Jetting adjustments for high altitude riding

[flyday58]

Well now that the scoots are in Nueva Mexico I am thinking about rejecting. The first ride from the warehouse the Venture was in to out new house went from 8600' down to about 7900', then up to almost 9000' at home. First noticed it idles a bunch lower so had to tweek that up a bit to get it to where it was in Ole Miss.

 

Throttle response seems about the same but couldn't really tell. I had to follow the wife hauling the other three bikes on a trailer downhill then up the rocky canyon road, so really just puttering along behind her in the dust.

 

During cold start it wouldn't take much choke before bogging, but too little and it wanted to quit. So all in all I'm thinking about going down one or two sizes on the jets. Most of my riding is going to be in the mountains at altitudes ranging from 4000 to 9500 feet. Don't know that I'll ever take it into the lowlands while I have it.

 

So should I just leave it be? Weigh in, one and all!

[Great White]

Run it, pull the plugs and see what they look like.

 

Brownish is good, black is rich, white is lean.

 

The thing about rejetting a carb is that it can be more than just the main jets. It can be the idle jets, or the throttle needle height.

 

Its waaay too much to discuss here, but if you google "jetting motorcycle carbs" there are many sites out there with good info on the process.

 

Jetting carbs is a bit of black magic, but with basic mechanical skills, a little patience and a bit of knowledge anyone can do it.

 

Good luck.

 

[videoarizona]

Our are carbs adjustable? As in..are the sliders grooved to allow different settings for mixture? If yes, there is your answer, If no, another ball game. Different jet sizes.

My guess would be contact a dealer in Albuquerque area...ask them about the problem. I would think they have the knowledge of jetting at higher altitude...

david

[cowpuc]

Flyday, the 1st Gen Mk1's (all I can speak for) have main jet control needles (mind went blank - cant remember there actual name) that are attached to the slides which are attached to the carb vacuum diaphrams. They hang from the bottom of the slides. If you pull the needles from the slides you will notice a fixed plastic platform on the needles them selves. I have done a fair amount of tuning on two stroke dirtbikes (and fourstrokes) and that little platform is not usually present - the needles were adjustable with and "E" clip on the needle. Tweeks (my 83) has got a fair amount of miles on her and a good while back, she developed the inability to handle the fuel effeciently coming thru the mains. I leaned her down by replacing that plastic platform with .015 nylon shims. A little bit of old fashion tuning and I found .065 of shim to be about what she liked. The old girl has been all over the country at that setting - she runs high and low altitudes like she used to and her fuel economy came back up. Plugs came to nice and tan.. I didnt pull the carbs and play with the jets but I have a hunch her mains are oblonged a smidgen..

In the course of that process I learned that one of our members, I think its Skydoc, sells a kit for doing exactly what I did. You could ask around a little and maybe Earl will chime in (HE IS AN AWESOME PERSON!!!)..

If I were doing what you are talking about, this is how I would fatten/lean for altitude changes - IMHO..

Hope this nonsense helps in some small way!

Puc

Edited May 20, 2015 by cowpuc

[Great White]

Flyday, the 1st Gen Mk1's (all I can speak for) have main jet control needles (mind went blank - cant remember there actual name) that are attached to the slides which are attached to the carb vacuum diaphrams. They hang from the bottom of the slides. If you pull the needles from the slides you will notice a fixed plastic platform on the needles them selves. I have done a fair amount of tuning on two stroke dirtbikes (and fourstrokes) and that little platform is not usually present - the needles were adjustable with and "E" clip on the needle. Tweeks (my 83) has got a fair amount of miles on her and a good while back, she developed the inability to handle the fuel effeciently coming thru the mains. I leaned her down by replacing that plastic platform with .015 nylon shims. A little bit of old fashion tuning and I found .065 of shim to be about what she liked. The old girl has been all over the country at that setting - she runs high and low altitudes like she used to and her fuel economy came back up. Plugs came to nice and tan.. I didnt pull the carbs and play with the jets but I have a hunch her mains are oblonged a smidgen..

In the course of that process I learned that one of our members, I think its Skydoc, sells a kit for doing exactly what I did. You could ask around a little and maybe Earl will chime in (HE IS AN AWESOME PERSON!!!)..

If I were doing what you are talking about, this is how I would fatten/lean for altitude changes - IMHO..

Hope this nonsense helps in some small way!

Puc

 

Yes sir, same way I adjust "non adjustable" needles. I add shims under the retainer to lift the needle. Unfortunately, you can only raise the needle in most bikes as the retainer usually rests directly on the slide.

