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Exasperated by stuttering SP250... all suggestions welcome!

Ian Hastings
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Re: Exasperated by stuttering SP250... all suggestions welcome!

Post by Ian Hastings » Mon May 27, 2019 11:53 am

I know you have changed the coil but have you checked that the low tension leads to the coil are the correct way round?

If they are incorrect you may get the symptoms that you describe, which are indicative of a weak spark.

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Re: Exasperated by stuttering SP250... all suggestions welcome!

Post by Agnello11 » Wed May 29, 2019 12:26 pm

Thank you for the continuing thoughts on our Dart problem... I had a good session on the car last night and, on the basis that a picture tells a thousand words, have captured a video of the symptoms!

But first, here's what I started with....

I started by checking the +ve and -ve wires were the right way round on the coil (they are) and verified that the spark wasn't leaking to earth via the rotor arm using the test outlined earlier (it wasn't). I also fitted my natty little spark tester device between one of the plug leads and insulating caps to check for spark. It has a massive fat spark on cranking. I checked the cap and rotor arm for any signs of tracking - none noted. There's no fuel leaks, no weeping float bowls, nothing obviously or even subtly erroneous that I can see....

With all checks done and everything confirmed as looking ok, I went for an evening test drive in the nice cool air...... As normal, it fired instantly on cranking and ran perfectly. Choke was used from cold and it benefits from that in fact. It runs SOoooo nicely when it's cold it's an absolute dream - it sounds fantastic, pulls like a train and is a massive pleasure to drive. I enjoyed a magical 5 miles of country roads at all throttle openings with it revving cleanly, pulling well and sounding ace! I'd dropped off the choke after a couple of miles as no longer required.

Then it began to miss the odd beat....

I elected to turn around and run for home again at that point, lest I experienced a 'failure to proceed' but popped the bonnet to check the spark on my tester first. The tester showed a decent spark still. I noted the Kenlowe had engaged to help cool the radiator, so engine temperature was spot on. As I retraced my steps it began to run rougher and rougher, feeling like it was dropping cylinders, not pulling, 'bellowing' if you could get it to the higher revs (I use the term bellowing as I don't know how else to describe it - it basically makes more noise but doesn't actually 'do' anything, no accelerative reaction). It's really not fun to drive when it's like this... or easy! It feels like it's going to die at any minute as you pull out of junctions, you can't use any more than minimal throttle opening and need to carefully feather the throttle to keep the engine from bogging completely. Moderation of the choke makes no difference to the way it runs at this time either.

Once half way back (about 2.5 miles) I stopped again where I knew I could safely wait if it died completely and that's when I took this video. As you can see, my little red spark tester flashes away on the far bank. There are NO signs of any spark tracking or leakage visible anywhere in the dark... I even remembered to check for vacuum in the fuel tank and pooped the cap - nothing. Fuel pump was still ticking normally and the glass bowl in the engine bay was full, so it doesn't look to be fuel starvation...... I have a 2nd coil fitted on the car as an emergency measure, so I switched off and swapped to the 'emergency' coil - but the symptoms were EXACTLY the same. Although it will start easily and at first flick of the key, it WILL NOT run properly - you can see in the video I am opening the throttle wide open and holding it WIDE OPEN and all it'll do is make that strangled gasping noise. It I open the throttle wide when it's cold and before these symptoms present, it would rev to the moon, so SOMETHING is breaking down as it warms up or after a few minutes use.... Checking the spark on cranking with the distributor cap off produces a big fat spark and it still wouldn't earth through the rotor arm when it was displaying these running symptoms....

The only thing I noticed was the drop in intensity of light produced by my spark tester as it revved, or tried to, with theses symptoms and I am now wondering if this is a clue? Could the spark be breaking down? Given what I saw last night I am suspecting the coil.... but it produced exactly the same symptoms on the 2nd emergency coil... I guess it could be TWO dead coils? but the chances? Going through the ignition components that have been replaced so far, it's everything..... literally. It's had new plugs, new insulator extensions, new leads, new cap, new rotor arm, new coil(s), and a new PowerSpark electronic ignition kit!

Irrespective, given that it's still not right nothing can be excluded from suspicion or examination - you can see why I'm exasperated! Any thoughts, suggestions, experience, comments very welcome! Would you concur that there might be a spark issue? The air temp was cool enough that vapourisation seems highly unlikely, no?

Here's the video - note the spark tester flashing away and my hand where you can see me opening the throttle to wide open and holding it all that time where it's bogging...


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Re: Exasperated by stuttering SP250... all suggestions welcome!

Post by Sydsmith » Wed May 29, 2019 2:35 pm


You and res-ponders appear to have covered almost all possibilities, must be very frustrating indeed.

I don't think it sounds like intermittent spark, if it was I guess you would have loud banging in the exhaust, but it could be timing, the engine sounds like it is retarded, though you have been very thorough and I guess you have checked and eliminated timing and the auto advance and retard.

It sounds to me like fuel starvation or weak mixture, perhaps an air leak, might be in the auto advance and retard feed pipe or between the carbs and the manifold or the manifold and block, something that opens up when hot sucks air in and weakens the mixture. Though I admit I would expect the choke to help if that is the case, and as you say it does not.

