RogerH Posted November 4, 2017 Report Share Posted November 4, 2017 Hi Folks, over the last couple of weeks my reliable TR4A has taken to doing a little cough now and then. Something is causing the sparks to go AWOL just a tad. In my mind I deduced it to the ignition. High revs above 2500 it all runs very well. At low revs 1500 - 2000 and it will miss about one cough per minute. At very low revs 800 -1000 it coughs much more - not enough to cause it to stop. Starting has been getting worse during this period. Today I was going to change parts one at a time. The rotor arm has been insitu for about 10 years or more and looked OK. I replaced this with a NOS rotor but not a Dizzy Doc one. The big brass rivet looked nice and clean The engine started easily and running at mixed revs was trouble free. It actually felt smoother. My theory is that at the lower engine revs the coil has a little more time to generate a decent (higher voltage) spark. As the Rotor arm is beginning to fail this higher (low Rev) voltage was arcing through and the spark fails. At the higher engine revs the coil voltage is a little lower and so stays where it is. As I was not inside the dizzy cap I can't guarantee my theory s spot on - but the replacement arm appears to have sorted it. Roger Quote Link to post Share on other sites
foster461 Posted November 5, 2017 Report Share Posted November 5, 2017 Good plan Roger. The path of least resistance. Quote Link to post Share on other sites
Peter Cobbold Posted November 5, 2017 Report Share Posted November 5, 2017 Hi Roger, It could also be the plugs' centre electrodes run hotter at higher rpm and so allow the spark to fire across their gap at lower voltages. So the rotor arm 'leak' cannot compete at higher rpm. Peter Quote Link to post Share on other sites
oldtuckunder Posted November 5, 2017 Report Share Posted November 5, 2017 Nice diagnosis Roger, one to lodge in the fading grey cells for future reference. Alan Quote Link to post Share on other sites
RogerH Posted November 5, 2017 Author Report Share Posted November 5, 2017 Hi Folks, on my long drive to the AGM today I have found yesterdays diagnosis is tosh - unless another problem has arisen. Tomorrow I will replace the plugs and see what happens. It would appear that things need to get really warm before the coughing. Roger Quote Link to post Share on other sites
boxofbits Posted November 5, 2017 Report Share Posted November 5, 2017 Hi Roger Could be the coil itself. They are not what they used to be! Mine started doing something like this. It actually packed up on the MOT ramp, and it turned out to be the coil. Regards Kevin Quote Link to post Share on other sites
RogerH Posted November 5, 2017 Author Report Share Posted November 5, 2017 Hi Kevin, it is a 20 year old Lucas Gold sports coil. I haven't ruled it out but will get round to it in a logical order so that I end up knowing which component actually is responsible. I'll be ordering some parts from Martin Jay tomorrow Roger Quote Link to post Share on other sites
boxofbits Posted November 5, 2017 Report Share Posted November 5, 2017 Oh if it's genuine Lucas then it'll probably go on forever. I often pick the old 1960's ones up at autojumbles or a dynamo with the coil still strapped to it like the old 'A' series BMC engines had. Nearly always still work perfectly! Kevin Quote Link to post Share on other sites
MichaelH Posted November 5, 2017 Report Share Posted November 5, 2017 Hi Roger Coughing engine which is worse at low revs makes me think of a leak of air on the intake side think check the manifold gaskets of carb/inlet manifod I have chatted about replacing manifold gaskets with high temp silicone before - but it cured my similar problem before Good luck either way Michael Quote Link to post Share on other sites
RogerH Posted November 5, 2017 Author Report Share Posted November 5, 2017 Hmm, that's interesting. If my ignition search does not get the result then I'll investigates the manifold gaskets. I'll check nut tightness tomorrow. Roger Quote Link to post Share on other sites
graeme Posted November 6, 2017 Report Share Posted November 6, 2017 Hi Roger, It could just be plugs, have you tried another set? Cheers Graeme Quote Link to post Share on other sites
Steves_TR6 Posted November 6, 2017 Report Share Posted November 6, 2017 Are you running points Roger, of so perhaps a condenser thats breaking down when hot? I once spent weeks looking for a fuel problem on a v8 in a boat that ruened out to be the condenser..... Steve Quote Link to post Share on other sites
RogerH Posted November 6, 2017 Author Report Share Posted November 6, 2017 Hi Graeme, plugs to be changed and manifold studs checked - all for this morning. Hi Steve, no CB's. I use a Lumitrition magnetrocic thingy. usually when they go it is instant. However It is on the list. Roger Quote Link to post Share on other sites
RogerH Posted November 6, 2017 Author Report Share Posted November 6, 2017 Hi Folks, I hope I'm not feeling smug too soon. I changed the plugs today and checked the manifold nuts (all nice and tight). Not sure what I was expecting. It all went well whilst warming up. When fully warmed up it drove faultlessly. Even taking off from a standstill in 2nd there was no coughing. Could it be that simple. I'll keep an eye on it and report back with any feedback Roger Quote Link to post Share on other sites
GT6M Posted November 6, 2017 Report Share Posted November 6, 2017 Ello Rodger, if ye still got yer olde plugs, get a magnifying glass, and have a good look at the porcelane area. specificly for a corona frae the metal end, and also for a wee crak /slit in it as plug get hotter, the crak get bigger, so earth oot It took some finding on mine, crak was barely see able and they wer a newish plug butt, whenst run int dark, all sorts of things come to light,!!! https://www.google.co.uk/search?q=cracks+in+a+spark+plug&client=firefox-b&dcr=0&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiUsazf56rXAhXiL8AKHadeCg0Q_AUICigB&biw=1368&bih=806&dpr=1.25#imgdii=jdGtBS77YZ9usM:&imgrc=mqKVLEfC8l-7EM: M Quote Link to post Share on other sites
RogerH Posted November 6, 2017 Author Report Share Posted November 6, 2017 Hi Marcus, I'll give that a go tomorrow. Roger Quote Link to post Share on other sites
pfenlon Posted November 6, 2017 Report Share Posted November 6, 2017 Rogger your diagnostic qualities are Manifold as they say, I would have had the carbs in bits, the Cam out, and head gasget changed before going for the Rotor Arm. Sue must have seen a good catch a bit ago! Glad its up to speed again. Quote Link to post Share on other sites
Lebro Posted November 7, 2017 Report Share Posted November 7, 2017 Why, was she into coarse fishing ? Bob. Quote Link to post Share on other sites
Dick Posted November 7, 2017 Report Share Posted November 7, 2017 I've been chasing a missfire on a non TR. I was getting a very hot ignition coil, if fact two failed...to cut a long story short, it was the voltage regulator that was regulating to 18volts. Not only did two coils fail but on their way to doing that, the extra HT was randomly breaking down various insulation in the ignition chain. It's a good idea to measure the resistance across the primary side of the coil......3.2 - 3.4 ohms is good for a non ballasted coil. Anything lower is a symptom of failing insulation inside the coil.....if you are down to 2 ohms, as one of mine was, then you will be pulling 6 amps instead of around 3 ish....Amps burn! Also measure the voltage with the engine running above the cut in speed. Dick. Quote Link to post Share on other sites
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