Runs better in cool, humid weather?

[FranksBMW]

My experience is not true? That's weird. (Ha. Ha.)

[Bonz]

I'm gonna throw in here, yes, jetting at elevation is different than sea level. Where I live in Palmer Lake is 7,200'. I run one size smaller mains in the 80SG, and run stock pilot jets with just 2.25 turns out from lightly seated.

ANYONE that has seen the bike ridden, seen the plugs, or ridden that bike will say it is dialed in. And that is at 4,000' up to over 14,000 feet. I have already met the altitude in the middle, per se, based on sea level-14,000+ feet of elevation. Sea level I would be lean, no doubt.

Brant, in theory then we should jet the bike however we want, and the forces of nature at work with vacuum and CV carbs would make it run fine. I.E. Everyone could run XXX size mains, XX size pilots and the laws of nature and CV carbs take care of the rest. That isn't so. There is absolutely a need to vary jetting based on elevation, and a need to vary jetting if two guys live at the same elevation but one is humid and cooler, the other hot and drier.

You gave me a "Colorado Rocky Mountain High" following you up Pikes Peak in 2014 at the Colorado Rally from your engine loading up and running huge rich. My 80SG ran very well. If you set up the Venturer to run in Grand Junction at 4,500', then by way below of what you said above, it should not have loaded up starting at 10,000' on the way up the Pikes Peak Highway. I would bet your jet sizes are stock, and pilots were set too rich also. It's not a knock on you or anyone else, but the carbs have to be jetted different to run at higher elevation.

[motoman]

Actually Howard, my bikes jetting is stock. Found when I returned home and actually hose checked fuel bowl levels with bike idleing, fuel levels were as I suspected.....way low at 7mm. So, just as I suspected, the low fuel levels in bowls acted on same principals as having a glass full of water with a straw stuck in glass and blowing across top of straw.......or glass half full of water and blowing across top of straw. Two totally different results ARE going to take place. The full glass of water and blowing across top of straw is going to result in a smooth even water flow out top of straw. The half glass of water and blowing across top of straw is going to draw a higher needed rate of vacuum, and just come out top of straw all at once instead of being metered based on vacuum draw. Results of fuel flow drawn were same results having fuel bowl levels too low(half-glass of water).
So, in this case, jetting anything down from stock jetting would have had absolutely nothing to do with bikes richness and how it was running at that rally. It was all about float bowl fuel levels as I suspected. After resolving those actual fuel bowl fuel levels 3 summers ago, getting all four identical at 3mm, the bike is as glassy smooth as it was when I bought it new. Had to remove carb bank three more times to get floats so all four actual fuel levels were the same......but that is just the nature of having to match up all four carbs doing the same work suppling identical amount of fuel/air mixture to each of the four cyls.
Hope that explanation was easy enough for you to follow. BTW, that resolve came from the fact that bikes running regularly at above 9-11+thousand ft. elev. with factory stock jetting all those decades prior had never been an issue, other than the noticeable 3% horsepower loss for every 1000ft. in elev.

[motoman]

My experience is not true? That's weird. (Ha. Ha.)

Yep....not true......but guess what......wasn't a wasted day......you learned something.

[Bonz]

It's not up for debate in my opinion whether going up or down a jet size based on elevation is merited or warranted. At 7,200 feet of elevation a bike does not need the same jetting as a bike at sea level. With the 80SG non-adjustable needles that means going down one size in main jets from 110 to 107.5. If the needles were adjustable I could raise the clip which lowers the needle and lean out with the stock main jets. I can't.

For example, at sea level the oxygen content in a given amount of air is 20.9%. At 7,000 feet that drops to 16% of any given amount of air around us. Keep in mind (for the sake of round numbers) that is not a 5% drop in oxygen, that is a 25% drop in oxygen. Go up to the top of the 14,000 foot mountain and the difference is almost 50%.

When the diaphragm slide lifts and lifts at a lesser rate at higher elevation because of the lower vacuum, the fuel drawn is still 100% fuel. But the air being moved has 25% less oxygen. Fuel and oxygen are what create internal combustion. When I lower the oxygen in the air by 25% and the fuel is not "diluted" accordingly it will run richer than it needs to. A guy can do that two ways. Lower the needle by raising the clip or put in a size smaller main jet. You also need to adjust fuel screws inward to lean out idle.

The pilot circuit is a great example of this. The engine has lower vacuum at idle which means it is drawing less air (less oxygen in that air by 25%) If these carburetors adjusted properly by themselves there would be no need to adjust the pilot screw. However, the pilot screw has the effect of changing the amount of fuel sent into the system to mix with air. You would think the lower vacuum would pull less air which with draws less fuel. Therefore you would think the pilot circuit would not be needing adjustment. But it does, it's a fact. By the same token a smaller main jet or a leaner needle setting is needed at higher elevation to run the best the bike can.

Last summer at the Colorado rally bikes from the Midwest ran great until they got to elevation and they started loading up and running rich. These carbs are not self adjusting to the extent stated. Yes they will run well... Run optimally? No.

[Bigfoot]

Yup, I was one of those that was running very rich. The bike was fine in Canyon City. As soon as we started going up, i was running like crap. The further up, the worse it ran. I ended up taking the baffles out of the supertrapp exhaust to lean it out some. That helped, but might have blown out some eardrums behind me.

[skids]

I agree with Bonz with my own experience and with experience of others. Although the carbs do compensate to some degree with lower air density, they are not perfect.

[Bonz]

Another way to look at this is vacuum is about 25% less at 7,000' vs sea level. Probably for similar or the same reasons there is 25% less oxygen (there is less atmospheric pressure keeping the air packed together, air is less dense thus less vacuum is attainable).

Less air volume is being moved (less vacuum means less draw through the carbs), and that lesser volume has 25% less oxygen. Even if the lesser volume of air being drawn through the carbs is drawing proportionally less fuel, the air is 25% lower on oxygen content. You have to make the jet smaller or adjust the needle.

I have been working for a number of years with my ZRX1200 and CVK40 carbs from the ZX11/ZZR1200. Guys in Florida are working on the same set up. 7000 feet difference in elevation. We can get away with using the same size main Jets however we must dramatically run richer or leaner need a clip positions and much different pilot screw settings. We have found dropping down one size in main jet allows one half clip position richer on the needle. Or staying with the same size main jet and raising the clip to lean the needle out at my elevation vs their sea level. We had to nail this jetting combination (and tendencies based on 40 deg temps like Colorado has at high elevation and 100 deg temps in Florida and everywhere in between) because one of the fellows is manufacturing very intricate velocity stacks to work specifically with the anatomy of these carbs to sell literally around the globe to the HUGE ZRX contingent. I feel I have learned more about jetting/elevation from this experience over the past three years in comparing notes with them than most guys have in a lifetime.