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Old 07-22-2012, 07:41 PM
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Default Ignition Timing Loncin 50cc 4-stroke quad

Hi all.

My daughter has a 50cc pink chinese quad known affectionately as "Pinkchink". The engine is a copy of the long running Honda ohc 4-stroke motor and is branded "Loncin".

These little quads seem to require continuous fettling just to keep them running with their dodgy carbs and ever-decreasing valve clearances but Pinkchink has just presented a brand new problem; the ignition timing seems to have "slipped" to about 30 degrees BTDC. The motor will still start albeit with much persuasion but will not take throttle. There is a strong enough spark, it's just too soon!

I've checked thoroughly the mechanical possibilites and the rotor is still correctly located on the crankshaft, indicating TDC when the piston is actually there (mechanical inspection) and the camshaft location also agrees. The stator plate is secure and the trigger for the CDI system is firmly located and does not offer adjustment in any case. The magnet is still welded to the rotor as it should be. The rotor has individual marks scribed for TDC (T) and firing point (F) with a pair of adjacent lines at around 30mm BTDC which I assume represent the maximum advance range for the standard CDI unit. My timing strobe shows these maximum advance marks when starting and running. The rotor happens to be around 360mm in circumference, so 1mm between timing marks represents approx 1 degree hence my estimation of firing at 30 degrees BTDC.

I have access to a second CDI unit which I know to be sound; it produces the same result as Pinkchink's own unit and on this basis I am not inclined to blame the CDI unit.

I have taken some photographs but am not sure how to attach these to a post so please take a look in my garage if you're interested.

Is it possible that a trigger coil can become "lazy"? Does anyone have experience with this particular issue? All responses would be very welcome.

Thanks to all in advance...
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Old 07-22-2012, 11:25 PM
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This is a strange problem.

I have to wonder out loud - and I'm not an expert in this - would a quad start and idle with the ignition timing at 30 degrees before TDC? I would not think so. I think it would backfire on compression and stop. I know a lot about electronics, and only some about engines, and since this is an engine question perhaps others better than me on this mechanical question can comment.

How are you measuring the timing? Timing light? Clamp on Inductive pickup?

I would discount a "lazy pickup coil" hypothesis. This coil has to sense the arrival of a raised bump on the flywheel. The leading edge of this bump "arms" the CDI, and the trailing edge "fires" the CDI. The "firing" point varies a bit with engine speed. Faraday induction raises the firing trigger voltage proportional with engine speed, so with the CDI trigger point at a fixed voltage the ignition iming advances a bit with increased engine speed. The point is though, the raised bump edges cannot be sensed early. The design of the pickup coil is delberately made to have a very myopic sense range. It cannot see more than a few millimeters out - else it would see the enormous battery charge magnets on the inner face of the flywheel and get confused.

How sure are you about this 30 degrees before TDC? I'm wondering if your CDI is firing on the "arm pulse" produced at the leading edge of the raised bump.

How sure are you that subsituting a working CDI *does not fix the problem, or change the symptoms*?

I still wonder about a quad idling with the ignition timing at 30 degrees before TDC. My gut instinct says it won't idle at this timing. Please recheck this.

I'm going away for 10 days starting tomorrow. I may not be able to get back to this forum untill then...
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Old 07-23-2012, 06:17 AM
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Hi LynnEdwards.

Yes, it is indeed strange.

30 deg BTDC is my best estimate, it may be a little less but still in that range. The motor will only start with much persuasion and then only for very short bursts, it will not idle and usually stalls with a backfire characteristic of overadvanced ignition timing.

I'm measuring the timing with a strobe light which I know to be sound; it is an older type with a physical pickup connection between the plug cap and spark plug head. It is powered by the 12V battery on the quad which shares ground with the stator, CDI unit etc.

"Lazy" was a poor choice of word on my part but in my defence it was 2am when I was typing the post; a better word would have been "premature".

The distance (around the rotor circumference) between the firing and maximum advance marks corresponds closely to the length of the raised bump sensed by the pickup coil; could this be significant?

Is it possible for the coil to become faulty in such a way as to produce an increased voltage to the extent where the CDI trigger voltage is reached almost immediately upon the arrival of the leading edge of the raised bump?

Fitting the second CDI unit does not alter the symptoms; it is known to be working properly.

I would really like to solve this through logical diagnosis but I'm just about stumped..!

All posts welcomed.
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Old 07-29-2012, 07:20 PM
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Did this ever get solved?

I'm on vacation at the moment with limited internet access. I'm trying to get a few posts in tonight, but I won't have full access until I'm back home on thursday.

Just to make sure - this is firing and running at slow speeds well in front of the "F" mark? If you spin the flywheel around and position it where you see it with the timing light strobe, where is the raised bump on the flywheel that passes under the pickup coil? Is it at the leading edge? Middle? Trailing edge?

Also, where is the raised bump on the flywheel when the "f" mark is lined up with the pointer on the engine?
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