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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 3 Dec 2006
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XT altitude ajustments

I have been over a few high passes and my engine is not as resposive as normal. I´m heading higher in the next week or so and know there is an ajustment that can be made to change the air/petrol mix. I can service a bike, but beyond that am a bit of a novice, so any advice in layman's terms would be very much appriciated.

Cheers,

Iain.
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  #2  
Old 3 Dec 2006
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At altitude there's less oxygen so even if your fuel:air mixture is correct you will never have as much as much power as you will at sea-level, that's just something you have to live with unfortunately. If you think you are running too rich becasue of the altitude (sooty plug, misfire under load) you can reduce the amount of petrol in the mixture in two ways: from about 1/4 throttle to about 3/4 open the mixture is mostly controlled by the needle and needle jet in the carb'. The needle is normally held in place by a little spring clip sitting in a groove, but ther are normally several grooves. By lowering the needle (putting the clip in a higher groove) you will weaken the mixture, by raising it you will richen it. Above about 3/4 throttle the main jet controls the amount of fuel. The ony way of altering this part of the carburation is to change the jet for one with a different size hole (big hole = more juice = richer, small hole = less juice = weaker). All jets are stamped with a number which corresponds to the size of the hole but obviously you need a supply of main jets first...

However, unless you are really sure you know what you are doing, or the bike starts to seriously misfire due to being over-rich at altitude. I'd leave everything alone and accept that altitide will rob your power until you get lower. If you mess with the carb' and run too lean you will also run hot and risk damage by overheating or promote detonation which will seriously spoil the party
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Old 4 Dec 2006
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There may be a recessed pilot screw on your carb which will allow you to lean the mix a bit all around. However, I don't know what year you've got, and if the recess is plugged or not, so I can't elaborate. Look for it under the carb. It'll be a brass screw in a tunnel.
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Old 4 Dec 2006
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Thanks the advice.

If I ajust the brass screw, could I ended up messing things up or is it fairly safe?

Iain.
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  #5  
Old 4 Dec 2006
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Quote:
Originally Posted by iain
Thanks the advice.

If I ajust the brass screw, could I ended up messing things up or is it fairly safe?

Iain.
It is completely reversible if your bike starts acting funny. If the screw is on the front of the carb (engine side), it will control fuel supply; more turns = more fuel. Screw it in all the way counting rotations, then back it as little as is required for smoother running. Don't get your hopes up, though. Some bikes will come stock with only 0.5 rotation on the screw.
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Old 5 Dec 2006
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Electric!sheep, are you sure you aren't mistaking this brass screw for the tickover adjuster? If so it'll allow you to adjust the fuel/air ratio but only for the pilot circuit which is only used at idle. It'll have no affect whatsoever on the fuel ratio when riding the bike.
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Old 6 Dec 2006
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Quote:
Originally Posted by rickg
Electric!sheep, are you sure you aren't mistaking this brass screw for the tickover adjuster? If so it'll allow you to adjust the fuel/air ratio but only for the pilot circuit which is only used at idle. It'll have no affect whatsoever on the fuel ratio when riding the bike.
No. Idle adjuster is a knob, the brass screw is the pilot screw which is key at idle, but supplies fuel over the whole range.
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Old 6 Dec 2006
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it won't make any real difference...
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  #9  
Old 6 Dec 2006
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2 points.

1 the idle mixture/ pilot mixture screw makes NO difference to the mixture past 1/4 throttle FACT

2 A little knowledge is dangerous, lean off your mixture and when you decend the other side BANG, goes too lean and heat siezes engine or holes piston.

Live with the loss of power unless you know what your doing. ive tuned lots of bikes and carbs and to be honest its a fine art.
if you start messing about now youll end up with a bike that doesnt run or one that runs at the wrong mixture destroying the engine

leave it as it is as others have advised. its the best and safest bet.
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Old 6 Dec 2006
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Thanks, I think I´ll just go slowly, rather than risk any changes.
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  #11  
Old 7 Dec 2006
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Quote:
Originally Posted by electric!sheep
No. Idle adjuster is a knob, the brass screw is the pilot screw which is key at idle, but supplies fuel over the whole range.
The pilot jet only effects the fueling at closed throttle (idle) to 1/4 throttle, then its effects are so slight, its not worth mentioning.
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