QT50/MJ50 vs YT60 CDI: The truth revealed!

I have been using a Yamaha YT-60 CDI on my MJ50 Towny for the past year or so. It is commonly thought that the YT-60 CDI is better, because

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

Same ohms on the ignition coil? Good to know. Just have to consider cdi boxes now

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

This relates to my interests. I am building up my qt and was almost about to switch CDIs. Thanks for this poast. Any thoughts on if another CDI might work better? pw80? I dunno just thinking out loud.

Are the cranks the same on the MJ and QT? Have you thought about inner rotor? I plan to do some measurements on my parts engine crank soon as I get the mothereffer opened. I need some fuel for my torch as the bolts are all locktite sealed and since they are phillips head bolts I strip them trying to break em loose.

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

^no heat necessary to get the case bolts loose only an impact screwdriver. MJ and Qt crankshafts are not the same, the main difference is the right side crank half. My old engine had the left side crank half of a qt pressed to a right side crank half of a mj, a left side case half of a qt bolted to a right side case half of mj. Frankenmotor, ran fine.

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

No impact driver either :(

I'm taking it to a friend's this weekend. He'll get it apart.

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

oh, but to get the bearings in you do need a propane torch.

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

Or an oven and a freezer.

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

Do they have that impact driver in store? Don't look as fun as the old hammer and screw driver wresting match, but I want one anyways!!! Good tip, thanks.

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

they do at mine and so I think they do everywhere

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

Tate in Madison /

Well I know QT50s and Razzi have interchangeable ignition coils and that leads me to believe lighting coils as well.

If Razzi and QT50s have interchangeable electric bits it is reasonable to believe they have interchangeable CDIs.

If memory serves me, Razzi and vertical minarelli engines have the same CDI,

So I am thinking maybe a CDI for a jog or early zuma would work on a QT.

I have a performance Malossi CDI here and I also have a Jog here so I will test out this theory tomorrow.

If it pans out then problem solved. American Jogs had unlimited CDI boxes.

This would be nice if it works.

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

well the qt's cdi was never limited. In fact the ones with the "start" function have a second cdi(you can see that in different year schematics) that does indeed have a limiter along with a higher voltage spark. The infamous qt "rev limiter" is the four stroking that occurs when performance anything is installed and the rpms are raised. This is because of the factory adjustment on the oil pump. This rumor's proponents have been new people to the qt. What I'm interested is programmable CDI boxes that will work, maybe even with a vacuum sensor so our bikes will stay cooler as it will full retard the spark when the throttle is closed and vacuum increases(some old vw centrifugal with vacuum diaphragm distys accomplished the same thing). As far as I know the yamaha's cdi is an ac type of 50 or so volts sent to it from the trigger coil. The cdi will run without a battery, voltage regulator or a rectifier. There are also DC CDI's that NEED a battery, voltage regulator and of course a rectifier. But it's kinda of simple to change out the coils and the CDI box for whatever you want so long as the parts are available and/or cheap. But a programmable AC CDI would be a sweet find. I would bet that any minarelli horizontal performance cdi would work(and there's a ton). I have one so I'll have to see if it works.

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

here's the diagram of the qt with the control box

Here's the diagram of the qt without the control box

Here's a couple of interesting diagrams of 5 wire and 4 wire cdi's

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

Ryan, you could get buckwild and get a HPI with programmable CDI. Or with the same cash, you could buy 3 qt50s. :\

Do qt cdi's have a curve?

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

Wow, what a thread this turned out to be!

Anyway, I thought that I'd chime in here and say a few things.

The CDI used in the QT50/MJ50/YT60 and early PW50s is a CDI in its most simple form. This is late 60's-Early 70's technology here - there is no computer or processor, so there is no "map" or real curve, per se. For the most part, the timing curve is determined by the pulser coil in the magneto. At low speeds, the voltage developed by the pulser coil triggers the CDI when the magnet has almost completely passed the pulser coil. At higher speeds, due to the nature of magnetic flux, the necessary trigger voltage is developed sooner, so the CDI fires the spark plug sooner - advancing the timing. Some pulser coils have a specially designed trigger head (the part that is closest to the passing magnet in the flywheel) that takes advantage of this, creating a quasi-advance curve. This is why there isn't much you can do to the CDI itself to gain more performance. There are, however, two components in the circuit that may be able to be tweaked a little in order to gain a little more performance. I am planning on building a copy of the Yamaha CDI from scratch to do some experimenting with, and am waiting on the parts to arrive. I will post back with my findings as soon as I get the CDI built and tested.

The other option, is of course to fit a different CDI from another machine, as is mentioned above. That may work, and be the easiest route if it does. One thing to keep in mind though, is that each CDI is designed to work with a specific type of charge, pulser, and ignition coils, not to mention some require a battery in the system as well. The pulser and charge coils provide a specific voltage level for each of their functions, and the ignition coil primary winding should be matched to the output of the CDI it is used with. A CDI that requires a battery could well likely have some sort of processor in it, which would be best from a performance standpoint, but some of these same units have the built-in restriction as well...