 

That little spacer in you pics is a nice little "find". By removing it and using shims, you could actually lower the needle if needed. Not many OEM carbs give you that option.

 

Here's a decent representation of what component effects what part of throttle opening:

 

http://www.jba.bc.ca/JBCycle/Resources/jet-chart.jpg

 

Its a little different for every manufacturer and even between a manufacturers set of carbs, but that's pretty close to what happens in general terms.

 

Edited May 20, 2015 by Great White

[flyday58]

Yes sir, same way I adjust "non adjustable" needles. I add shims under the retainer to lift the needle. Unfortunately, you can only raise the needle in most bikes as the retainer usually rests directly on the slide.

 

That little spacer in you pics is a nice little "find". By removing it and using shims, you could actually lower the needle if needed. Not many OEM carbs give you that option.

 

Here's a decent representation of what component effects what part of throttle opening:

 

http://www.jba.bc.ca/JBCycle/Resources/jet-chart.jpg

 

Its a little different for every manufacturer and even between a manufacturers set of carbs, but that's pretty close to what happens in general terms.

 

Love this chart, thanks for posting it. Never knew the 'voodoo' surrounding these carbs. Even has 'cutout' for my 2-stroke Yams. That's cool.

 

Hey ScottPuc, where'dja get them shims, or wha'dja use? Maybe you can just donate your slide needles from Tweeks after you win that Indian in Sandusky, Illinois!

[Prairiehammer]

Dick,

The CV carbs are intrinsically self adjusting for altitude. Something to do with vacuum, negative and positive pressure and interaction with atmospheric pressure and altitude. It means for us flatlanders who might occasionally head for the hills, that our bikes don't suffer as much as non-CV carbed bikes with "altitude sickness".

Having said that, if you (and you are) were spending all your time in the mountains, then jet modifications will be a good, albeit finicky, endeavor.

Here is a copy and paste of one guy's take on jetting the Vmax/Venture carbs for high altitude. Keep in mind, that the actual size of the jets he talks about are for a Vmax.

"Hope you don't mind me jumping in here, but we recently redid the carbs on my V-max (I assume the Venture is very similar) and got some clarification from one of my friends that is a technical expert on the Max...keep in mind this is for an adjustment to Colorado altitude:

 

What we're going to do is replace the main jets in your float bowls with a 145 Mikuni jet. The stock jetting is a 152.5 which is a bit rich at sea level for a stock bike and WAY rich for a bike at 5000+ feet altitude. This will make the bike run more crisp along with giving it a better top end and better fuel mileage. Shane, you know what I mean by top end after seeing how easily I take my bike to 140+. It never pulled like that when I had the stock jetting in it. There is actually a calculation for determining jetting at altitude. Anyway, that's the story behind the main jet selection. The mains always have to be selected first. Without a good main jet selected, all the other 'circuits' of the carb will never be right. Now the 'pilot fuel jet' is the one that is size 42.5 Stock on a 49 state bike is 37.5 I don't know what a California bike has but I'll bet it's leaner if anything. With the 42.5 you won't have to have the idle mixture screws out so far and you'll also have a better crossover from idle to midrange. All in all, this means no more popping out the carb throats when you gas it from idle. The 'pilot air jet' is part of the same circuit that the we use the 'pilot fuel jet' in. Fuel jets let fuel in and air jets let air in to make a mixture that is fed down the carb throat at idle and slightly above. The 'pilot air jet' and the 'pilot fuel jet' both work together to give a proper mixture at idle and also up through midrange when using small throttle openings. In other words when you are cruising with the throttle held open just a bit. Like when you're going 30, 40, 50, or 60 MPH. It's this jetting arrangement that is too lean from the factory. This helps the factory get past missions testing. It's also the reason that the main is too big. They try and use a larger, too large in fact for best power, main jet to help compensate for a very lean idle mixture. As you can see from the popping that every stock jetted Vmax is plagued with, it's not a very good compromise. This is what we're attempting to fix. Now, this air jet I just mentioned is the same size (not the hole, but the physical size and the threads) as the main jet. That's why I told you guys to get 95 main jets. But Yamaha probably doesn't have a listing for a main of that size. Soooo, what you do is just order a 95 'pilot air jet,' because that is what it really is. You'll see when you get it that it's the EXACT same as your new main jets, just a number 95 and not a 145. The other 'pilot air jet' is called PAJ2,,, meaning 'pilot air jet number 2. It has an effect through the midrange before the jet needle takes over. This also is the same size as a main jet and that's also why I told you to get a 160 main jet. We just won't be using it in the main jet location. In this case, the stock jet (PAJ2) is a 170 and we are going to a 160. When fuel jets get larger, it means you're going richer. When air jets get larger, it means you're going leaner. Because fuel jets let in raw gas and air jets let in air. Basically, more fuel= richer,,, more air= leaner. What we really are dealing with is:

