Ignition issue... dull / bright / dull

[aidey2000]

Hi All...

Anyone got any ideas on this.

I've been testing my ignition spark by using sparkrites plug testers, the ones that light up.

From cold they light up but are dull but the engine starts

When the engine is running they are really bright

Turn engine off when at temperature and she won't restart and the spark is showing as dull again.

When the engine cools down it starts again

Any ideas 💡

[TooLateForAName]

Would help if you said what the engine was and what history there is with the problem.

my default answer to a no start when hot is camshaft/crankshaft position sensor

[aidey2000]

long issue with hot starting

old school jaguar xj6
no electronics like throttle sensors, camshaft sensors or crank sensors.

Twin 2 inch Su carbs
Recently rebuilt by burlen

Ignition system is currently electronic but there's no difference when I put the points distributor back on.

It's only today we noticed with the new sparkrite testers the difference in spark brightness with engine turning over and engine running

[healeyneil]

Does it have a ballast resisted coil ?

[aidey2000]

yes it has a resistor coil

[FMOB]

Sounds like the coil or something in the ignition is incorrectly wired, the ballast resistor needs to be in circuit when the engine is running normally and out of circuit during engine starting.

The spark brightness should be similar during starting and normal running IIRC.

[GreenV8S]

FMOB said:

Sounds like the coil or something in the ignition is incorrectly wired, the ballast resistor needs to be in circuit when the engine is running normally and out of circuit during engine starting.

The spark brightness should be similar during starting and normal running IIRC.

Ballasted ignition systems are typically set up to deliver a higher voltage while cranking. For example the coil might be designed to run on 6V but see full battery voltage (maybe 9V) while cranking.

To diagnose this I suggest you measure the voltage at the supply side of the coil while stopped, cranking and running. Also check what voltage the coil is designed to run at. A non-ballasted coil in a ballasted circuit is likely to be a pig to start. Also remember to check the dwell - that would also affect spark strength.

[Peter-jforf]

From ex Auto sparks now 80 years....as GreenV8s says and certainly old school Jags do have the ballast system....so when engine running voltage at the coil + positive terminal will be 7 / 9 volt.....starting from cold this voltage jumps up to Starter cranking voltage...ie...10/12volt depending on good battery....what causes this jump comes from the starter solinoid...apart from the main large big lead you will see 2 smal push on terminals...one is as i remember colure "white/red"...ignore that one........the other one is for the ballast thats the ONE you need to cheke.....so once the starter is engaging this wire increases voltage....with out this voltage engine will as others have said.....give poor starting as very low HT voltage at coil. so check when cranking you see voltage at the terminal ....of couse ensure some one has not fitted a non-ballast coil in the past...they wont work corectly on a ballast system. Peter

[richhead]

you dont really need a ballasted coil if you arent running points, as said, check what voltage you have at the coil live when cranking.

[aidey2000]

Thanks all ...

It was the 12v not going direct to the 9v coil whilst cranking.

Very happy to report that today she's gone from not wanting to start
to starting on the button cold or hot