel are you using?
If you are using 87, this will not work well above 3000 feet.
You may be losing as much as 3 octane at that elevation your running.
Put in 92 or 93 and take a ride.

What temperature was it, at that elevation?
At 60 you will be ok. This will lean you motor and thus balance out the richness from the elevation.

If below 60 and using 87 octane, you will run lean and lack of cooling from the low octane will cause pre-ignition.

 

I thought as you go up in altitude you need lower octane fuel - less compression PSIA?

As for the leaner at high altitude. . .

Basics of the carb. . .

Fuel is drawn into the carb via vacuum. Vacuum is created by a pitot effect of the carb. Basic theory is air moving has less pressure than air at a standstill. So, atmospheric conditions have more air pressure than the air passing through your carb which causes the fuel to in a sense be pushed out your tank and drawn into the carb through the jets.

As for air - at altitude, the number of OXYGEN molecules per Cu/Ft. of air is less than that of air at sea-level. In boilers we de-rate about 4% per 1000' ASL (above sea level.) The reson for this is air is pushed together due to the weight of the air above it. At higher altitude, their is less air pushing down on the air so the atmospheric pressure is less and the number of OXYGEN molecules per Cu/Ft. is less.

NOW, IF you were to be able to draw in the same number of OXYGEN molecules at sea level as you were able to draw in at altitude, you would actually be drawing in more CFM (Cubic Feet per Minute) of air. So, your velocity would be greater through your carb resulting in a greater differential pressure resulting in more fuel being drawn in.

So, the greater the velocity in your carb the more you will have to jet down - all things being equal.

If your mechanic was thinking you would get the same number of OXYGEN molecules at altitude I can see where he is going but, unless you change porting, cams, etc. you won't see any difference in velocity so, you will see a loss of Oxygen into your bike and you will need to reduce the fuel to it as well or you will be running rich.

 

I was running 91 octane, and if I were to guess it was about 60 degrees maybe a bit warmer. It was running fine for a good hour of hard riding(in the morning cool weather), then intermitently started cutting in and out of power and then full on cutting in and out of power with pops and backfires.

After the DS sat for a good long while I noticed how HOT the engine felt around the intake and carb, hotter than the rest fo the engine, this seemed unusual for how long it had sat without running. The spark plugs where very dry almost chalky.

While I'm no mechanic it seemed like it was choking for gas???? Would that have anything to do with it being to lean and cutting in and out of power like that?

 

If you have a vacuum advance ignition, and thus as elevation increases, you lose vacuum and thus will run very rich. By running lower octane will help in that application.

In this application he is running lean.

 

Is the backfiring when you do a high speed downshift?
If so, turn out the pilot screw 4 turns from seating.
Because the end of your plug is chalky/gray. Your bottom pilot adjustment is probably too lean. You want to see gray and some black on end of plug.

Is the humidity very low their?
Lack of humidity also makes the motor run hotter.
You said it ran good in the early morning. Thats when the air is usually most humid.

If with pilot screw out 4 turns, you still have popping, you will probably need to jetup one size.
Stock is 165, try a 170.
If you rejet, ask the mechanic to check float height at that time. If floats have dropped because of bouncing during breakin, they may be too low and this will cause motor to run lean in low/mid range.

 

ya there there is not much humidity here normally, and it was cloudy during the morning(certainly more humid) I have had the high speed down shift backfire happen followed by a motor shut down on more than one occasion, but in this particular case it was in assosiation with the power cutting in and out abruptly while trying to accelerate. 1st gear, 2nd gear, etc. I couldn't get it to run without bucking like a bronco because of power cutting in and out popping sounds and occasional backfire... the descriptions of the rectifier problems sound just like my problem... I did just roll my DS 5 times down a hill maybe the rectifier wires are loose? But if thats the case it should have ran bad for my mechanic at sea level too... Too wierd..... But I cant thank everyone enough for the on going ideas!

 

Usually if the motor is cutting out HARD like a JERK!! then it will be electrically( is that a word?) I would bet it since you did roll your DS previous to these symtoms it could be the battery/rectifier or possibly a loos wire to one of your coils...



bigger.

 

You need less octane when raising elevation, thats why we have 85 octane here and not at sea level. . If you are above 4000' and have a completely stock machine you should be jetted around 160. With an end cap and no lid 167.5. That will run good up to 8000' but be a little doggy at that elevation. I do run 91 at all times in them machines and that jetting is to that octane. You might need to jump up one half size for 87. If its 60 degrees out that jetting will be about one half size lean also, so increase for that.