Jetting for 5500'+ Confused

[bobfab]

Hello,

First time poster long time reader! I have an 89 GS500 that was "tuned" for sea level San Diego California (40 Pilot, 122.5 Main, 3.5 Turns out) which ran great for the 1.6 years i have owned it. I recently moved to Boulder CO area and the bike was having real issues idling and a big powerloss/flat spot. The intake and exhaust are both stock and as far as i know the needles are too. Figuring the simple relationship between air and fuel that needs to be maintained i figured that the pilot jet was too large so tonight i replaced them with a 37.5 Pilot and left the rest of the stock config the same. The bike actually was gasping for throttle and would not hold an idle at all.

Shouldnt you be adding less fuel for thinner air? 

Any jetting recommendations, i plan on taking the bike to higher elevations mountain riding.

[burning1]

I'm not a jetting expert by any means... TheBuddha or Google are the people you want... But I'd think that it would be the mains that you'd most likely need to lean out a little. Maybe just revert to stock jetting? Dunno, but I'm sure there is a good guide somewhere.

[bobfab]

I should say that stock settings for the 89 GS500 are (37.5/122.5/3T) which is what i have it at now.

[twocool]

Hello,

First time poster long time reader! I have an 89 GS500 that was "tuned" for sea level San Diego California (40 Pilot, 122.5 Main, 3.5 Turns out) which ran great for the 1.6 years i have owned it. I recently moved to Boulder CO area and the bike was having real issues idling and a big powerloss/flat spot. The intake and exhaust are both stock and as far as i know the needles are too. Figuring the simple relationship between air and fuel that needs to be maintained i figured that the pilot jet was too large so tonight i replaced them with a 37.5 Pilot and left the rest of the stock config the same. The bike actually was gasping for throttle and would not hold an idle at all.

Shouldnt you be adding less fuel for thinner air? 

Any jetting recommendations, i plan on taking the bike to higher elevations mountain riding.

Yes...as you go higher in altitude, the ambient air pressure becomes less.........THIS MEANS LESS MASS OF AIR FOR A GIVEN VOLUME....(in rough terms, you loss 2% power for each 1000' in altitude, even if you change the mixture......if you don't change the mixture, you will loose some more because the bike will be runing too rich...and waste gas, and possibly foul the plugs, etc.)

In order to maintain the correct air / fuel ratio....you would need to introduce LESS gasoline...

In other words you need to "lean" the gasoline.........to match the "thin" air...

In airplanes we have a "manual" mixture control and "lean" with altitude......we use sometimes just RPM as a guide but most use Cylinder head temperature guage, or better, exhaust gas temp guage,  in order to set the mixture correctly..........(lean burns hotter, rich burns colder)

Probably no change needed up to say 3000' or so...but at 5000' leaning should be necessary....

BUT..........some carburettors, like the ones that ROTAX uses on their airplane engines, are AUTOMATICALLY ALTITUDE COMPENSATING!!!

I don't know about he carbs on GS500...

Cookie

[The Buddha]

Bobfab - put the 37.5 stockers back in for the pilots, set your floats right and try it.
37.5 - you need some I'll send you send me address, these things are lying all over the place, I must have 500 I have pulled off carbs ...
Cool.
Buddha.

[bobfab]

Thanks for Chiming in guys. As physics would have it, i agree you need to lean the mixture for it to run properly. That is where the confusion in lies.

I have reinstalled the stock 37.5 Pilots but it will not hold and idle without alot of throttle and will not rev up higher than idle when given throttle.

For some reason, the guy at the dealership was telling me yesterday that everything needs to be richer to run correctly, that the thinnner air does not pull the same vacuum through the jets and hence the need for bigger jets. That goes against everything i have read, studied and observed; but then again now the bike wont idle at all 

So as of right now, the floats and everything were left untouched, and the 37.5 Pilot and 122.5 Main are installed. Bike still will not idle, nor run off of idle. I am tempted to put the 40 Pilots back because at least they will allow the bike to run (albeit poorly).

I am going to get a couple additional jets from the store today (i mangled one of the 40's, and want to try to go richer with a 42.5; It just seems so counter intuitive  )

[The Buddha]

42.5 pilots = death below 3500, take offs will be a nightmare.
I think you got a different issue. BTW you get that crappie 85 octane in that area do you ... then jet it like the rest of us do, 40/125 ...
Cool.
buddha.

[bobfab]

Death below 3500? Meaning that i need to be below 3500 for that to be a decent jetting?

I havent gassed up since moving here it still got Chicago-87 octane in there. But good food for thought.

Thanks for all the help guys.