My understanding on how carburetors should be "tuned" is that on a warm bike you need to play with the pilot screws to find the "highest idle" on the bike when adjusting each carburetor screw, resetting the main idle after each is done. As you progress down the cylinders it gets a bit harder. This is very time consuming, and it relies on someone knowing what the highest idle is "by ear" (also challenging). I have a theory that you should be able to tune a carburetor by using the AC current coming from the stator, and I am wondering if anyone else would like to experiment with me to prove this theory. Here is my logic: 1. Highest idle = Highest RPM 2. Highest RPM = Highest crankshaft rotational velocity 3. As the flywheel is attached to the crankshaft, the magnetic field (and consequently AC current) generated by its rotation around the stator should therefore proportionally increase with RPM. So the guts of it is, if we can measure the AC current being sent to the regulator rectifier from the stator as we mess with the pilot screws, we should be able to find the "highest current" which would proportionally translate to "highest RPM" therefore eliminating the need to try and do it "by ear". Thoughts...? I am yet to test this as I have not had the time yet but over the Christmas break I intend to give it a shot. All theory at the moment but it seems to make logical sense to me...
Sounds like it may work if you have a meter sensitive enough to detect the variance in the current flow. Would it not be simpler to use a tacho of the correct low rev scale connected to the engine output?
Well that's what I don't know. I have a cheap multimeter that works ok for most applications but I know it is better at detecting step changes. If someone had a better one, a fluke or similar I'd be interested to hear how it goes. I'm sure as you get "finer and finer" in the tuning the changes in RPM will be minimal if not hardly noticable, but if that was the case you wouldn't hear it by ear anyway I wouldn't think. You'd then be test riding and checking performance on that particular setting. The other thing is not all bikes have tachos. That may work in some instances but I am trying to find a more "universal" method - the majority of bikes would have a charging system.
Hmm well when I lean out the fuel supply to my bike, the RPM slows until it eventually cuts out. What logic was given when it was explained to you mate? interested to hear the thinking for that one.
Isn't a tachometer the same thing? If you use a unit where you wrap the wire around the spark or coil lead, you can pretty much use it on any engine too. Nonetheless, I usually have enough luck doing it by ear although I normally spend time tuning single or twin cylinder bikes so it's fairly easy.
So I had a good crack at this yesterday with the tacho. I am definitely getting a current, but it does not seem to change proportionally to RPM. I was connecting the multimeter at the rear of the tacho. Next opportunity I'll try the direct output from the stator, alas I fear I will need a much more precise way of measuring the current to achieve this effectively.
I have tried in the past to record the RPM with an Arduino. It's not as simple as just measuring the current because the tacho signal is a square wave (so current is roughly the same for all RPMs, except maybe at low RPM where generally the whole bike has a slight voltage drop). So it shouldn't really change too much in terms of voltage, but the frequency should change. Which makes sense because the time it takes for the crank to turn once is one revolution, so then the period (or frequency which is 1/T) is then converted to RPM. I did have a working concept but I don't know enough about electronics yet or have an oscilloscope to check what I'm doing thus wasn't particularly accurate and has a lot of noise (needed to filter the signal). It's also pretty much how any aftermarket tacho works but you get the same signal by measuring the firing of the sparkplug (same concept, measured at a different point) whereas the actual tacho might get the signal from another sensor or the ECU/CDI etc. For your reference, this is what I had come up with: /* FreqMeasure - Example with serial output * http://www.pjrc.com/teensy/td_libs_FreqMeasure.html * * This example code is in the public domain. */ #include <FreqMeasure.h> void setup() { Serial.begin(57600); FreqMeasure.begin(); } int RPM = 0; double sum=0; int count=0; void loop() { if (FreqMeasure.available()) { // average several reading together sum = sum + FreqMeasure.read(); count = count + 1; if (count > 60) { float frequency = FreqMeasure.countToFrequency(sum / count); //Serial.println(frequency); sum = 0; count = 0; RPM = frequency * 30; Serial.println(RPM); } } }
Hmm interesting... didn't know it was a square wave though. I wonder if the signal directly out of the stator is a common AC voltage one. Will have to test that. Either way it sounds like it is getting complicated. I might have to do some more reading to refresh my memory on all this engineering stuff and see if I can come up with a simple way to do it. I'm not motivated enough to make it complicated, by that stage I may as well just borrow a noise meter and do it by (electronic) "ear"!
Well depending on your multimeter it might have the option to measure the frequency of the signal (a higher end one should have that feature). Not too sure if measuring the stator voltage will mean much but there is the pick ups you could tap into and grab that signal. You really need something to interpret whatever signal you get though (which is what the multimeter can possibly do). It's really simple to convert Hz to RPM as well. You could even work out the RPM from the sound of the exhaust (which is what competing racing teams sometimes do to see what the other teams are doing with engine development) but not sure exactly what is needed there, probably couldn't be done in real time either.
