High Altitude Carb adjustment

[rakeshogi]

Hello all,

I'll be taking my Firestorm to Mt. Fuji this weekend. There is a nice scenic road which goes upto 2500m above Mean Sea Level. Now, the problem is that I will be starting the climb after almost 200kms of motorway riding at normal altitudes. Which means I can't get the Pilot Screw adjusted before I leave home.

Because I'm a newbie, I really don't have any experience setting up the carb on my FireStorm. I checked up the Service manual and it looks like it's a stretch to set up the carb for high altitudes. It says you need to change the main jet and then adjust the pilot screw. I don't think I will have enough resoures to do something like that in the middle of the journey. Anyway, I will do just under 50kms at those high altitudes and I am wondering if the carb adjustment is required at all.

Is it possible to just adjust the pilot screw (without changing the jets) and still ride with decent power at about 2000m? And I was thinking I can adjust the screw with my normal screw driver but it looks like a special tool (pilot screw wrench) is definitely required (Noob! ). Is that so?

Thanks for reading! And please excuse me for my trivial questions. I am just starting to learn (through experience on my VTR) motorcycle maintenance.

[8541Hawk]

For that short of a time you will be fine just leaving your carbs the way they are....... no need to mess with anything.

[leevtr]

For that short of a time you will be fine just leaving your carbs the way they are....... no need to mess with anything.

+1

[VTRDark]

I agree for a short time you should be fine.

The affect on carbs is that there is less air pressure so less air enters the engine and the air itself is less dense so has less air molecules which both cause things to run rich. So basically you will use a bit more fuel and the bike will feel a little flat and not so responsive. A bit like us as altitude, it gets to the point where we can't breath properly and need some Oxygen.

Something else that often gets missed and can happen at altitude is that the float in your carbs can swell up which can also cause a slight over-richness. The reason being that when the floats are made the majority of them are sealed at Sea level and set at a specific internal pressure. When you go up a mountain the external pressure drops and the internal pressure inside the sealed float cannot equalise. The internal pressure can make the float swell up which makes it slightly rounder and not such a flat bottom, therefore the float can sit slightly higher in the fuel bowl causing more richness. With extreme amount of pressures for any long periods of time this can also lead to the float distorting to the point of rupturing and causing a leak wich will give erratic fuel metering.

If I was you I would either set the carbs to run slightly lean at sea level so compensating for when you climb higher when things richen up. Or maybe if you have your carbs set up with a standard air filter. Take a hi flow filter with you and swap over as you require a bit more air.

Something else to take into consideration. If you are running an aftermarket de-baffled exhaust/race can and have not had the bike jetted to suit, then there is a good chance that you are already running slightly lean especially if you have put a k&N fitter in or equivalent. In which case you will find things will run a little better at altitude anyway.

If you are going to adjust the carbs, I wouldn't bother changing the mains as it's very unlikely that you will be running at high speeds above 7,500 rpm at the edge of a mountain. Maybe just a 1/4 to 1/2 turn on the fuel mixture screw would do the job. Turn in to lean it off or turn out to richen up. But I have a feeling that if you have a Jap model VTR then the fuel screws will be sealed off for emissions purposes like the US models, and would require drilling the caps off and then a D tool to adjust them. Here in the UK we have a straight forward slot head screw so all that is required is an angle screwdriver to get adjust them.

Have good trip and take some pics to post up.

(:-})

[rakeshogi]

Thanks for the comprehensive reply, Carl! It's a shame that I can't do any of the solutions which you said. Don't have a free flow filter, or jets or for that matter, don't have many tools. And yeah, because mine is a J-spec, the pilot screw is almost inaccessible.

