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Jetting 101- The Basics


georgestr

Originally Posted By: PA_Kevin in the TTR forum.

Jetting 101:

Question:

I thought my Yamaha TTR 125 would idle without the choke. But with when the choke was off it would not idle. But the opposite happened with the choke.

Is there any particular reason for this?

Answer:

Your carb meters the fuel to your engine. The carb has three different sub systems:

Pilot Jetting - for up to 1/4 throttle position

Needle Jetting - 1/4 > 3/4 throttle position

Main Jetting - 3/4 > full throttle

These jets allow a fixed amount of fuel, the needle allowing an "operating band" of fuel metering.

To alter the amount of fuel, you can either increase or decrease the orifice (the hole in the jet). This applies to your pilot and main jet.

The needle jet is actually a needle. The fuel first passes through the main jet, then the needle limits the fuel flow (kinda like putting your pinky finger into a garden hose. Water still comes out, just not as much).

The reason your bike won't idle:

Fuel Screw

The Fuel Screw is mis-labeled in the Yamaha manual. It is labeled as an "Air Screw". This is 100% backwards. Air screws are on two stroke carbs. The last I checked, this carb has been on this 4 stroke engine.

The fuel screw allows more fuel through the pilot circuit. This very definitely affects low speed operation. To allow more fuel flow or richen up the bottom end, the fuel screw is turned counter-clockwise (or turned OUT). This IS richening up the bottom end.

To lean it out, the fuel screw must be turned clockwise (or turned in). The problem with this low speed/idle jetting is it is too lean. The only time I think you would turn it in is if you decide to ride your TTR into the Rockie Mountains, where bikes are starving for air, which ain't there!

Pilot Jet

First, your pilot jet is absolutely too small. You need to go up in size to a 17.5. The fuel screw on your carb allows a VARIABLE amount of fuel to pass through the pilot jet. This fuel screw WILL ABSOLUTELY affect your idle. The fuel screw has a spring behind it that allows you to turn it without it falling out of your carb. Turning it in REDUCES the amount of fuel (leaning the fuel/air mixture) and conversely turning it out INCREASES the amount of fuel (richening the fuel/air mixture). The bike IS starving for fuel at idle because the pilot jet is too small. Regardless of the fuel screw position, your bike NEEDS MORE FUEL. If you end up turning your fuel screw out 3 turns or more, your pilot jet IS TOO SMALL.

Needle Jet

The needle jet can be changed as well, or raised or lowered. So far, I have not seen any recommendations to swap out needles. Needles vary in diameter, AND taper in size, i.e. the tip of the needle is skinnier than the diameter half way up the needle. So as the needle is being pulled up, the needle gets skinnier. This allows MORE fuel as the needle is raised up via the throttle cable.

There is a clip on the needle that allows you to raise or lower the needle, effectively changing the amount of fuel flowing in this 1/4 > 3/4 throttle position. So, if you move the clip down, this effectively raises the needle. Since the needle is now higher in the carb, and we know the needle is skinnier at the bottom, MORE fuel will pass through in the midrange than before you moved the clip.

As for swapping out the needle, the skinnier the needle, the more fuel allowed into your engine.

Main Jet

At 3/4 throttle and higher, the ONLY thing metering fuel is the main jet. If your bike has problems at wide open throttle, the main jet is your culprit (unless it's your ignition...???). A larger main allows more fuel.

When you modify your bike, i.e. cut your airbox lid, install a high air flow airfilter, you are changing the amount of air into the engine.

When you install an aftermarket exhaust pipe, more air (exhaust) is coming out of your engine.

In BOTH of these conditions, you MUST increase the fuel going to your engine to offset the air going into (or out of) your engine.

Moving more air means you have to move more fuel.

Elevation & Weather Changes

When there is a change in the weather or riding elevation, the amount of Oxygen or moisture will be the culprit. At higher elevations, there is less O2 available. You will need to jet lean to decrease the amount of fuel to compensate for the lower O2.

If it is humid, there is MORE water in the air, you MAY have to jet lean.

In the winter, bikes run lean. Because of the lower temps and humidity, there is more O2 due to density changes. Your bike will run lean, and you will benefit w/ RICHER jetting.

I hope this helps you understand carbs better.

If you have ANY recomendations/comments/questions to IMPROVE or DISPUTE this post in any way, please PM me. I am wide open to critique!!

Kevin




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