A forum for owners of Yamaha TTR250 trail and enduro bikes!

KamJam · Newbie

Jets For High Elevation

brindabella · Senior Member

if your bike is fairly standard and runs ok at near sea level, it certainly needs to be leaned at 7000ft.
(even at half that altitude, it may need, eg a leaner needle position).

leaner pilot and main are (of course) smaller numbered jets.
leaner needle position is moving the clip AWAY from the pointy end.

even if jetted so it runs cleanly at altitude it WILL have LESS power.

jetting needed depends on "oxygen density".
lower oxygen density means bike will run richer and jetting needs leaning to compensate.

higher altitude lowers the oxygen density (altitude is the most important variable).
higher temperature lowers the oxygen density.
higher humidity lowers the oxygen density;
an approaching storm (lower local barometric pressure) lowers the oxygen density.

there is a complicated formula for working out oxygen density but it is of no use because it cannot be related easily to the needed jetting changes.

why not just check the needle is at the lean setting, and go to the next smaller available main jet number?
and tell us all how you went, please.

TT-R250M · Super Guru

Before re-jetting you should be 100% sure that your carburettor is clean. This includes both the pilot and main jet, both the air jets (main jet and pilot) and your accelerator pump is working correctly. If you are unsure, see here- http://ttr250.activeboard.com/t55374798/rebuilding-a-ttr250-teikei-y30p-carburettor/

Without taking into account what mods the bike has, what fuel is used, average temp, humidity etc., there is no way to tell you what is best. What is good for one bike may not be for another, so there will not be a ''one size fits all''.

You say ''I dropped the needle down two notches in the carb'' but does that mean that you lowered the jet needle clip position towards the sharp end or raised it towards the blunt end? If it at the top setting closest to the blunt end (leanest) you will need to fit a smaller main jet and adjust the needle accordingly. Do this until the right main jet and jet needle clip position is found.

I seem to find that people in countries that use an Ethanol blend of fuel, do not need to enlarge the main jets as much as countries that do not use Ethanol blend. It seems the Ethanol blend is harmful to performance and a decrease in jetting sizes is the end result. Also, different countries have different climates, so this will help to confuse things.

If the jetting is not right to begin with then re-jetting can be hard to know what setting is best for your particular climate, elevation, fuel quality and modifications made to your bike. In this situation you would be ''stabbing in the dark'' so to speak. If the bike is standard, it would be best to start with the standard jets sizes that YAMAHA brought the bike out with and then go down a size or two smaller (if needed).

For a reference- I am at situated 930 metres (3,050 ft.) above sea level, Ravenshoe is the highest town in Queensland. It gets as high as 4,000 ft. in places. I had no problems running the standard jets with a standard bike (my old OE) and have no problems using larger jets in my modified bike. The following is a link to the highest road in Qld (near where I ride regularly), of course it is only the highest declared ''main'' road though and there are higher backroads - http://herberton.qld.au/attractions/highest-declared-road-in-queensland

Just to add a little more insight into re-jetting-

Symptoms from 3/4 throttle through to full throttle- Main Jet

Please note that some of the following symptoms can be from the wrong pilot jet installed, so if you have any trouble at idle to 1/3 throttle, see Symptoms from idle to 1/3 throttle- Pilot Jet. Also, not all sypmtoms will be present at once, so unless way out of specification, you will not have all sypmtoms described in the topics below.

Lean symptoms

Choke helps at wide open throttle.

Header pipe gets red at the header pipe (even at night).

Backfires/pops in carb when throttle is applied quickly at WOT (wide open throttle).

Feels like it's running out of fuel (not to be confused with splutter-rich symptom).

Runs good but lacks power-slightly lean

Spark plug is clean but has a whitish deposit- extremely lean

If you let off the throttle at top speed and it picks up in revs slightly it suggests a lean symptom.

Spark plug is very clean or white- lean

Rich symptoms

Starts up without choke (it should take quite a few turns to start when cold without choke usually).

Runs good until warm & then has flat spots at WOT.

Splutters at 3/4, unless the throttle is slowly rolled on.

Bike stalls easy at low revs.

Flat spot in carb at WOT.

Engine does not idle down fast enough when at roughly 4000 RPM & the throttle is let off.

Smells like un-burnt fuel.

Black smoke that has an unburnt fuel smell.

Hits the rev limiter too easily.

Sounds a bit like a two-stroke.

Carb pops when throttle is let off.

Black, soft,sooty deposit on plug.

Perfect

Bike runs great through all gears & idles smoothly.

Spark plug is a chocolate brown colour.

If you let me know more information about your bike i.e. whether it is standard and if not what mods, average humidity and temp, what jets you currently have installed I can be more specific.

Jarrah

dlowenst · Member

I checked in with a friend with whom I ride regularly in Breckenridge, Colorado (10,000 to 14,000 ft above sea level) and Moab, Utah (4,500 to 8,000 feet ASL). I tend to ride my CRF-250X on these trips, but he typically brings his TT-R250 (which runs great). Here's the jetting info he provided:

Breckenridge: 130 main, stock needle on the leanest setting, stock pilot jet with the fuel mixture screw set at 1 3/4. Tune based on throttle response off idle / crackling on engine braking.

Moab: 135 main, stock needle on the one leaner from middle grove, stock pilot jet with the fuel mixture screw set at 2 turns out. Tune based on throttle response off idle / crackling on engine braking.