Sounds daft but I had an A35 which suffered a similar problem and it turned out to be a faulty new gasket between the carb and manifold letting air in when the manifold heated up.

Real head scratcher, hope you solve it soon. Syd.

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Re: Exasperated by stuttering SP250... all suggestions welcome!

Post by johnwager » Wed May 29, 2019 4:34 pm

After watching the video my feeling is that it is fuel starvation, for one reason or another.
I would rig up a separate fuel supply to eliminate this.
Have a can with a pipe attached that you can swap over when the problem starts.
Of course it would have to be a gravity feed direct to the carbs.
There could be a partial blockage somewhere from the tank to the front.
Worth a try.

Ian Hastings
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Re: Exasperated by stuttering SP250... all suggestions welcome!

Post by Ian Hastings » Wed May 29, 2019 7:27 pm

When I removed the petrol feed pipe from the tank to carbs on my car there was a basin full of crud lying in the lower part of the pipe.
Maybe in yours there is crud which builds up and restricts the pipe as you drive along. if so a blast through in both directions with an airline may clear it out.
You didn't say that you checked the fuel flow by removing the pipe at the fuel filter, you only mentioned that the glass was full which does not really show that fuel is getting through in sufficient quantity.

Big Col
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Re: Exasperated by stuttering SP250... all suggestions welcome!

Post by Big Col » Wed May 29, 2019 9:18 pm

It all points to fuel. The fact that the fuel filter bowl is full doesn’t mean fuel is getting through. As suggested you may want to try a completely separate fuel supply to the carbs, probably by gravity or a squeegee bottle, when the problem occurs.
I may be slow but I’m rough as well !

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Re: Exasperated by stuttering SP250... all suggestions welcome!

Post by JohnM » Wed May 29, 2019 10:05 pm

Just one other thing I can think of that hasn't been mentioned - you may have a hairline split in the fuel pickup pipe in the tank ( a known, reported problem in the past) that's opening slightly as the fuel temp increases and allowing the pump to pull more air than fuel. If you have the reserve solenoid setup fitted, try switching to the reserve pickup and see if it helps. If not, does the problem disappear when the fuel tank is full?

BTW - I'm a bit surprised your cooling fan is coming on after only 5 miles unless you were running slowly at high revs - my engine would be barely up to running temp by that time at normal road speeds..........

Best of luck,
John M in Middlesex, NW London
1962 SP250 "B" Spec

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Re: Exasperated by stuttering SP250... all suggestions welcome!

Post by Paulkennedy » Wed May 29, 2019 11:29 pm

I don't remember reading that you had changed the distributor capacitor which can cause this problem. If you have I apologise

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Re: Exasperated by stuttering SP250... all suggestions welcome!

Post by bop » Thu May 30, 2019 2:35 am

Just wondering if you notice this problem happening after your Kenlowe fan engages. If your electronic ignition and your fan are fed from the same source, maybe fan is dropping voltage at ignition enough to affect its workings.

Another thought is put your points back in temporarily and see how it runs. That way electronics are out of the way.


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Re: Exasperated by stuttering SP250... all suggestions welcome!

Post by Agnello11 » Thu May 30, 2019 10:26 am

Gentlemen, thank you for your further useful and insightful comments. I take on board the wise observations over fuel starvation and will investigate this further - you're quite right that a full fuel bowl does not necessarily equate to adequate fuel flow and I have been hasty to assume otherwise......
On that basis I stripped off the fuel bowl last night, cleaned it all out (and there was a fair amount of fine silt and debris in it) checked it for flow and refitted. On energising the ignition, the pump quickly ticked away to refill the bowl, so that bit worked and I next need to establish if flow remains when symptoms of poor running present..... I'll rig up a temporary supply tank in due course too and see if that produces any insights.

Does anyone know about the tank pick up? is there a gauze filter on the tank pick up? I am just wondering if that is slowly choking to the point of being fully obstructed as debris / silt in the fuel sloshes back and forth whilst driving. Wondering if I might be getting starvation from that which then dissipates when the car is left, the suction on the pick up ebbs away and the crud drops back off the gauze.... but I don't know if there is a gauze even fitted....

I don't have the reserve solenoid on my tank and the problem seems to be there whether the fuel tank is full or empty.

I haven't noted a correlation between the Kenlowe coming on and the issues presenting, but I will observe and see if there is one - good suggestion.
I have the Kenlowe set pretty cold, so it runs quite a lot of the time. I think it was on in the video as I had been driving 'spiritedly' ;) and had then stopped and popped the bonnet to check on the spark tester etc. I guess the poor running is evidence of improper combustion too, so that may be having an impact on engine temperatures. I am reasonably confident that the thermostat and cooling system are functioning correctly, but welcome any suggestions or procedures to verify same.

Distributor has a PowerSpark electronic ignition set up, so no capacitor to consider. I am in contact with PowerSpark currently in a further effort to verify that the ignition system is working correctly, but again any suggestions on this welcome. For the record, when cold the coil (in fact there are two fitted, one as an 'emergency' spare) measures 3.3 ohms across the LT circuit / primary windings and 10,300 ohms across the secondary windings and I think those figures are within specification ('emergency' coil measures the same). I haven't measured it hot, so that's something to check.

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