Jetting for altitude

[JoeW]

The V7 Sport I'm finishing up is going to Littleton CO. I'd like to re-jet the carbs for the client. Stock is 142 main and 50 idle. I'm thinking of a 140 main and 48 idle. Any of you rocky mountain high guys have a recommendation?

[bigbikerrick]

Joe, I am at 4200 ft elevation, and regularly ride to  5100 ft ( Bisbee AZ.)  my eldo runs best with 135 main jet, and 45 idle jet. I have the  stock airbox with UNI foam filter, and Mac slash cut mufflers.
I think the altitude in Bisbee is almost the same as Denver.
I hope this helps,
Rick.

[Arizona Wayne]

I wouldn't worry about it.  Back in `98 2 of us Guzzisti had to make a cross country stop  so my partner on his `97 Cali. could get a new rear tire, me on my CX100.     We ended getting the Dunlop 491 @ a Yamaha, etc. dealer in Colorado Springs, Co.  While there I asked the dealer if they had jet issues because of their altitude. He said no.  Neither did my CX until I got to higher altitudes and then it was only some loss of power then.  I'd say it takes 7k'+ altitude before you feel any real power loss w/o CV carbs like my VX800 Suzuki has which never feels any power losses even @ 10K' altitude.

[Rough Edge racing]

I wouldn't worry about it.  Back in `98 2 of us Guzzisti had to make a cross country stop  so my partner on his `97 Cali. could get a new rear tire, me on my CX100.     We ended getting the Dunlop 491 @ a Yamaha, etc. dealer in Colorado Springs, Co.  While there I asked the dealer if they had jet issues because of their altitude. He said no.  Neither did my CX until I got to higher altitudes and then it was only some loss of power then.  I'd say it takes 7k'+ altitude before you feel any real power loss w/o CV carbs like my VX800 Suzuki has which never feels any power losses even @ 10K' altitude.

 I'm gonna have to disagree based on my LSR racing experience and the many guys I know racing at Bonneville and El mirage..Power loss on a race bike is noticeable at 4000 feet and careful jetting can minimalize it but some power is lost.  I lived in Colorado Springs for several years and driving east and dropping down to maybe 1500 foot altitude made the car or bike feel like it gained 10 percent more power than at 5000 foot.
   So you're saying a CV carb has the ability to mask the lowered oxygen at 10K altitude?

[Testarossa]

Running 10% ethanol and unifilter pods, I wound up going back to stock jetting on the T. Running regularly 5,000 to 10,000 feet.

[Curtis Harper]

Joe, make sure the bike has the adjustable piston domes and dial-a-gap plugs. Put thumbscrew caps on the air ports for twist and tune riding. It'll all be dandy.

[kirby1923]

As most aircraft (light normal aspiration) pilots will tell you that the power loss is about 3% per 1000 above MSL regardless of carburetion and the ability to manually lean by EGT and smoothness.

Of course this is a "rule of thumb" but very close.

My CX jetted for sea level ran fine this year all the way to the top of pikes peak, (14,000'+) power loss was not too noticeable 'cause of the low speeds up the mountain but you sure could feel it in the high country of CO NM.

Started and idled normally.

[Arizona Wayne]

I'm gonna have to disagree based on my LSR racing experience and the many guys I know racing at Bonneville and El mirage..Power loss on a race bike is noticeable at 4000 feet and careful jetting can minimalize it but some power is lost.  I lived in Colorado Springs for several years and driving east and dropping down to maybe 1500 foot altitude made the car or bike feel like it gained 10 percent more power than at 5000 foot.
   So you're saying a CV carb has the ability to mask the lowered oxygen at 10K altitude?

Yeah, we rode by Denver heading east 2up 1 time and nothing including a Merecedes diesel car could keep our pace on the VX800 w/it's CV(constant velocity) carbs.  As I recall we went to around 11K' on that trip.  I got light headed and my stomach felt funny but the VX never missed a beat. Maybe you lose some power but at say up to 80 mph you don't really feel the loss that much.  At  sea level that bike has a top speed of 127 mph.  Whereas my CX100 w/DelOrto PHF carbs definitely felt the power loss say from 7K' on up with stock jetting when passing a vehicle.

[Arizona Wayne]

As most aircraft (light normal aspiration) pilots will tell you that the power loss is about 3% per 1000 above MSL regardless of carburetion and the ability to manually lean by EGT and smoothness.

Of course this is a "rule of thumb" but very close.

My CX jetted for sea level ran fine this year all the way to the top of pikes peak, (14,000'+) power loss was not too noticeable 'cause of the low speeds up the mountain but you sure could feel it in the high country of CO NM.

Started and idled normally.

But as I recall YOUR CX has Mikuni CV carbs on it.

[charlie b]

I live at almost 7000ft.  Bike is jetted to run at sea level without being too lean.  Why?  I'd rather run rich at higher elevations than too lean at lower.  Yes, I get down to Phx and Tucson at least once a year and the CA coast every other year.  Bike runs fine all the way from sea level to 12000ft.

I did lean out the carbs one year.  When I had to go back for a trip then I decided it just wasn't worth the fuss.

So, before you start swapping, make sure the owner is not going to spend a lot of time at lower altitudes.

[JoeW]

Joe, make sure the bike has the adjustable piston domes and dial-a-gap plugs. Put thumbscrew caps on the air ports for twist and tune riding. It'll all be dandy.

Curtis, I have new screaming yellow zonkers on the side pipes and I've re calibrated the kanootan valve and the muffler bearings are NOS. I should be good, right?

[kirby1923]

But as I recall YOUR CX has Mikuni CV carbs on it.

