General Rough running while hot

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General Rough running while hot

PointFiveO

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Mar 24, 2012
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Hi All, sorry to raise this issue as I know it has been raised in one form or other recently. Last week while driving my 500 which has a 650 engine with 123 electronic ignition I noticed after a short while that it was starting to run roughly and was noisy and quite jerky. I stopped and saw that the lead connected to the spark plug at the back of the engine bay seemed a bit loose, although connected you could wiggle the lead about a fair bit. On returning home I tried to tighten up the lead to no avail so changed spark plugs and leads. this seemed to run ok for about 5 minutes but as before once hot started to run erratically getting hesitant to the point of almost cutting out at times, hardly pulling above 5-10 mph and sounding almost like it was running on one cylinder. After some trawling through the forum and convinced it was a sparking issue I ordered a new coil, duly fitted it and started the car. To my amazement the car started first turn, previously it alway took a bit of churning and it also idled more smoothly and quietly than ever before. After letting it idle for 10 minutes to be sure no hesitancy was creeping in once warmed I decided it was safe to drive around the block. Again it pulled more smoothly and cleanly that it had in the last seven years since the engine was rebuilt. the sight jerkiness that was previously there while flooring the accelerator or lifting off abruptly, which I had put down to it having been fitted with a larger carburettor (weber 30 dic) during the rebuild, was now gone. Unfortunately after 5 minutes of trouble free driving the jerkiness and hesitancy started to return so I quickly returned home.
I'm now left wondering if the coil had been my problem or not. Is it possible for an ignition coil to run badly but still run? The car ran so much better than it had in years initially on the new coil that I was convinced that the old coil had been the issue. Could it be possible that the old coil was the issue but that the new coil became damaged after just 10 minute running? I've just read a thread that says you need to run these twin spark coils with a resistor and I don't have one fitted. However neither did the previous coil and it ran for 7 years. Sorry for the long winded ramblings, any opinions from the wise and experienced would be appreciated. My other thought as it runs ok until hot was that maybe it was an issue withe the carb; blockage/air leak...

Thanks,
Colm.
 
The problem could be the '123' unit itself. All these electronic units have a problem with heat---they don't like it. Unlike almost all other cars, the distributor on the 500/126 engines is directly in-line with a blast of hot air---the 'cooling' air from the engine. I had a major problem with a 'simple' electronic ignition (hall effect type) in that eventually, the car was undriveable---it back-fired violently and there were flames coming out of the exhaust. I have (hopefully) got round the problem by fittind a small 'air-dam' inside the thermostat housing to prevent the hot air flowing over the distributor. When I checked the temperatures of the distributor body vis-a-vie the thermostat housing after a longish run, the temperature on the distributor was about 12 degrees lower than the thermostat housing, and under 100C
If you still have your 'points' distributor, try fitting that back on whilst still using the same coil. Depending on whether the problem goes away or not,will show you where the fault lies.
 
Thanks Ian, I was wondering if it might be the 123 itself. I had read they can eventually go. I don't have the original points as the car came into my possession with the 123 fitted. You've given me the idea though to try to divert/shield the direct blast of heat from the unit. If this results in taking a bit longer than the usual 7 or 8 minutes before stating to misbehave it might point to the 123/distributor being the problem.

Colm.
 
Be careful Colm, when these electronic units start going wrong, they are not 'self-healing'! So, although you might have created an effective heat-shield, the unit could still (and possibly will) play silly buggers! I have to admit to carrying a complete 'points' distributor (with a high quality condenser), and little test lamp, in my car along with the tools. The idea being that if the 'electronic' ignition fails, it is only a few minutes work to swap the distributors at the side of the road. The working temperature of the "123" units is -30C to +100C---I can assure you, after a longish run, the distributor body can be at above 100C!
 
Thanks Ian, the idea to construct a heat shield would just be to see if it takes any longer to act up thereby pointing more conclusively to the 123 being the problem. Although from what you say it seems to be the most likely culprit, I'd just like to rule other potential issues out before going to the expense of replacing the unit. I have the 123 plus unit. It was on it when I bought the car in 2009 so it's done it's time I suppose. I know if it's now reacting to the heat after that length of time the damage would be permanent so replacement would be the only option. One more silly question, should a separate resistor be fitted with the coil or does the 123 possibly have an inbuilt resistor negating the need for a separate one?

Colm
 
I have read that the 123 and most electronic ignitions are also sensitive to arcing of the leads. You didn't say if your loose connection was doing this but it could also have damaged the unit. I don't personally know of a way to test them but theres a few remarks about this in other car forums.
 
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