 

4 main jets in the size of '145' (3G2-14231-29-00)

4 pilot fuel jets in the size of '42.5' (4G0-14142-42-A0)

4 pilot air jets number 1 (PAJ1) in the size of '95' (3G2-14231-19-00)

4 pilot air jets number 2 (PAJ2) in the size of '160'" (3G2-14231-32-

00)

 

In addition you may also need the jet block gaskets (3JB-14199-00-00) depending on the age of your bike and the condition of your current gaskets. Nimalex "

[flyday58]

WOW. That's a lot of jets! Guess I'll look into Puc's ideas first on the needle shim thing. Thanks for the post, Kevin.

[cowpuc]

Dick, I just took the needle, my calipers and went to Ace Hardware, found the coolest little nylon shims that worked perfect.. Shimmed it down till I could tell it had a small starvation hesitation, added .030 off shim to fatten it back up and bingo - worked great!! She has been doing great every since (plug color, mileage, and still pulls good even a the miles on her).. I would also advise taking a very close look at your Diaphrams while you got her down.. I have tried LOTS of stuff for bringing "iffy" diaphrams back to life (I like testing products) and never had much luck with anything UNTIL,, I discovered "Yamabond #5 ".. Stuff is amazing!! Thought I would toss that your way for programming my name/location into those airliner cockpit controls and gauges!!

Anyway, here is a link to the shim thread, LOTS of great info here from LOTS of folks wayyyyyyyyyyyyyyyyyyyyyyyyyyyy more knowledgable about this stuff than I!! Hope it helps Dick!!

 

http://www.venturerider.org/forum/showthread.php?86325-Carb-metering-rod-question&highlight=carb+shims

[Patch]

I'll likely come off somewhat cynical but here are some thoughts about re jetting. I am not a fan, I liken it to figuring valve timing, with a stretched chain.

Of course there are instances where jets are simply wrong or the cause of poor throttle response, or over heating; usually for us we find this in some random melt down formulated by some brochure for a slip-on and intake filter for a magical 10 + 10 HP gain promised if we re-jet? Of course we also have to spend Dino testing dollars to prove the manufactures less than truthful when they slipped the hard earned from our wallets; kind of like the 2nd mortgage needed to tour with a HD although I've heard one can now without the need of a trailer following, I wonder?

 

So I like GW wouldn't re-jet unless after riding for a while and the existing setup proves to be a problem. It may just need a pilot adjustment as you mentioned the pull seems the same. I enjoy predictable power and reliable muscle climbing a mountain road on a touring bike.

A note about shimming, what works for one carb may not for another, this is mostly due to wear. If experimenting is on the menu it is best to start with fresh needles and tubes.

 

Patch

[cowpuc]

I'll likely come off somewhat cynical but here are some thoughts about re jetting. I am not a fan, I liken it to figuring valve timing, with a stretched chain.

Of course there are instances where jets are simply wrong or the cause of poor throttle response, or over heating; usually for us we find this in some random melt down formulated by some brochure for a slip-on and intake filter for a magical 10 + 10 HP gain promised if we re-jet? Of course we also have to spend Dino testing dollars to prove the manufactures less than truthful when they slipped the hard earned from our wallets; kind of like the 2nd mortgage needed to tour with a HD although I've heard one can now without the need of a trailer following, I wonder?

 

So I like GW wouldn't re-jet unless after riding for a while and the existing setup proves to be a problem. It may just need a pilot adjustment as you mentioned the pull seems the same. I enjoy predictable power and reliable muscle climbing a mountain road on a touring bike.

A note about shimming, what works for one carb may not for another, this is mostly due to wear. If experimenting is on the menu it is best to start with fresh needles and tubes.