Actually, just to clarify the point, I would be riding my VTR till about 2400m and then hike up the mountain. It would take me 12 hours to return to the motorcycle. A friend, who did the same with his NSR250 (also carb), said his bike never started when returned after the hike! He tried adjusting the air screws but to no avail. Once I was riding at about 1800m in a nearby mountain. When I tried starting my VTR after a small break, it struggled to ignite up. I'm worried I'll have the same problem. Guessing I will just leave my VTR at the base station

[lloydie]

If it don't start when you get back to it at least you have a 2400m down hill slop to bump start it :-)

[VTRDark]

2400m, so that's almost 8000ft. The problem starting could be a loss in cylinder compression which is also another side affect of altitude with less air pressure which at those altitudes could be about 25% down in power. In some mountainous regions where there are fuel stations they will supply a lower octane fuel to compensate for this. So in theory one could make carb changes at seal level using their regular fuel, get to a higher altitude and re fuel from the local filling station and this messes up any changes that where made at sea level. Ideally changes should be made once one gets to altitude and the engine starts to feel the effects of altitude and also use local fuel.

With regards to cylinder compression and starting, make sure you have a good battery to turn the motor over and you have some good spark plugs. Also I wouldn't bother using the choke as this will just adds fuel to already rich mix if not jetted to suit. You may want to consider temporally removing the air filter just to help balance things out and get you started. Unfortunately carburetted engines struggle to breath like we do hence the reason most people would use a donkey or sherpa to carry all their gear.

(:-})

[8541Hawk]

All I can add is when I lived in New Mexico I used to ride to the top of Sandia Mountain.

It was the only real road to ride in the area, so I was there rather often.

At the south peak you are at 9,702 ft (2,957 m) and the main peak is 10,678 feet (3,255 m).

The only issue I ever had was that the bike did not like to idle at the peak. No starting issues or anything else.

This is why I stated that you should be fine for the day riding there.

[VTRDark]

I guess it all comes down to how ones bike runs at seal level to begin with, Also temperature and humidity come into to play as well. Maybe it's a bit like altitude sickness where one person may suffer more than the next. Obviously it's slightly different with engines but I expect different states of wear and tear may have an effect.

have no experience of running at high altitudes so all I have to go on is the theory and from what I have read. But I have experience from Scuba diving which is the exact opposite as pressures increase. Enclosed spaces need to be equalised to ambient pressure and more oxygen is used the deeper you go as more atmospheric pressure is being forced down on you.

It will be interesting to hear how rakeshogi's trip goes and what if any problems he runs into. Along with some nice pics.

(:-})

[rakeshogi]

Hello guys,

Sorry for the late reply.

I went to Mt. Fuji last weekend. Took my VTR till about 2400m (After that, no road ). There was no problem whatsoever! Of course, the engine was slightly rougher, but no hiccup. Was really surprised!

I left the motorcycle and hiked up the mountain to watch sunrise from the summit. And it was almost 10AM when I got back to the parking lot. And believe it or not, in the very first crank, the Firestorm was up and firing

All my high altitude worries are cleared now! Many thanks for all your advice. And I'm adding a few pics from the "ride & climb".

[AMCQ46]

wow, great photos, thanks for sharing your trip with us...........glad the bike started

[VTRDark]

Ran out of road. now there's a surprise. That's funny.

Nice pics, up in the clouds. That will be an experience you will have for life. What a lovely thing to do. What's with all the face masks and how did you get on up there. Was it any harder to breath. Good to hear everything was fine with the bike. I'll keep my mouth shut next time Oh well, at least you went in prepared.

(:-})

[rakeshogi]

Actually I knew the road is only till 2400m My plan was to ride up to 2400m and then hike up to the summit (about 3800m). I have a craze for high altitudes

The temperature at the summit was -1 or -2 deg C while at the base, it's like 35 deg C and it's so humid in Japan! That's why people wear all sorts of stuff to help in the acclimatization process

It's not that hard to breath. But, some people do struggle and use O2 canisters for ease of breathing. BTW, I just thought it would be a nice idea to share my Himalayan motorcycle ride from last year. If someone has a similar craze for altitudes, it will be useful!
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