Coil / ballast / ignition issues

[Sam_C]

I’m normally a whizz at electrics but I’m baffled at the moment I am running a standard 240Z set up with a 12v coil and ballast resistor. It was running beautiful and starting first turn until I changed a plug lead to check something and now it runs like a dog at idle despite swapping back to the original lead. Once off idle - no problem, revs and runs as sweet as. I decided it must be something else that I disturbed by accident or just pure coincidence. So I’ve swapped the distributor cap, plug leads and coil lead in turn, coil, ballast resistor, reset the points cleaned everything in sight. Still no joy. One thing I’m confused with is the ballast resistor circuit. If I have this right. the coil gets 12v for starting (from ignition switch start pos) and then drops down to 6v to protect the points, through the resistor. This is wired using a common line through the tacho, with the constant 12v power supplied when the ignition is turned on running through the resistor (down to 6v) and the temporary 12v start power supplying the main coil +ve.
When I turn on the ignition and check voltage it’s 12v at one side of the resistor, 6 at the other and 6 at the coil. So far so as expected. When I crank it the voltage at the coil goes up to 12v (again, expected) but when I check the voltages when running I get 12.6 at the resistor input, 10.5 at the output and around the same at the coil. So yes it’s gone down a bit, but nowhere near the 6 I expected. I have tried 3 different resistors but all give the same result. I may try taking the resistor output direct to the coil instead of routing it back up to the tacho and back down the to the coil via the common line. That might help but wouldn’t really explain the lack of voltage drop. Any thoughts or similar experiences. I know I’m probably one of a very small minority in here still on a standard points / carb setup but hopefully…
If I can’t sort it, it’s off to Bogg Brothers at Malton and hope that Dave is still alive to work his “old school” magic

 

[Farmer42]

Its been a while since I had a mechanical points dizzy but from my recollection, the issues I had were mainly due to the condenser. They don't last very long and anything that changes in the distribution through the spark leads seems to upset it and stop it working properly. I would try changing that as well if you have tried everything else. If your ballast resistor wasnt working you would get no Low Tension current to the dizzy and it wouldn't start.

The other thing to check first is the state of the spark plugs. If you were running with a lead off, it could have mucked up the plug so you are no longer getting a good spark on that cylinder. If the condenser was sh***ed, it either wouldnt run at all or it would pop & bang when you revved it under load.

 

[Sam_C]

Thanks Farmer, sorry for the late acknowledgement. Plugs are all new so that shouldn’t be the issue. No misfiring or popping and banging, in fact it runs very well above about 1200rpm, it’s just the idle and setting off where it’s lacking. Interestingly, if I start it just with the key, no throttle or choke, it starts straight up and runs very smoothly for a few seconds (although slightly faster than tick-over). It then slows down to idle and the lumpiness kicks in. I’m thinking that I have the idle set slightly high to offset the rpm reduction when the missing begins, but on initial start-up all is well so it runs a little fast, then whatever is wrong kicks in and the problem occurs. Not really sure what could be causing it though??

 

[Farmer42]

That doesn't sound like ignition to me. Those symptoms start pointing to carbs if you are running them. A common occurrence for SUs is a sticky float valve and flooding the carb after a short while which allows it to start ok then as the float chamber overfills, it cuts the idle & starts running lumpy. It will be ok on higher revs as that uses more fuel & stops the float chamber from overfilling. It only needs a small bit of dirt to cause it to stick.

You might wish to take the float lids off and check the valve operation and if there is any muck in the chamber. Then check the float level. It should be around 4.5mm between the lid and the float face with the tang resting on the valve needle. A drill bit is a good way of measuring this.

 

[Davidsw]

I think check your earths . Are you getting the voltage drop at the coil and respective to the battery negative . I assuming you are running a box standard set up and haven't by mistake put in a coil that needs more than the standard voltage . Any one device not seeing the chassis where it needs to will give you additional resistance and voltage drop and issues .

if You don’t drive the car a lot take it out for a good warm up and then see what she is doing after a cool down .

 

[Sam_C]

That doesn't sound like ignition to me. Those symptoms start pointing to carbs if you are running them. A common occurrence for SUs is a sticky float valve and flooding the carb after a short while which allows it to start ok then as the float chamber overfills, it cuts the idle & starts running lumpy. It will be ok on higher revs as that uses more fuel & stops the float chamber from overfilling. It only needs a small bit of dirt to cause it to stick.

You might wish to take the float lids off and check the valve operation and if there is any muck in the chamber. Then check the float level. It should be around 4.5mm between the lid and the float face with the tang resting on the valve needle. A drill bit is a good way of measuring this.

That’s a good shout, I’ll look at that next. Is it possible to remove the lids with the carbs in-situ?

 

[Farmer42]

That’s a good shout, I’ll look at that next. Is it possible to remove the lids with the carbs in-situ?

Yes it should be if you have the round top Hitachi SUs. Remove the fuel lines first then undo the screws. Be careful that you don't wreck the gasket as they are difficult to get hold of. When checking the float level, you need to turn the lid upside down so the float is resting on the valve needle. Test the valve the right way up first by blowing through the inlet pipe and pushing the float so it opens & shuts. If air goes through when you have pushed the float closed then you have some dirt in the valve or the seal has broken and it would have been stuck open giving you the idling symptoms. Take the valve out and soak it in carb cleaner before reinstalling it. The other thing to check is that the float has not let fuel into it which stops it rising and shutting the valve.

 

[Sam_C]

That doesn't sound like ignition to me. Those symptoms start pointing to carbs if you are running them. A common occurrence for SUs is a sticky float valve and flooding the carb after a short while which allows it to start ok then as the float chamber overfills, it cuts the idle & starts running lumpy. It will be ok on higher revs as that uses more fuel & stops the float chamber from overfilling. It only needs a small bit of dirt to cause it to stick.

You might wish to take the float lids off and check the valve operation and if there is any muck in the chamber. Then check the float level. It should be around 4.5mm between the lid and the float face with the tang resting on the valve needle. A drill bit is a good way of measuring this.

Just a quickie - should that be 14.5 mm rather than 4.5mm?

 

[Sam_C]

Ah maybe not 14.5… I think your measurement is between the ungasketed float bowl lid and the edge of the float when sat on the needle (which would be fully closed at this point?). I got (still am) a bit confused because the front and rear bowls have different height mounting posts and different length needle valves so wouldn’t the front bowl with the longer valve have a different dimension? I think I got the 14.5 from sitting a between the lid and the underside of the float valve.
I have a few other q’s about my set-up so I switch over to the engine forum as they are carb rather than electric based. Please feel free to answer this post on here though