Decaying spark - any pointers?

larrylamb11 · Post by **larrylamb11** » Wed Dec 27, 2006 10:32 am

I have been running the ignition upgrade as outlined here for quite a while now without any problems, but i now have a headscratching problem.....
I have a good fat spark on start up and tickover and for normal pottering, but the harder the engine revs, the weaker the spark gets until it breaks down and the engine starts to miss. It will keep going but is just not happy and having had a spark tester on it while revving you can see the spark get weaker and weaker as the revs rise higher and higher.... anyone got any ideas what could be causing this breakdown? So far i have changed the ignition amp for a known good one and fiddled about swapping components, but nothing i have swapped has made a significant difference. It seems that the breakdown is in the HT side, as the injection seems to keep working fine and this is taking its signal off the LT side, but any thoughts from you guys are welcome.

Post by **Coops** » Wed Dec 27, 2006 11:06 am

you tried another coil???

larrylamb11 · Post by **larrylamb11** » Wed Dec 27, 2006 12:10 pm

I tried a spare one, but i don't have a new one lurking around. The spare made no difference. Do you reckon it is that?

Post by **Coops** » Wed Dec 27, 2006 12:13 pm

i would check the cap and rotor arm also,
i had a similar thing on my capri 4yrs ago,
i changed the cap and rotor arm and it was fine,

Post by **katanaman** » Wed Dec 27, 2006 12:51 pm

might help if you listed what you had fiddled with and swapped around otherwise you could get a whole load of suggestions for what you have already done. The injection doesn't pick up on the HT side by the way so the fact your injection still works only means the coil is getting a trigger pulse not that its working properly.

ian.stewart · Post by **ian.stewart** » Wed Dec 27, 2006 3:59 pm

I did have similar problems last year, where the engine would rev to a certain speed and then miss until the engine would splutter,, if you tried driving it the problem only got worse,
If you have changed the Rotor arm and cap, and the engine was runnung fine, go back to the original pieces and see if that makes any diference, some of the rotor arms seem to be of an indiscrinate quality and can cause no end of problems, and this is also compounded by there being 2 different size rotor arms

larrylamb11 · Post by **larrylamb11** » Wed Dec 27, 2006 4:57 pm

I have tried rotor arms and caps, both genuine used Lucas items- same problem with the substitutes. Also tried swapping king lead and swapping a couple of individual spark plug leads (with spares) and it didnt seem to make any difference. I had always assumed this type of dizzy trigger was pretty reliable - from what i can see it is a microswitch tapped by the chopper on the dizzy shaft - is that right? Either way i wondered if there was anything any of you guys have had that has given similar problems before i start buying new stuff and eliminating by virtue of new components. I still think i am on the right lines with an HT failure of some form, it is just a matter of narrowing it down as i havent had a failure like this on any of motors before.

I did have similar problems last year, where the engine would rev to a certain speed and then miss until the engine would splutter,, if you tried driving it the problem only got worse,

That is exactly it Ian, it will rev to say 4000rpm , then go no further no matter what throttle you use, wide, trailed, feathered etc. it just coughs and spits and wont go any higher.

jrv8 · Post by **jrv8** » Wed Dec 27, 2006 5:39 pm

larrylamb11 wrote: - from what i can see it is a microswitch tapped by the chopper on the dizzy shaft - is that right? .

Hi,
If it's a 35DM8 or DLM8 the rotor shouldn't touch the pick up head. There should be an air gap.

Jim

larrylamb11 · Post by **larrylamb11** » Thu Dec 28, 2006 7:29 am

Hi,
If it's a 35DM8 or DLM8 the rotor shouldn't touch the pick up head. There should be an air gap.

Jim

OK, i can't really see the pick up as the dizzy has the factory cover on so had guessed it was some sort of microswitch - what you say above makes more sense. I will check that there is an air gap.