Need some details on the ignition system

The SR CDI unit is much more robust than most of us give it credit
had lots of trouble with ignition on my 81 SR, I swore it was the CDI but cured that problem
with an old XT carby, recently it became hard to start and would kick me back and backfire.
Finally found the problem, The mountings on the rear mudguard had crushed up and the plastic guard was touching the back of the battery box which is insulated from earth along with the CDI, you would probably say the same thing as me...IT CAN'T EARTH THROUGH PLASTIC...well the guard was abit dirty so goodbye some of the spark, have spaced the guard out and glued some rubber to the batt box...starts easy now, so EVERYBODY make sure you have clearance between the guard and the batt box.

singlespinner

It runs! but only on starting fluid. I don't think the carb is working at all. But I sprayed starting fluid in the carb, and it runs rather smoothly for a few seconds, so the ignition system is working. I feel confident in investing for a replacement carb, so I'll be ordering that in a few weeks, then hopefully dial in the ignition system. I'll post the part numbers soon for what i used in case someone wants to play around with it.

What's the status, Nick? Is it ready for Prime Time yet? I think I need a CDI.

Yes, and now you will have to run 98 octane gas when going over 36 degrees full advance to take full advantage of the advance, [ like me with the 39 degrees and XT250 ignition on our HL500] as for the two stroke carbs, I've just signed up to the pre copernican obscrurants club to fully appreciate the finer points of going backwards.

Due to failing to get a replacement ignition box, I been sitting down with one our avionics guys and going through this circuit and think we've sussed it. While it looks simple it is in fact a finely balanced bit of kit.

Basically this unit is using the rise in voltage level from the triggering coil, as the engine speed increases to create a synthetic advance. You need to draw this out on a graph to see it clearly.

The main factors are the size of the capacitor and the type of SCR. These combined with the primary coils in the ignition coil form basically what is a tuned circuit. To stop it ringing, or becoming an oscillator there is a damping diode.

The capacitor is charged by one set of coils, whose output is rectified by the diode network. The second set of coils trigger the SCR. you can alter the timing by changing the capacitor or the SCR, but by far the easiest way (if you remeber your capacitor charging time constants) is to mess about with the resistor network to change the advance curve. The thyistor is there to stabilise the circuits resistance with temperature, which shows how balanced this set up has to be to work properly.

We sat down and down some head scratching and came up with some componant values so I'm going to have a go at building one of these too. Although its simple in appearance there is a lot of thought gone in to this unit.

I love all this simple electronic stuff.

I wouldn't know, but it sounds messy!


Just as an up date I've rejigged the signal conditioning stage, opted for a heavy duty motor control thyristor and am using standard rectifier diodes. I've been doing some research and it turns out this type of ignition is typical of what is fitted to your average lawnmower engine (if it has CDi)!

I think the problems with the Yamaha CDI are caused by the parallel resistor networks. If one of the resistors packs up it will alter the ignition timing and pop goes your engine. I'm guessing it was either the thermal compensating resistor or one in the parallel pair that failed.

Just a word of caution: there are some surprisingly high voltages coming off the generator to power this thing! I haven't yet measured them, as I want to use the calibrated Fluke at work to map the pulse coil's output (it occured to me that you can build thiswith a rev limiter too), but I think it would be about 100 volts or more.