There has been mention of the "second CDI", called the "control unit" by Yamaha. Its main function is to limit the engine RPMs when the bike is started, so the bike doesn't accidentally run away from the rider. (I can't confirm nor deny that the unit also provides a stronger spark for starting...) At any rate, for what its worth, you _can_ run with the control unit completely disconnected. I discovered this last year while troubleshooting an ignition problem and studying the wiring diagrams. I have had mine unhooked and physically removed from my MJ50 for over a year with no issues...

I am curious to see the results from other people's CDI "experiments". Keep us posted!

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

^you sir know your shit.

I've seen sudo programmable cdi's on ebay with dip switches. Just a guess but I doubt that the switches do anything but change resistance in the circuit or something similar and are in fact not digital. Sterret, is this the path that you are talking about when you talk about you modifying a stock repro cdi? I would love to get my hands on a schematic of one of those dip switch cdi's. I bet I could build one with all the crap in my garage and maybe hit up electronic goldmine or some other online electronic surplus store for what I don't have. Sounds super sick.

here's the cheezy ebay dip switch cdi http://cgi.ebay.com/ebaymotors/ADJUSTABLE-RACE-CDi-BOX-APRILIA-Area-51-Sonic-SR50-_W0QQcmdZViewItemQQhashZitem2a088c5b59QQitemZ180532042585QQptZMotorcyclesQ5fPartsQ5fAccessories

Here's a neat site with schematics of diy true digital cdi's

http://peony888.com/VS/programmableCDI.htm

super neat stuff.

MA is the new NASA. That's right scrap the shuttle program we'll do it with a puch ridden with a dude covered in tin foil.

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

awesome! Would you mind posting the schematic?

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

yeah sterret can you post the schematic of the stock cdi?

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

Sure, no problem. I'll try to get it posted up later today or tomorrow. I don't have it on the computer I am using right now.

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

another quick question if you don't mind, (sort of related)

I'm having issues with an over-heating stator coil, I have decided to wire power resistors in series with it to act as a ballast. Will this help alleviate the load and thus the heat level of my stator coil? Or am I completely backwards in my thinking?

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

bump

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

Sorry gang, I got unexpectedly busy over the weekend.

Here is the schematic

1280759035_yamaha_cdi_schematic.jpg

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

Yeah, amazingly, that's all there is to it!

wiffleball - as for your stator coil overheating, adding a power resistor in series will do nothing more than limit the current (amps) available to your electrical stuff, as well as cause the available voltage to drop (depending on what value resistor(s) you use...). The added resistor(s) will still be seen as an additional load by your stator coil, and I don't think that it will help your situation.

Last summer, I had ignition problems with my MJ50, which I traced to a "bad" charge coil in the magneto. It would cause ignition breakup when it got hot. Well, long story short, I put in a new charge coil and all was well, or so I thought. Apparently, my engine was running hot - too hot, (due to some mods I did and not getting the jetting right) and I ended up seizing the damn thing soon after. Had I been jetted properly, it would have been running at the proper temperature, and I wouldn't have had the problem with the charge coil to begin with... I mention this in passing, in case your situation is similar to what mine was, and maybe your engine is running too hot as well - and causing your stator coil to overheat. Hopefully some of this info helps...

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

Awesome! Thanks so much. What is the purpose of D1 in that schematic? Is it there to scavenge extra current and feed it back into C2 to shorten the cycle time for higher RPM? The reason I ask is that I built my own CDI for my Jawa and it has nothing that feeds back to the capacitor like that.

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

sterret, in what way are you planning on modifying the cdi?

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

hopefully it's for SPEED!

Re: QT50/MJ50 vs YT60 CDI: The truth revealed!

You asked about the purpose of D1. After the CDI "fires", C2 discharges into the ignition coil primary which generates high voltage on the secondary (the plug wire) and the plug sparks. After the plug sparks, the collapsing magnetic field in the ignition coil develops a reverse current flow via D1, which partially charges capacitor C2. So, yes you are essentially correct on your thinking.

As for the potential modification, that would involve changing the value of either C1 or R3 (or both). These two components are the "conditioning" circuit for the triggering of the SCR. Their primary purpose is to screen out any electrical noise that would cause the CDI unit to fire at the wrong times, or other issues that would cause false triggering. A secondary purpose of this part of the circuit is to retard the timing. CDI ignitions will advance automatically, and will advance too far if left to do so. The retard function of this part of the circuit _may_ also be a speed limiting device, not allowing the timing to advance as far as it should and thereby reducing power at higher engine RPM. The only way to know for sure is to experiment, and that is what I intend to do. I am going to build a CDI from scratch as soon as I can find the time, and see what happens. I have all the parts and will get to it as soon as I can. Hopefully by the end of the week.

The results are finally in!

Sorry it has taken me so long to get back in here on this. I have been trying to find time to post back about my findings for a while, but things have been very busy on my end...

Anyway, I spoke with the (now retired) engineer where I work, and he suggested leaving C1 alone and experimenting with R3. I built a CDI from scratch, and made it so I could

Re: The results are finally in!

big hill. jk

ever think about putting a rheostat in there to have adjustable resistance? Tweak till good then measure and replace with appropriate resistor.

Re: The results are finally in!

this is awesome stuff

are you going to fry the transistor by pushing more current through it with a weaker resistor?

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