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Jets recomendation for higher elevation?

  • Thread starter Thread starter Crazy_Russian
  • Start date Start date
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Crazy_Russian

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78 GS750E. its got a stock exhaust and intake, the only thing thats not stock is the air filter element, it got a K&N unit in there, im also at 5400 feet elevation. what woudl you guys recommend for jets? i found a 15 Pilot jet and 102.5 Main jet, what would you guys recomend? and a good source for them?:o
 
The higher elevation is going to make the VM carburetors run richer. A little more air with the K&N is heading in the right direction. You could try dropping the needles a notch to begin with and do some plug chops.
 
When I had VM equipped bikes up here,(Colorado) I ran them slightly lean here at nearly 5,700ft. I would be careful not to ride it too hard until I got up to 7,000 or so, they started to run perfectly there. At 7,000 to 9,000 they were great, then started to get rich above that. At eleven thousand or above they were almost unrideable again, you have to keep the RPMs up and the throttle almost closed or it would bog down, too rich. If I tuned them perfectly for 5,700, they would be almost unrideable by 8,000-9,000, which is not much use around here.

I have since switched to all CV carbs or FI, it is nice to be able to just open the throttle at any elevation and have the bike go like it was supposed to. The elevation compensation on the CV carburetors works very well, stock jetting works great at all elevations. VM carbs really suck with big elevation changes.

If the 102.5 is the stock jet for your bike, try maybe a 97.5 main. Drop the needle a notch for starters. Leave the pilot jet alone, but tune the screws, get the fuel screw closed as far as you can without causing poor throttle response at the tiniest throttle openings. This is just a guess, you will of course have to fine tune it. If you ride it to a sea level place, put the stock jets back in it before you get there.
Buy the jets at Z1, they are cheap, and fast.
 
Crazy_Russian said:
would it be batter just to get pods instead?
Click to expand...

It would not have anything to do with the elevation changes, could possibly make it worse. It would likely give you less low end torque which is nice to have on mountain roads. You can change jets with the carbs in place so being easier to get the carbs in and out with pods is not much of an advantage.
 
thank you, i will order a set of mains :) im rebuilding my carbs right now, and considering im already there, might as well get the jetting redone
 
tkent02 said:
It would not have anything to do with the elevation changes, could possibly make it worse. It would likely give you less low end torque which is nice to have on mountain roads. You can change jets with the carbs in place so being easier to get the carbs in and out with pods is not much of an advantage.
Click to expand...

Agreed.

You could probably get away with the stock jetting for now with the K&N (oiled of course) and just see how it does.

You don't want to go SO lean as to hurt the motor... A sound investment may also be to get an oil-cooler if you're going to be pounding the elevation and mountain roads. Lean bikes tend to run hot. A cooler spark-plug may help as well (cooler = 1 or 2 numbers higher than on the stock plug).
 
DannyMotor said:
Agreed.

You could probably get away with the stock jetting for now with the K&N (oiled of course) and just see how it does.

You don't want to go SO lean as to hurt the motor... A sound investment may also be to get an oil-cooler if you're going to be pounding the elevation and mountain roads. Lean bikes tend to run hot. A cooler spark-plug may help as well (cooler = 1 or 2 numbers higher than on the stock plug).
Click to expand...

No ones running lean, it's rich from high elevation. There's no extra heat, there's less, as there's less air to make heat. Normal spark plugs are fine, oil coolers are not needed.
 
I agree. I'm at 5700' elevation and the trick at altitude is to get air in. Stock jetting is usually close if not a touch on the rich side. Usually when I pick up a new-to-me bike from some flat-lander that was in good running condition, it needs the needle clip moved one notch and MAYBE lean the main one step.
 
tkent02 said:
No ones running lean, it's rich from high elevation. There's no extra heat, there's less, as there's less air to make heat. Normal spark plugs are fine, oil coolers are not needed.
Click to expand...

I'm saying.... if you're running lean to match elevation requirments... but you're not constantly at 7000 feet... say you're riding around at 5000 feet 30% of the time and plus 8000 60% of the time and plus 9000 10% of the time... wouldnt you jet your bike for 8000 feet? Thus being lean at 5000 feet 30% of the time?
 
OK, I see what you are getting at. Yes I set my bikes for 7000 or so, here in Denver they were lean, so I was careful not to ride them hard down here. Up in the hills where I do most of my riding, they ran great. They never got hot at all, because i wasn't riding them hard down low where they were lean. An oil cooler would do no good, the oil wasn't getting hot. If I went for a top speed run down low it would have likely burnt some engine parts up. This is the reason for the caution to put stock jets back in before riding down to lower elevations.
Really there is no correct answer, except dump the VM carburetors and go with CV or FI bikes. The CVs work very well, sea level jetting works great up to 14,000 feet. You can just whack the throttle open and go at any elevation. Same with FI.
 
i wish it was as simple as bolting up a set of CV carbs, i do have a set sitting form an 81 750
 
Crazy_Russian said:
i wish it was as simple as bolting up a set of CV carbs, i do have a set sitting form an 81 750
Click to expand...

Not that hard, you would need the head from a 1980 or later GS 850. Airbox and intake boots, too. Hell, you might as well throw the pistons and cylinders from the 850 on there too while you are at it.:D

15% increase in displacement for a song is a nice idea.
 
yeah, unfortunately i dont have the resources to do that at the moment, and yeah, thats what i have those carbs for, to do a conversion to 850 :) slowly gathering up parts :o
 
It will make a great winter project.
Oh Wait, I forgot. We ride all winter. :D
 
LOL, yeah, just about, i park my for about 3-4 month out of the year, the winds just get way to brutal for me to be on a bike :rolleyes:
 
so now got a question about the needle, where is the stock position for it? I have 1 more space to drop it, and thats it. also what is the stock jetting for these bikes?
 
Most of the VM equipped 750s it was 102.5, some of the early ones it was 105 I think?
15 pilot jet on all of them I believe.
The needle clip in the middle position is stock.
 
Hey guys, I know this is an old thread of my, but now i got everything adjusted, and rebuilt, but still having problems with carb adjustment.
I got a K&N filter in a stock box, Kurkle 4-1 exhaust and Dyna S ignition.
Main jet is 97.5, Air is 15, Needle is in the middle, i did sink them. My in town and cold starts are fine, 15-30sec in the morning, and its ready to go :dancing:
The problem is when im on the highways, as soon as i get off the highway after an hour of 80-85mph, it will idle at 3k for a few minutes. To me, that indicates that its running lean at about 5k RPM (average cruising speed) From my understanding of these carbs, i need to lift the needle one step up? should i change anything else?
 
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