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
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If performed incorrectly,
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|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?
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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