 

Patch

 

Well put Patch, I certainly would NEVER follow an "experimenter" like my selfs advice unless you happen to be willing to suffer the consequenses of a failed experiments. I gotta say though, concerning the shimming.. I was actually surprised to open the slide up on my 83 and find the Metering Rods not equipped with "e" clips for adjustments. Most bikes that I have played with in the past have come equipped with Metering Rod adjustability via this clip and (may be wrong with this) I always assumed moving the meter rod up and down was part of the tuning process for areas between right off closed throttle (where the fuel from the pilot jet is, again - IMHO, feeding the engine) to where the tapered needle is no longer effecting fuel flow thru the main = at WOT the main jet is in total control - the area's where people spend most of there time and where altitude changes seem to have the most effect.. I used to use the metering rod adjustments to get my dirt bikes and snowmobiles spot on and it really really seemed to help in plug fouling BUT, and reiderate,, its a huge BUT - I am farrr from anymore than an experimenter with this stuff and certainly am ALWAYS willing to acknowledge that!!

Thinking about all this,, years ago I worked in a Honda shop and got involved in setting up brand new CT70's and Z50's.. A big part of my time back then was in final carb adjustments in tweeking out little hesitations and the like (they were sensitive). I always used the meter rod to crispen them up in final tune and these were brand new bikes.. Now ya got me wondering, hope I didnt mess up a bunch of new minibikes in the process:yikes:

Edited May 20, 2015 by cowpuc

[Patch]

Thanks for the comment, truth is guys I sweat every post every forum! At the lineup for languages, their proper use and spelling; my thought's must have been on the ladies, so I missed out on those gifts the angles were equipping innocent souls with -and- by the time I landed here on earth I hit the ground falling out of a wheel chair, in an elevator, on our way to....! I mention this laughing as I was reminded often there was no slowin me down! Also my thinking and spelling go hand in hand, I like to keep it simple;)

 

Back to carbs, nothing wrong with shimming the needle providing you know the base line and are aiming for an objective. If the tubes are worn, replacing just the needle will deliver disappointing results, and visa versa. If you are ready to shim start with the potential for success, start with a good base line, then adjust, remember most of us do this by the seat of our pants. Dino testing show results based on pre-programmed inputs; seat of the pants testing is scaled by a grin, and feedback, that works for me.

 

The other thing I might add is, the CV Carb is a unit that must pull together each circuit, one circuit will influence the next one above it. Now we often fault a carb before knowing other base line requirements, like compression and vacuum, if they are out of spec no amount of carb fiddling will get you the performance you may be looking for!

 

So Kevin was helping me with some electrical stuff, after finishing up I took the bike out for a short spin. Now I have a slight flat spot, not sure how, but I know it must be Kev's fault? Of course that would be a stretch to blame Kev for his help, so the fault falls back to me, or does it? Some of you may know we had an intruder who made it a point to visit each bike, we had 2 open at the time, I wondered????????? Not Kev, can't be me? Must be the intruder! So I call this a W.T.F. moment, right there on the left, you guested it choke was slightly on, turned it to off and flat spot gone. Back to base line, can we solve a problem, can you shim or tune without knowing the complete picture, no we can't-- not properly!

 

Of course I know we can always go buy 10 horses in a box, but extracting those ponies from the barn they arrived in, that takes a dino to prove and special lighting:)

 

Patch

[videoarizona]

Puc...shims...that's what I was trying to say! Thanks!

I couldn't remember the terms.

Anywho..I did that with my 650 after changing pipes and air intake.

 

And Kevin...yea our CV carbs can somewhat compensate on their own...but not from sea level to 8,000 feet. They will run rich regardless. Need to make some changes..especially if you are running up about 9K feet in the hills.

I remember that well with my Bugeye and those 2 SU's. So easy to change mixture on those with a knob at bottom of each carb as well as another knob on the distributor to change advance! Ahead of their time, those Brits in 1960!

david

[flyday58]

Thanks again for all the input. Sadly, all this is about to become academic as I return to Ole Miss Monday, leaving the Venture to await my return in the garage. Hopefully it won't be any later than the end of summer. Scott, diaphragms are new, but gotta make a Lowes Depot run in a day or two. I'll take a needle with me and see if I can find some shims for it.

 

My seat is still in the hands of the folks at Rich's Custom Seats, so can't put any hard miles on it anyway. And still have the COPs in a box waiting to be installed. Lots to do, no time.

[cowpuc]

Oh man Flyday,, end of summer is a long time to be without your scoot brother!! Mississippi is a foreign country to me but, what da heck,, they gotta have bikes down there that you could pick up.. I know one thing,, I would have a hard time leaving Tweeks for the summer BUT, I would at least have a Helix or something hidden down there if I were you.. JUST DONT MENTION IT TO YOUR 1st Gen cause they have feelings that easily get hurt!!

Anyway, have a GREAT summer no matter where you land my friend!!!! We can pick up the carb discussion as soon as you get back (or sooner)..