Information: On how the fueling system works on the XT660

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TitanKing

Puppy
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Jul 14, 2016
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Location
Cape Town
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Yamaha XT 660 Z
I have noticed a lot of confusion surrounding the fueling system of the last XT660 models. Gaining recent knowledge around this subject from the XT660.com (Kev) forums I would like to share that knowledge here and clear some things up. Any additional info is welcome.

The 2007-2015 XT range:
Most would agree that the XT has a superb chassis, a great engine and is such an enjoyable bike, I drive a 2014 XTZ. It does have a few small annoyances (little top heavy, etc.) but the greatness of the bike makes you forget about this easily.

The main problem:
The bike does not ride smooth if you are picky, it can feel jerky and sometimes feels like a wild stallion. This really is not only the bikes design but certain emission laws that cripples the engine, which luckily can be fixed at a low cost.

Why does a law cause an issue?:
The law applicable in many countries requires the engine to run lean with mixture ration of (14.7:1), this means more of the gasses are burned out before it leaves the exhaust. This is the first problem, engines don't like running lean, they like running perfect.

What does this cause?:
The question is more, at what point does this cause a jerky engine. To explain this we need to delve in a little deeper.

There are two sides to the fuel system, it is commonly called closed loop and open loop.
The ECU switches to open loop and closed loop when certain conditions are met.
This results in the fuel mixture suddenly switching over to a different ratio, which makes the engine surge and behave differently upon switching, which in turns causes the jerky riding conditions at certain revs or when certain conditions are met. This is directly contributed to emission control.

When does the loops switch:


Open Loop:

  • ECU detects sudden acceleration
  • Any RPM above 4500
  • All throttle openings above>20%


Closed Loop (emision control kicks in):

  • Coolant temp of 70c is reached
  • Any RPM above 4500
  • All throttle openings below<20%

Now the ECU takes a voltage reading from the O2 sensor and will lean the A/F ration out until sees a lean ratio of 14.7:1 = JURKY!

What is common thought solutions?:
If you think a power commander alone can fix this mixture issue, you are wrong. The issue is that any fuel mod can ONLY adjust the mapping on closes loop, which will NOT make the jerkiness go away, also only a few people can afford a power commander. So in my eyes not a solutions.

Yamaha thought it good to make it extremely difficult to set the mixtures (almost all modern bikes and cars do actually make it difficult, because they all have open and closed loop if they have fuel injection and emision control), its almost as if it is secured. (My opinion): The Yamaha XT660 would have been an almost perfect bike if it had a carburetor, now performance is the the hand of people making laws that is self serving and does not address the issue of what really pollutes the air.

What about the Yamaha F1 Diag tool, there you can set O2 mixtures right?
This is simply a useless setting, this is just for idling throttle below<2% adjusting this will be of no help at all.

What would have been ideal?
We know more air to the engine is more power right, but that does not help us if the bike is running this lean already, more air is even more lean if we cant enrich the fuel mixture (for more power), which means an even warmer flame and burned valves after a while. You can't even run DNA stage 2 or 3 filters with this silly mixtures and the inability to adjust it will hurt the engine if you do have these filter setup. You will note that most newer models did come with a DNA stage 2 filter, but it also came with a DNA air restrictor which limits the air intake to almost stock so it does not burn even leaner, this is called DNA stage 1.5

How can we fix this at an affordable price then?
First of all, I fixed mine to a perfect ratio of 13.6:1 over the closed and open loop range and at an affordable price. The bike is a different beast, smooth, the engine purrs better, offroad you have way more control, in general, its probably the best mod on my bike. It feels like a different bike and adds an amazing experience. So it is worth while.

We need to fix the two different loops independently then.

Closed loop

This is the easy one, a cheap fix is to have a 10k ohm variable resistor inline with the airbox sensor which increases the resistance of the sensor making the bike think it is colder than what it is and enriches the engine.  This is where a power commander wil work (for the closed loop only).


Open loop

Fool the O2 sensor that is mounting on the exhaust, this sensor takes a voltage reading in open loop and leans the engine out. Its not easy to fool this sensor, the ECU detects faulty signals, for instance the sensor eliminators does not work.

Kev:
The ECU constantly looks for 0 to 1 volt square wave signal, by unplugging the O2 sensor
& fitting a resistor it sends a constant voltage signal to the ECU, after a few min's the
ECU see this as an corrupt signal & then constantly switches the A/F ratio up & down, in
this state the bike is not tuneable.

By controlling the closed loop circuit we are able to reduce the surging when you are
cruising, with one of the latest O2 devices when the ECU switches between the open &
closed loop circuits the transition is much smoother as the A/F ratios are closer to each
other, see table below.

The only device that works then is the electronic mod Kev build that plugs between the O2 sensor which regulates the signal to ensure a 13.6:1 ration.

To sum it up:
You need two mods, one to richen the closed loop and one that richens the open loop which makes the changes between open and closed loop the same giving you a super smooth ride.

How do I get these mods?
We cant really, Kevin does not ship to South Africa because we have people that steals all our post. Though, I asked nicely and he couriered it to me, hes mods are of great quality, sealed and watertight with proper wiring (plug and play). It comes with decent documentation. I bough both mods from him for R1900 excl. customs and courier.

Instead of everyone asking him independently, if there is enough interest then I will speak with Kev, we should then have Flying Brick import 20 mods or so.

It can be even better:
There are two more free mods (I did both) you can do to make the bike even smoother, these are very much recommended:
The twitchy throttle mod:
https://www.xt660.com/showthread.php?p=154983
The cush drive mod
https://www.xt660.com/showthread.php?t=12422

Hope this thread can clear up some fueling questions regarding the fueling and solve your jerkyness issues.


 
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