Jet it... And forget it?

[Screaming Meemie]

Hey guys, jetting/elevation question here. Bought the bike in MN (elev.950ft) all stock, ran great (which I later couldn't believe due to the rotten air filters, oil filters, sparkies...etc). I hit 102 speedo-indicated mph with stock gearing. Didn't check the tach at that time, but I'm sure it was high. Brought it to MT (elev.5000), didn't re-jet, switched to UNI pods, cut out the rear exhaust baffles and it runs great, with one caveat... I went to 18/30 gearing and am only able to get to about 85-87mph in top gear. And, even in 4th, she doesn't like to rev above 6k rpms. Aaaand I plan on bringing the bike back to MN in 3 days.

My question(s) to the all-knowing collective that is XS650.com:

1. Jetting suggestions for going back down 4000 feet? I'll start with stock of course, but it'd be nice to have a vague idea for direction when I inevitably need to tune.

2. Is the hesitancy to rev up high due to my high gearing, high elevation or both? It really runs sweet from low idle right up through 5k.. but has a tough time gaining them after about 5700rpm. It gains & looses revs real nice and idles like a peach. 1 kick starter. Plugs look like god breathed on em.. great color.

Im hoping to re-join the century club if possible... But mostly just want to hit the redline for a sec if I see fit. haha.


Thanks as always!

 

[5twins]

I think you're probably geared too high. You have to realize, we're only dealing with about a 50HP bike here. Unless you weigh something like 110 pounds, it's not going to pull 18-30 gearing to redline in the upper gears. It just doesn't have the horsepower.

 

[Screaming Meemie]

Thanks 5twins, thats honestly what the bike was feeling like... I was wondering if the elev. maybe just added to it? Either way, I'm most likely going to gear it back down a bit, maybe try the stock rear & 18t front, in MN because the speed limits are lower & I'd like a bit of extra giddy up.

Any 'tried & mostly-true' adages when dealing with big changes in elevation? 1 main for every 1200 feet or anything like that? It seems to me at least a size or two on the mains & 1 on the pilot would be about right.. but that's just going off of Banshee tuning.

 

[retiredgentleman]

In my experience the CV carbs on these bikes, handle elevation changes with no problems at all. I live at 3400 ft ASL, and quite often ride into the mountains up around 7400 ft ASL. My bike drives the same at both elevations.

Don't forget there's less air/oxygen at 5000 ft. IIRC for every 1000 ft you go up, your engine loses 3% power. When you went up 4000 ft, you lost 12% power from the engine. Now combine that with 18/30 gearing, and you have a bike that runs like a 350 Honda.

Go back to 17/32 or 17/33 gearing and put some fun back into your bike.

 

[scabber]

The Stock gearing is the best choice,

 

[Screaming Meemie]

Thanks all! I'm definitely going to change some sprockets around when I get back to MN.. The stock rear with the 18t front seems like it'd be a decent middle ground while still leaving 5th gear good for some higher speed cruising on county backroads.

 

[scabber]

The 17 T is a better choice. I tried the 18 T Chain rubs shifter and the 17 wont kill your top end.

 

[blue900]

I am very happy with 18-33. at 4500 ft I have found I needed to be leaner on the jets and needles than expected, You will need to try some different ones with your setup to get it correct.

With hills and bucking big wind I wont go any higher on gearing.

These sure run nice when jetting is sorted and carbs are synced and all the other stuff that effects tune and ride are correct.

 

[jefft]

Remember the percentage of oxygen at elevation is exactly the same as at sea level. The molecules are just further apart the higher you go.
jefft

 

[retiredgentleman]

Remember the percentage of oxygen at elevation is exactly the same as at sea level. The molecules are just further apart the higher you go.
jefft

That's a moot point. His engine will produce less power at 5000 ft, simply because there is less air for combustion. It goes without saying, that oxygen is still 21% of air regardless of elevation.

 

[Screaming Meemie]

Yeah definitely, Gentleman. Same composition, just less dense/less of it to be had per 'breath' of the engine. Going to keep my jetting the same, fingers crossed, and do a plug chop or two.