Nothing wrong with your memory ha! but no CV.
Mikuni flat slide
38mm TM 85
A very basic carb doesn't even have an air circuit just the pilot and main.
Never had the heads off but the first owner (I think I am the third) put dual plug heads on it. I don't think it has big valves and the compression probably stock (9.2) as it runs fine with cheap gas gets 50+ mpg on the road without pinging. Plugs always look clean and light honey brown.

mike

[Arizona Wayne]

Nothing wrong with your memory ha! but no CV.
Mikuni flat slide
38mm TM 85
A very basic carb doesn't even have an air circuit just the pilot and main.
Never had the heads off but the first owner (I think I am the third) put dual plug heads on it. I don't think it has big valves and the compression probably stock (9.2) as it runs fine with cheap gas gets 50+ mpg on the road without pinging. Plugs always look clean and light honey brown.

mike

Well, my memory was 1/2 right.    Visually flatslides are far different looking than CVs.  Doh

[Curtis Harper]

Curtis, I have new screaming yellow zonkers on the side pipes and I've re calibrated the kanootan valve and the muffler bearings are NOS. I should be good, right?

As long as you don't over torque the cable adjusters, and remember to not use truck air in the tires. I also think that running the headlight on low beam helps performance due to the forward pressure of the higher wattage output of high beam.

[JoeW]

Joe, I am at 4200 ft elevation, and regularly ride to  5100 ft ( Bisbee AZ.)  my eldo runs best with 135 main jet, and 45 idle jet. I have the  stock airbox with UNI foam filter, and Mac slash cut mufflers.
I think the altitude in Bisbee is almost the same as Denver.
I hope this helps,
Rick.

Rick, 135 main from stock 145. That's a bigger jump than I would have suspected. I'll consult the buyer and see what his feelings are. Thanks Joe

[Arizona Wayne]

As long as you don't over torque the cable adjusters, and remember to not use truck air in the tires. I also think that running the headlight on low beam helps performance due to the forward pressure of the higher wattage output of high beam.

You're a Godsent of wisdom for us shadetree Guzzi mechanics Curtis. 

[oldbike54]

 Two words . Thrunge grommet .

  Dusty

[Rough Edge racing]

Nothing wrong with your memory ha! but no CV.
Mikuni flat slide
38mm TM 85
A very basic carb doesn't even have an air circuit just the pilot and main.
Never had the heads off but the first owner (I think I am the third) put dual plug heads on it. I don't think it has big valves and the compression probably stock (9.2) as it runs fine with cheap gas gets 50+ mpg on the road without pinging. Plugs always look clean and light honey brown.

mike
 

I tune TM flatslides on race bikes... They have FOUR adjustable features affecting air fuel mixture like most every slide carb...idle jet, throttle slide cutaway, needle and main jet...
 No carburetor and no type of fuel injection can compensate for the loss of air density at altitude. Electronic FI can adjust the mixture for best power but the fact remains the power is less ,a lot less at 10,000 fet than at sea level...A CV carburetor is still a variable venturi like any slide valve bike carb and at wide open throttle still suffers the same loss in power..

[Chuck in Indiana]

Yep. Naturally aspirated engines will produce right at 75% power at 7500 feet.

[Two Checks]

The reason that X didn't suffer much at altitude is because it was jetted lean in the first place.
In 98 I rode a C10 Kaw over Beartooth Pass. On the way up you could feel it lose power. At the top (over 10k ft) it was really soft and starting even became somewhat difficult.
BTW, it had CV type carbs. All they do is make for smoother on/off throtle transitions. If you whack the grip the slides move with engine vacuum. If anything it would actually be richer due to the slides lifting slower.
Back down the hill into Cooke City and Yellowstone the bike acted like it gained another cylinder.

[Curtis Harper]

You're a Godsent of wisdom for us shadetree Guzzi mechanics Curtis. 

I do what I can. You should hear some of the real conversations I have...............

[Arizona Wayne]

The reason that X didn't suffer much at altitude is because it was jetted lean in the first place.
In 98 I rode a C10 Kaw over Beartooth Pass. On the way up you could feel it lose power. At the top (over 10k ft) it was really soft and starting even became somewhat difficult.
BTW, it had CV type carbs. All they do is make for smoother on/off throtle transitions. If you whack the grip the slides move with engine vacuum. If anything it would actually be richer due to the slides lifting slower.
Back down the hill into Cooke City and Yellowstone the bike acted like it gained another cylinder.

All I know is my VX keeps it's power far better at high altitudes than any other MC I've ever ridden there.  When I'm doing that I don't have the throttle on wide open and the VX is only supposed to have like 55hp tho it sure feels like a lot more.  Back then they didn't give hp ratings but I challenged a new Buell 1200? once and we were dead even on acceleration to 100+ mph.  He had to stop for gas, I didn't.     The stock VX is definitely faster than my stock CX ever was.

[kirby1923]

The reason that X didn't suffer much at altitude is because it was jetted lean in the first place.
In 98 I rode a C10 Kaw over Beartooth Pass. On the way up you could feel it lose power. At the top (over 10k ft) it was really soft and starting even became somewhat difficult.
BTW, it had CV type carbs. All they do is make for smoother on/off throtle transitions. If you whack the grip the slides move with engine vacuum. If anything it would actually be richer due to the slides lifting slower.
Back down the hill into Cooke City and Yellowstone the bike acted like it gained another cylinder.

Not to beat a dead horse but my X has 38mm mikuni  flatslide carbs and is defiantly not jetted lean at SL
!4,000 feet is about? as high as you can get on a road in the States.

:-)