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High altitude jetting...carbs, not planes...


flyday58
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Hey all, been a while. Did a long distance move to the mountains of the American Southwest and the Enduros have been languishing in the garage.

But Spring is upon us, most of my new home projects are either complete or well under way or shelved pending time & funds. So time to dust off the scoots and see how things are. I now have a 1971 AT1 and a 1974 GT80 in addition to the 1978 DT175. 

I've already tinkered a bit with the jetting on the 175 since I had the jets from when I was trying to sort the lean heat seize issues I was having. I've got a 140 in now, but the top end is lacking, only getting to around 5500 rpm. It also pops quite a bit, very 2-stroke sounding.

The 80 and the 125 are stock and don't run well at all up here. My house is at 9000 feet, and all of my riding is done between 7500 and 9500 feet (whatever that works out to in REAL measurements!). So I'm looking for advice on jetting and airflow.

I realize these are fairly extreme altitudes for most riders, but any suggestions will be helpful. Cheers!

Richard

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Standard options count for everybody.

High altitude, high temp. Needs leaner mixture.

Lower altitude, lower temperature needs richer mixture.

 

Don't forget that at those kinds of heights you will lose a hell of a lot of performance from little engines like those as they have little spare horsepower. Losing 20% or worse of the power from the 80 will be really obvious. Just from the fact there is less air to burn giving less power no matter what you do.

Unless you want to start supercharging your engines..;)

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Yep, glad to be back, although never really gone. The bikes have been languishing since I last checked in, and I have been gone from home much more than I ever used to be. Additionally I traded a pile of Honda junk that I had gotten it into my headed to mess with, basically 3 CL Scramblers in boxes, for a Yamaha XVZ1200, 1983 vintage. That one was in terrible shape but at least was in one piece. Parts were plentiful on fleabay USA, so that's where the focus has been for two years. It has CV carbies but I still had to fiddle with the needle jet to lean the high altitude end enough to where I didn't soot up the plugs Idling down our VERY rocky canyon road.

Mr. A. Head sent a pm about jetting that I will read just as soon as I get done with this online yearly training course I'm working on for my job. Guess I will just take a day and one of the scoots and see if I can get it to run  better choking or jetting down. I'll let everyone know how it goes. :)

Richard

 

 

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  • 9 months later...

um...yeah...Heeeey everyone! It's the return of the prodigal Dick! Sorry (again) for the lengthy absence, no excuse this time. Well, that ain't true, there are many, but I don't wanna tell them and you folks don't wanna hear them, so bollocks on that.

Finally got some free time to mess with the enduros. Had to take the carb off the AT1 as it wouldn't run without the choke on. The pilot jet was clogged so that was the issue, but after reassembly and warm up it was very boggy on the low end. Quickly lost power going uphill and had to continuously downshift. If the revs dropped below about 2700-2800 it would bog down rapidly until downshifting and getting the revs back up.

The main jet is a 140, and the online parts shop lists sizes from 140 to 200. I bought the bike in Denver so it was already living at the higher altitudes, and I suspect the PO put in the smallest jet listed. So is it possibly too lean? The needle is on the middle clip setting, so I can try changing it but I understand that it is more for1/4 throttle and on. 

One more q: in the pic of my carb shown here, the hole on the right at the 5 o'clock position leads to the fuel tap for the choke, but doesn't seem to go all the way through. I tried several of my carb cleaner probes but nothing would go all the way in. Does anyone know if this is a blind hole? If not and it is, in fact clogged, that would be part of the problem. Thanks everyone and Happy New Year!

s-l500.jpg

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Hey good to see you:wavey: Flybee,. whats the plug colour saying with all this jetting malarkey ,  ,  ,,  not shure what hole is the  AIr bleed,

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I'll pull the carb back off and take some pics. As for the plug, it is darkish tan and dry. And this is on the AT1/ 125. And to be clear, I have yet to malarkey with the jets!:P

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I'll pull the carb back off and take some pics. As for the plug, it is darkish tan and dry. And this is on the AT1/ 125. And to be clear, I have yet to malarkey with the jets!:P

IMG_3007.jpg

I'll pull the carb back off and take some pics. As for the plug, it is darkish tan and dry. And this is on the AT1/ 125. And to be clear, I have yet to malarkey with the jets!:P

IMG_3008.jpg

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Okay, bizarre behavior on the site with pics since I last visited, but got it done. Was finally able to get a good view down the bore at about the 5 o'clock position and it does look like a blind hole on this model Mikuni. I have a spare and they are the same.

Replaced the 140 main jet with a 150 and dropped the needle one notch leaner, plus emptied the old gas and put in fresh. No extra pilot jet so it is still stock. Seems a little better but it may just be our old nemesis, age. I think I will remove the motor and start a core rebuild with all new crankcase seals and bearings, maybe go to oversize on the piston. Biggest issue I will have is I now live in an area with even fewer prospects for bore jobs and the like. Only 3100 miles on it, so maybe just new rings.

I will document the teardown if I decide to go that route and post to the forum, so look for that.

Richard

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Not sure if this helps but it may help with what you expect.

HP Loss = (elevation x 0.03 x horsepower @ sea level)/1000

Acording to this from doctor google you will lose around 40% of your hp at 9000ft.

That would slap my dt 175 down to around 11hp. If it made 17hp to start with.

You bike may be running fine. Brown plug is hardly a warning.

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On ‎09‎/‎01‎/‎2017 at 11:15 PM, flyday58 said:

 And to be clear, I have yet to malarkey with the jets!:P

:crylaugh:..pms Luaghing at that,  Plug colour sounds good, , crank seals can be done with engine in frame,  a High idle when engine warm, is the first sign of hard seals,  or white smoke, [gear oil sucking thru, ]

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  • 2 weeks later...

What I really should do is take the young lady down to airier climes and see how she runs with atmospherics she can sink her teeth into. The PO was riding it in the mountains of Colorado but he was. You know. Old.:rolleyes: May not have noticed it running poorly.

My truck currently resides under almost a metre of the white frozen stuff so will have to dig it out first, then unload said stuff from the bed before I can load up the AT1. Unfortunately my back is on vacation somewhere and hasn't told me when it's planning on returning so it may be a while before anything gets done round here. Getting old ain't for wimps, apparently...:thumbdown:

 

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  • 2 weeks later...

Decided to give the old mill an extreme makeover. Started the teardown and will post the pics when I return home from snowy NYC in a few days. So far all looks okay. The armature popped right off the left side of the crank, so that concern is past. There IS pretty good vertical play on that side so new main bearings are in the offing. The piston head is well carboned, and there is definite wear and some blow-by at two of the intake ports on opposite sides of the cylinder.

Sadly on this side of the Big Water it is getting harder to find shops or garages to do the sort of work I may need, like cylinder boring. Don't EVEN get me started on rechroming! However, I am sure I will overcome!

Again, pics to follow...

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Don't forget with this small world we now live in you could post the pot here, or to say pjme and then get it shipped back. I'm sure folk on here, me included would not mind helping out with the language if needed :P.

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