Home (sea level to 4,000 ft): Stock (USA blue spec TTR) 137 main, stock needle on one richer from middle groove, stock pilot jet with the fuel mixture screw set at 2.3 turns out. Tune based on throttle response off idle / crackling on engine braking.

He says that if you are starting at 7K and staying there, you'll be OK with the Moab settings. If you are running up past 8 or 9K the bike will start breaking up, so switch to the Breckenridge jetting.

Hope this helps.

Dave

-- Edited by dlowenst on Tuesday 14th of January 2014 09:52:22 PM

TT-R250M · Super Guru

dlowenst wrote:
I checked in with a friend with whom I ride regularly in Breckenridge, Colorado (10,000 to 14,000 ft above sea level) and Moab, Utah (4,500 to 8,000 feet ASL). I tend to ride my CRF-250X on these trips, but he typically brings his TT-R250 (which runs great). Here's the jetting info he provided:
Breckenridge: 130 main, stock needle on the leanest setting, stock pilot jet with the fuel mixture screw set at 1 3/4. Tune based on throttle response off idle / crackling on engine braking.
Moab: 135 main, stock needle on the one leaner from middle grove, stock pilot jet with the fuel mixture screw set at 2 turns out. Tune based on throttle response off idle / crackling on engine braking.
Home (sea level to 4,000 ft): Stock (USA blue spec TTR) 138 main, stock needle on one richer from middle groove, stock pilot jet with the fuel mixture screw set at 2.3 turns out. Tune based on throttle response off idle / crackling on engine braking.
He says that if you are starting at 7K and staying there, you'll be OK with the Moab settings. If you are running up past 8 or 9K the bike will start breaking up, so switch to the Breckenridge jetting.
Hope this helps.
Dave

Hey Dave, nice write-up

I think there may have been a misprint in info as US standard spec is #137. Other than that, great work!

Jarrah

Maniot · Veteran Member

Such useful advice from the contributors above.....I'm only located at 1150 feet riding down to sea level and up to some 5500 feet but am sure some of the tips will be of great benefit to me for one....thanks!

dlowenst · Member

You are correct, thanks! I've edited my original post to avoid propagating incorrect info

Dave

TT-R250M wrote:

Hey Dave, nice write-up
I think there may have been a misprint in info as US standard spec is #137. Other than that, great work!
Jarrah

TT-R250M · Super Guru

dlowenst wrote:
You are correct, thanks! I've edited my original post to avoid propagating incorrect info
Dave
TT-R250M wrote:

Hey Dave, nice write-up
I think there may have been a misprint in info as US standard spec is #137. Other than that, great work!
Jarrah

Nice work!

zzrdd · Member

Good info guys. Here's my challenge with my 2002 TTR 250.

Altitude: 7500-8500 feet (2300-2600 meters, moderate temps <80F (<27C)
Main Jet = 128
Pilot = 50
Needle = 5C9C
Needle position = 2nd slot from leanest (two slots down from the top/most lean slot).
Accelerator pump = new diaphragm and post. (have not visually tested yet but no stumble when throttle snapped open).
Mixture screw: out 1.75 - 2.75 turns (seems little difference)

Airbox = OEM not modified

Exhaust pipe = Stock w/baffle removed.

Carb cleaned all orifices open, engine rebuilt, good compression. Runs strong and starts well cold (choke hardly needed) and hot. Runs well from idle to WOT. Pulls reasonably strong.

ISSUE: After WOT, when the throttle is closed and re-applied to about 1/3 - 1/2 throttle, the engine stumbles. (I assumed main jet rich or lean?)
Also, on whoops (closely spaced 2-3 foot bumps at moderate speed), the bike stumbles (carb bowl splash?). Carb bowl has plastic piece installed around main jet location. (bowl level too low?, seems odd if rich).

Spark plug is lightly sooty (black) after WOT then kill switch and coast to the garage (I suspected the main jet was too rich and was testing WOT rich/lean condition).

Based on the spark plug color (black and slightly sooty), easy start cold w/little choke, I assuming too rich, therefore:

Attempt #1: Leaned needle one slot to leanest position = top slot. Result: Poor idle, poor running.

Odd: Spark plug says too rich but when we leaned the needle, engine was not happy.
Weird: I did not expect the needle to impact idle and <1/3 throttle (much).

What should I try next? Many thanks.

-- Edited by zzrdd on Tuesday 11th of August 2015 05:27:03 PM

-- Edited by zzrdd on Tuesday 11th of August 2015 07:11:59 PM

PlumasDude · Senior Member

I'm riding a US 2003 model. Only thing on your list I can think of is the airbox and the condition of the jets. Or something more complicated with your carb that I wouldn't understand.

Maybe pull the snorkel and cut open the airbox (or test it by riding with the side of the box removed). The stock snorkel/box is absurdly restrictive, seems to me.

New main jet? If a prior owner tried to clean your main with a drill bit, for example ... . But if you have a 128 you must have replaced it already?

New idle jet (they're hard to clean)? Sounds totally wrong, and I'm probably displaying my ignorance by saying this, but what you say about idle issues and the whoops (where you're trying to pop it off the very low end) ... OK that can't be right, inconsistent with what you say about easy starting with no/little choke.

If you rebuilt then you've recently done valves and seals ...

After all sorts of fussing with a rich carb and idle problems I ended up back at stock 50/137, new idle jet, 2nd leanest needle setting, a big hole in the top of the airbox, and it finally ran like a charm. Clear gas too, but that probably makes no real difference.