John L Posted June 22, 2018 Report Share Posted June 22, 2018 Is there a way to test if the leads are good or not. The resistance seems to vary quite a bit with the different length of the leads so its difficult to compare one lead with another. I didnt really want to change to a new complete set. They are the green silicone core ones. I'm getting an odd misfire after the engine is hot, but starts and runs ok. What other possibilities could there be, I'm running an accuspark hall effect, with red rotor arm. John Quote Link to post Share on other sites
Hawk Posted June 22, 2018 Report Share Posted June 22, 2018 Is there a way to test if the leads are good or not. The resistance seems to vary quite a bit with the different length of the leads so its difficult to compare one lead with another. I didnt really want to change to a new complete set. They are the green silicone core ones. I'm getting an odd misfire after the engine is hot, but starts and runs ok. What other possibilities could there be, I'm running an accuspark hall effect, with red rotor arm. John I had a similar problem and resolved by changing out the HT leads and plugs (touch wood) Quote Link to post Share on other sites
Paul Down Posted June 22, 2018 Report Share Posted June 22, 2018 Basically two types of HT leads: 1. Solid copper (i.e. no radio suppression) that should normally be used with a resistive plug connector to avoid radio interference. 2. Resistive leads that provide suppression and do not need a resistive spark plug connector. These as you have found have a resistance that depends on their length, roughly 10-50Kohms. If either type measure open circuit , or a very high resistance then they are faulty. Worth looking at the spark plugs after a misfire to see if any look a bit odd. Capacitors and ignition coils can both fail when hot especially coils. Best of luck Quote Link to post Share on other sites
HSM Posted June 22, 2018 Report Share Posted June 22, 2018 For information HT leads that are resistive have a value of 5000 ohms per foot. Harvey S. Maitland Quote Link to post Share on other sites
John L Posted June 22, 2018 Author Report Share Posted June 22, 2018 Perhaps a silly question, does it matter is the wires are at different lengths, or should they be nearly the same length within a few mm's? John Quote Link to post Share on other sites
HSM Posted June 22, 2018 Report Share Posted June 22, 2018 In general all plug leads on conventional ignition systems are all different lengths. Harvey S. Maitland Quote Link to post Share on other sites
RobH Posted June 22, 2018 Report Share Posted June 22, 2018 No the length makes little difference. Just make sure you don't use resistive spark plugs if the leads are resistive too. You don't need both forms of suppression and too much resistance can weaken the spark. Quote Link to post Share on other sites
Waldi Posted June 22, 2018 Report Share Posted June 22, 2018 What a wealth of knowledge on here:) Are resistive leads everything else except the old style copper core leads? If so, are the copper cored leads the only ones requiring resistive spark plugs? And is the 5kOhm/ft a common value for all these leads? Thanks, Waldi Quote Link to post Share on other sites
RobH Posted June 22, 2018 Report Share Posted June 22, 2018 Only plain copper-cored leads need suppressed spark plugs. Other types of lead are resistive but the actual value will vary between manufacturers to some degree. NGK quote 10k to 23k Ohms per metre for carbon-doped cable. Although its good practice and EMC directive notwithstanding, there really isn't the same imperative for suppression as there once was. Digital radios and TVs are much less susceptible to impulse interference than the old analogue ones were and whereas once your neighbours might complain about 'snow' on the picture, nowadays they are unlikely to even notice. Only a radio ham using analogue kit is likely to object. Quote Link to post Share on other sites
Waldi Posted June 23, 2018 Report Share Posted June 23, 2018 Thanks Rob. Waldi Quote Link to post Share on other sites
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