Ignition timing weirdness on my Chinese engine

[lingeringstin...]

Wow, I don’t understand this. Here’s the two alternator rotors I have.

One has a smaller sensor bump on it than the other one. They’re both from Chinese Honda twin clone engines, 125 and 250, are both physically the same size and have the keyway in the same spot. Both of the rotors only have one lug on them.


https://s26.postimg.org/ea5pl4nd5/rotors_1.jpg


https://s26.postimg.org/91aon94y1/rotors_2.jpg


The one with the smaller bump is from a 250 twin (253FMM) with a 360 crank. It uses one pulse sensor at the top of the alternator case. The other one is from a Jinlun 125 twin engine (244FMI I think) that had a 180 crank. It has two sensors, one in exactly the same place as the 250 and then another sensor 180 degrees opposite at the bottom of the alternator case. The 250 fires once every revolution of the crank and the 125 fires twice per revolution.

The alternator cases are made the same and are interchangeable on either engine, it’s just that one has two sensors in it because it has to fire twice per revolution, but the sensor at the top on both of them is the same part and is positioned in the same place.

The question I have is why are the sensor bumps on these things in different positions? I would have thought the sensor lugs on the rotors would have been in exactly the same spot for both of them since the cranks both need to fire at least once per revolution in the same spot.



https://s26.postimg.org/hxlgr6vk9/timing_marks_1.jpg


https://s26.postimg.org/hm40eff49/timing_marks_2.jpg


https://s26.postimg.org/70k52f8sp/timing_marks_3.jpg



Also notice that the timing marks on them are slightly off. That makes no sense to me. The 125 crank has to also fire 180 degrees out because it’s a 180 crank but that shouldn’t make any difference whatsoever to the timing of the other piston.

The 125 rotor also has timing marks on the opposite side for the other piston but this seems pointless since if you get the first one right the second one can’t help but be correct also, but there you are.

The 250 engine uses a 53mm stroke and the 125 engine uses a 44mm stroke but I can’t see why that would make any difference whatsoever to the timing of TDC.

I know the cranks are basically the same because I’ve even had the 180 degree 125 crank taken apart and pressed back together as a 360 crank for another engine I’m making that I wanted a 360 crank for. There was no problem doing this so I know the design of the cranks between the 180 and the 360 are basically the same apart from how long the con rods are, which as I said should make no damn difference whatsoever to where TDC is.

I don't know if using an AC or a DC fired CDI box would make any difference whatsoever to the timing of the spark. I can't find any information that implies there's any time difference between the two types of CDI. To my mind electricity happens faster than a junkyard hound nips at dog pecker gnats so they should fire at the same time.

The rotor with the smaller lug is the one that came off the 250 engine I'm currently using so I know it's the right one. The engine came off a quadbike so I'm wondering do they have more sedate timing for some reason compared to an engine designed for bike use?

My next question is what would happen if I fitted the the “wrong” rotor to my 250 engine? The timing for the 125 rotor with the 180 crank is clearly advanced compared to the 250 one, but by how much and why?

If you look at the timing marks and put the “T” in the same place, where I assume it means “top”, then you can see the sensor lugs are slightly off and the one for the 125 is even longer than the other one.

Ignoring the length of the lug for the moment, I would assume using the 125 rotor would make the timing more advanced than the other one. Would this serve any purpose? Would I get any performance enhancement from having more advanced timing? The whole thing is a mystery to me.

[Teflon-Mike]

Look at the C/R numbers for 125 motors vs 233's.. there in is your clue.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[lingeringstin...]

Compression ratio? Who? What? WHY? This sounds like four stroke electronic ignition wizardry to me which I know very little about. I do wish I could alter the timing of this ignition on the fly so I could set it the old school way like I used to do with points.

[Robby]

On both of them the bump is before TDC. The CDI should also be controlling the ignition advance, so decides how much delay to put in before sending the spark.

[Teflon-Mike]

The 125's all run I think 10.5:1C/R, where the 233's run something slightly over 9:1; the 125's 'just' rev out a little more; the 180 timed versions a tad extra thanks to cam-timing...

The igniter-boxes are pretty clever, but also pretty simple bits of kit; they have a pre-programmed 'advance' mechanism known as advance by retard' in them; the pick-up is 'static' timed like a 2T for maximum advance, B-U-T clever bit is that thanks to the voodoo of A/C electricals rather than even electronics, the amplfied output can be convinced to 'lag' behind the input, and the 'lag' tuned to reduce as the input frequency increases.. I think that the static timing of the trigger is something daft like 20Deg before TDC, but if you put a strobe on the 'lag' the black box offers gradually reduces towards the static advance as revs rise... no fiddly bob-weights and springs required.

For something designed in the late 70's/early 80's, then expensive programmable electronics; it could all be done with greek-elecrickery, and varying proportions or earth wind and fire.. sorry, capacitance, inductance and impedance, provided by an analogue circuit of capacitors and coils and resistors, while only the amplification need be done with a simple transistor. It was a pretty clever solution, utilizing 'cheap' electrickery of the day to minimise mechanical complexity and keep down electronic costs.

A four-stroke car engine of the old sckool, like a Mini's A-Series would run a distributor, that had a mechanical advance system, that comprised usually a back-plate shifted points around the opening cam, depending on engine rpm, and a vacuum advance, that would shift it a bit more depending on throttle opening.

On bikes this refinement was oft omitted, and they were static timed for max power, as part throttle response was usually not so important n a lighter vehicle, and economy not so critical.

Two-Smokes? Curiously don't have a particularly wide rev-range.. so rev-varied advance isn't 'so' critical as on a 4T; may have very narrow power-bands when tuned, and a long way up the rev range, and that's where the motor is going t be most oft made to work, so why worry about the fluffiness of lower speed running? Your piston-ported MZ motor, has peak power at what, 6K revs? I think that my old Cota, that is peculiarly soft tuned for trials makes it's full quota of poke at barely 5K... so with an effective operating rev-range of only a few 1ooo revs, they can quite happily dispense with complicated advance mechanisms, and run static timed points that are close enough for low speed running and peak power, for simplicity/cost/reliability.

BUT.. answer is that 'optimal' igntion timing changes with the state of tune, ie cam-shaft or porting; the compression ratio, and the cylinder 'fill' as well as whether the motor is in the static state at constant RPM or accelerating; hence why advance curves and vacuum-advance mechanisms can be useful to re-optimise the ignition for best-power or best-ecconomy, on the fly, in operation.

So that is the likely explanation; if you check CMSL part numbers, you'll find that Honda used the same igniter box, with the same advance by retard curve in it, pretty much across all the CDI equipped Benly motor bikes, as well as a fair few of the CG and XL derived small singles, and merely modded the pick-up position on the rotor to set static timing to better suit.

It was more refined than static timed points, and cheaper and more reliable than bob-weight advance mechanisms.
____________________
My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[stevo as b4]

Simple fixed 2T ignition timing works ok as a cheap compromise on certain machines. But wait until you start playing with expansion chamber design and development, as then it throws a big spanner in the works regarding ignition timing, and I'm told has a huge effect on both performance and exhaust design.

Looking forward to seeing how it works myself when I get my bike back on the dyno for some set up.

[lingeringstin...]