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Weak Spark On A 3.5 V8


landyliam

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friend of mine has a 3.5 carb which ran for about a mile after he bought it, restarted once an hour later then cut again and hasn't fired up since (about 2 weeks now), previous owner insists it always started on the button!

getting a very weak spark at the plugs and the coil, have changed points, condensor, dizzy cap, coil, ht leads with no success.

any ideas?

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Liam,

Does it still have the original amp under the coil? I have a feeling these are renowned for unreliability and you may want to change that. I removed one of mine and replaced it with the IWEMA amp and on the other car, tried the RPi Power Amp. If I was going to do it again, I'd stick with IWEMA.

It made a fair difference and they have both been running for a coule of years although I must point out that I'm on LPG thus needed the advance/retard feature, not just the fatter spark. I'm sure there are other replacement "plain" amps available that would do fine on single fuel. ;)

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Hang on Donald, Liam states he has changed the points so I figure there is no amp.

Have you chacked that the ballast resistor(pink/white wire to coil) is good?

Try a 12v coil wired direct to the battery to eliminate this item. I had to modify mine after the ballast lead failed, by using a Fiesta ballast lead wired into the system.

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When you say the ballast resistor has been removed, how does he know this. I ask because the ballast resistance is built into the loom itself and is not easy to find let alone remove.

If the resistance is still there then that would explain why you are getting a weak spark with the 12V coil.

I suggest you put a meter on the + side with the ignition switch in the run position and see what the voltage is, if the ballast has been removed you should have 12V at least. If it's still in there then you will have, dare I say it, around 6-7V.

You can always run a line directly from the battery to see if that improves things.

The only other thing I can think of is the capacitor inside the distributor, I seem to remember the early 3.5 carbs had them.

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i assumed the ballast resistor is the small block adjacent to the coil, getting around 12v at the coil + anyway so it must be bypassed even if there is another in the loom, also tried the 12v direct from battery with no success but getting to the stage where we have tried so many things we could have created another fault, can't tell you how many times we've checked it and then forgotten to put the rotor arm back on :rolleyes: !!!!

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>>>> getting a very weak spark at the plugs and the coil,

Firstly how do you come to the conclusion you have a weak spark? In other words, how do you know you don't have fuel supply/carb problem?

Try this; remove the dizzy cap and the lead at the cap from the coil. Short the points if they are open by putting in a metal object between them e.g. screw driver then open thyem by removing the screwdriver - this fires the coil - and hold the metal end of the lead near an earth e.g. inlet manifold or cylinder head. You should see a bluish spark that should jump a gap of 2 or 3 mm quite easily. If the points are closed, you may be able to rotate the base plate or else need to put in gear and rock engine so they open - I don't think you can force them open.

You can buy spark testers, I've seen 2 types, aa neaon type and other an arrangement of 2 metal contacts that can be moved to de/increase distance between them; I've not used either so can't comment but pretty cheap they are.

It would be worth while checking the earthing to the battery and the positive too - lead goes to starter motor then follow on from there.

Is you battery in good condition?

Have you fitted correct coil? The primary i.e. LT resistance is 3 ohm for 12 V coils, for EFI and for ballasted it is 1.5 ohm.

>>> The only other thing I can think of is the capacitor inside the distributor, I seem to remember the early 3.5 carbs had them.

AFAIK Points distributors had capacitor/condensor fitted. IT is possible that the new one fitted is duff so worth a try.

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I suggest you put a meter on the + side with the ignition switch in the run position and see what the voltage is, if the ballast has been removed you should have 12V at least. If it's still in there then you will have, dare I say it, around 6-7V.

Niall - that test won't work, as the coil charges up it will eventually hit 12v whether the resistance is there or not. Resistors limit current, not voltage.

If the points happen to be open, then with no load (so no voltage drop) again you will see 12v, resistance or not. You could see it with the engine running, but then you'd need an oscilloscope :blink:

The way to tell is to measure the resistance from the coil to the points.

The only other thing I can think of is the capacitor inside the distributor, I seem to remember the early 3.5 carbs had them.

Do you mean the condensor? I think almost all points systems have those :P

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Niall - that test won't work, as the coil charges up it will eventually hit 12v whether the resistance is there or not. Resistors limit current, not voltage.

If the points happen to be open, then with no load (so no voltage drop) again you will see 12v, resistance or not. You could see it with the engine running, but then you'd need an oscilloscope 

The way to tell is to measure the resistance from the coil to the points.

If the points are open, wedge the aforementioned screwdriver between them to close the circuit. Once the circuit has reached its final state, (probs less than a second?) a current will be flowing through the LT coil, in proportion to its resistance - so surely there will be a voltage drop across the ballast resistor????

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explain what you have inside the distributor under the rotor arm....

is it like this.... ( dont worry folks, i made the piccie yonks ago <_< )

dizzypickup.jpg

and then from the bit labelled 2, do the wires then just run straight into the silver box under the coil?

If so, thats what mine is like. Not all electronic ignition has an amp bolted to the side of the dizzy, mine runs from the pickup straight into the box under the coil and i guess that must be the amp + gubbins to drive the coil.

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not quite, can't be sure what age it is but looks very early (4 spd manual gearbox and stromberg carbs)

will take some photos tonight to clear up any confusion but there is definitely no amp fitted

It may look different as you may have the yellowed plastic cover.

Either you have points, 36D8 or something akin to what Fisha drew - reluctance pick up, 38DE8 or 35DM8.

So, if the square silver box that my coil is bolted to is not an amp.... what is it?

it is a small box for keeping your crisps nice and dry :lol:

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