The WOT test (wide open throttle)

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Written by Steve Graham   
Saturday, 26 June 2004

This is a living document and will be ammended from time to time, as needed to make corrections, and as appropriate to accomodate new things as we learn them.  We are still learning the benefits and potential misreads that can come from this test, but at this point the feedback has been very good, and it has helped to identify lean conditions in many main jets.

This is a test that is fairly well known in Harley performance circles, and is now being adopted and adapted to the Road Star.

The test is as follows, and comes with the standard disclaimer in that it requires high speed for best reads and is potentially very dangerous.  If you elect to use it, it is at your own risk.  This is for informational purposes only.  It will give insight into how some have elected to tune their bikes.  It is not a recommendation that you use this method.  That choice is yours alone and you will be fully responsible should you choose to use it.

To get best results, we need both load and RPMs.  Given that the needed RPMs would equal extremely high speeds in fifth gear, I prefer to do it in 4th.

On the freeway in 4th gear, accelerate at WOT (wide open throttle) to 75 or 80 mph (the higher the better).  Acceleration needs to maintain for a couple of seconds to use up the squirt supplied by the accelerator pump.  I like to start around 55 or 60 mph.  After getting up to the appropriate speed, back off of the thottle 1/8th turn.  While doing so, pay attention to your RATE of acceleration.  You will still be speeding up, as you are still in a nearly full throttle position, so rate of acceleration is the key (feeling of acceleration or momentum).

  • Bike wants to increase rate of accelleration.  Main jet is lean and you need to increase main jet size
  • Bike wants to maintain accelleration, or feels like you have done nothing different when backing off.  Your main is right on the cusp, slightly lean.  Many factory Road Stars will react like this.  Dyna needles should be set to 4th clip for this jet if you keep the main, and Baron's should be set to 5th clip.
  • Bike looses some momentum (may still be accerating, but at a slower rate).  The main is not lean.

This test will not help to identify a rich main, only a lean one.  To get best results set needle clip on the 5th clip before trying this.  Clip positions above the 5th clip have mixed results, and some folks ended up getting reads on the needle instead of the main.

The Road Star will tolerate fairly rich running environments.  Because of this it can be hard to identify rich mains.  This test gives us a way to identify lean mains, and thus create a reference point to use in tuning our bikes.  If we know the main is lean, we can go to the next jet size, and keep doing so until we identify the first step up on the main that is not too lean.  That main is the right one for the bike and its accessories.

At this point we can move forward confidently with that main and proceed with adjusting the clip on the needle (if needed), and dialing in the PMS to balance out the other circuits in the carb.

Since we are still learning about this test ands its implications on tuning, please take the time to add feedback to this article should you decide to use it.



Questions should be asked in our forum (Use discuss link below). The forum is very active and you stand a good chance of getting your questions answered there. If you would like to leave feedback for the author, or have additional information you think will benefit others, please use the comment section at the bottom of this page.

Discuss this article on the forums. (6 posts)


DISCLAIMER: This information and procedure is provided as a courtesy and is for informational purposes only.  Neither the publishers nor the authors accept any responsibility for the accuracy, applicability, or suitability of this procedure.  You assume all risks associated with the use of this information.  NEITHER THE PUBLISHERs NOR THE AUTHORs SHALL IN ANY EVENT BE LIABLE FOR ANY DIRECT, INDIRECT, PUNITIVE, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, OF ANY NATURE ARISING OUT OF OR IN ANY WAY CONNECTED WITH THE USE OR MISUSE OF THIS INFORMATION OR LACK OF INFORMATION.  Any type of modification or service work on your motorcycle should always be performed by a professional mechanic. If performed incorrectly, this procedure may endanger the safety of you and others on your motorcycle and possibly invalidate your manufacturer’s warranty.


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  Comments (3)
Answer to the O2 question posted by twos
Written by pc_doc, on 11-05-2017 19:09
I do a lot of tuning using air flow sensors, (I usually use a Zirconium sensor which is commonly called and 02 sensor as opposed to a Titania type which requires more elctronics to test) I say this because Zirconia sensors are commonly called 02 sensors and Titania sensors are generally called Air flow sensors when in reality they are both air flow sensors. The actual name of both of these sensors is the LAMBDA sensor because this is what Bosch named it when they invented this. We are attempting to achieve LAMBDA which is the Stoichiometric ratio for maximum efficiency for a particular purpose, (generally 14.7 parts air to 1 part fuel 14.7:1) Coomon Zirconia sensors read the air fuel mixture in a properly heated exhaust stream approximately 17 times per second. Though this might sound fast even a two cylinder engine that idles at 850 rpm has 425 firing pulses per minute or about 7 times per second, (per cylinder). At 5,000 RPM the engine has 2500 firing pulses per minute or about 41 firing pulses per second. This is normally multiplied by 4, 6, or 8, cylinders that are filling the exhaust pipe. This is to say that the 02 sensor is a, "broad range" sensor. Zirconia sensors generally produce approximately 1 volt at a ratio of about 8:1 and will produce .01 volts at 26:1. (It's been a while since I went to school for this so the numbers are close but not exact). A properly running engine will generally increase the voltage to full rich upon acceleration and then level out pretty close to .5 volts when running at a steady speed such as idle or cruising speed. Generally you want to test the exhaust stream as far to the back of the exhaust flame but still within the exhaust flame, ( during my tests that's about 10" from the exhaust port on a 2007 Roadstar XV 1700 AT. Stock mixture is generally a bit lean at idle, full rich during acceleration, (accelerator pump) goes just a bit lean through the RPM range and goes lean when the throttle is held wide open for 7 seconds in 5th gear from 70 MPH. It is REALLY EASY to crash your brains out trying to watch the voltage changes through RPM ranges in a car. This multiplies by ten at least on a bike. Yes an air/ fuel sensor is a great tool to test for what is actually happening in your exhaust it requires drilling and welding an O2, "bung" into your exhaust and then using a graphing multi-meter with recording capabilities to properly diagnose and tune with. I have two welded into my exhausts because I am sort of psychotic. The readings that i gave you were taken from my completely stock 2007 Road* Silverado. The lean conditions got better at high RPM after the Stage 2 jet kit which used a 195 jet and a richer needle and jet. I will say that the Roadie ran exactly the same but showed rich from idle to full after i blocked off the AIR system from allowing extra air into the exhaust which means that Yamaha either had to use the A.I.R. system of fuel inject the bike in order to run rich enough for good acceleration and still pass emissions. They chose the latter. 14.7:1 is the correct ratio for proper combustion but not for acceleration, (needs to be richer) or for emissions, (can get 85% of performance with lower emissions at about 18:1) I hope that this answers your questions. sorry about the length of the answer. Joe :)
Written by twostroke, on 09-23-2008 20:00
I have found while undertaking, or attempting to perform the above test, that the bike performs well up to the middle, upper middle rev range, and then bottoms out, stutters and refuses to accelerate and bogs down.Is this a condition of overly rich mixture?
I wonder?
Written by vwtech0, on 05-24-2008 17:42
If you stuck an o2 sensor one of the exhausts and hooked it up to a decent ohm meter, what readings would i be looking for.

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