Jet Sizing?

[TearlessTom]

OK, its me again with MORE questions.

 

This time as I basically reading the repair manual from front to back I notice that the RSTD and I assume the Venture show different size main jets for each cylinder.

 

Main jet (M.J)

#1, #122.5

#2 #122.5

#3: #117.5

#4: #120

 

Why is this. I know on my wife's HD Fatboy the front cylinder gets different spark plugs from the rear due to the difference in the heat range of the cylinders as the front will run cooler due to getting colder air as you ride, the back gets the warm air from the front cylinder.

 

But this is a water cooled engine, soooo all the cylinders should be within a few degrees of each other. It uses the same spark plug front and rear why not the same jets?

 

Just doesn't make sense to me, can anyone explain?

 

Tom

 

[darthandy]

Even with water cooling, there are some differences in operating temps and conditions among the cylinders. My Triumph 900 triple has a different jet size for the middle carb for this very reason. All in the name of efficiency - better power, mileage and smoother running - oh...and cleaner emissions too. Fuel injection does a better job of balancing fuel distribution, which is why we're seeing it on more and more new bikes.

[stuckonstupid]

All of you brainiacs....

 

I live at 6K feet, I love to ride over the continital divide ( roughly 11K-12K feet ), yes I live in God's country ( Colorado ) just kidding...

 

What my question is, and I've been looking for a couple of years, how should I set up my jets for this altitude? Different jets, needles and all that. It seems to run rich now and I've tried the needle mods with the small washers...and it did help, but it still is not perfect and if you would know me...you know how anal I am. I seek perfection. If anybody can steer me in the right direction, I'd be forever grateful.

 

BTW, I bought a 86 XVZ1300 DSC from a guy from Los Angeles sight unseen, I've taken off the cannister and rerouted the vacuum lines so it is set up much like a "normal" 86 XVZ1300 DS. New plugs, wires, caps, KN air filter, sync'ed the carbs, I run Sea foam every other tank, very heavy right wrist ( I love to go fast )

 

Again, any help or direction would be grateful

 

Stuck-on-stupid Ken

[Freebird]

I may be wrong but I'm afraid you aren't going to get a lot of help with this one. I haven't heard of anybody who has changed the jets due to altitude issues. Maybe somebody has though, we'll see.

 

I've ridden the highest peaks in Colorado and never felt that I needed any carb adjustments. Maybe if I lived there and rode at those elevations all the time I would notice something.

[darthandy]

Normally you need to lean out your mixture at higher altitudes since the air is thinner and pressure tends to be lower. What this means is that at altitudes of about 5,000 to 6,000 feet above sea level, you are getting less air through the carbs and therefore you are running "rich" with normal jetting. You may therefore have to drop jet sizes to compensate. How much? Good question...wish I knew! Try to find someone with a similar bike in your area who has re-jetted or perhaps see if you can find a knowledgeable (and friendly) mechanic with good carb experience. Unless you can find someone with useful experience and/or knowledge, it will be a lot of trial and error to get it right. Good luck!