Intermitent engine miss

[olzed]

Hey guys.

72 240 stock points ignition, stock SUs. Usually pulls hard.

She just started an annoying and sudden miss which occurs mainly under light acceleration. Almost as though someone is turning the key on and off.

The tach. flutters at the same time, and sometimes the car will stall completely. Then sometimes it starts straight up, then another time it would not start till a few hours later.

My assistant was cranking it over, then as soon as I shorted out a plug wire , and it started. The spark at the plugs is weak and yellow.

Plugs are clean.

I changed the coil, but no difference. Whats next? Condenser?

I am leaning towards ignition, because of the fluttering tach.

Any ideas?

There is heaps of fuel at the carbs, but I have not inspected the carbs themselves. As yet.

Brian.

Edited December 15, 2012 by olzed

[djwarner]

I suggest you examine the inside of the distributor cap looking for cracks, carbon traces where sparks jumped where they shouldn't. Look for burn marks on the rotor and verify the center contact point is making good contact.

Then I'd look for sloppy linkages on the points mechanicals.

Last I'd look for a vacuum leak or loose electrical connections up to and including corrosion in the negative battery terminal wiring.

[olzed]

New battery a month ago. All the connections are tight and clean.

I'll check the distributor cap and points again, although I did give the cap a wipe out, a couple of days ago.

I just now bypassed the ignition with a hot wire from the battery, took it for a mile drive, and she ran as smooth as silk. I'll give it a rest and try that again.

Could this be trouble in the ignition switch?

Brian.

[beermanpete]

I'll second the condensor. Also, check the points gonditon and gap.

[AWAUDIO]

Has a similar problem on my 1972 240Z. Turned out to be a bad connection between the coil secondary and the condenser/points at the distributor. Redoing the spade connector and tightening screw solved that one. Good Luck.

Andy

[djwarner]

I just now bypassed the ignition with a hot wire from the battery, took it for a mile drive, and she ran as smooth as silk. I'll give it a rest and try that again.

Could this be trouble in the ignition switch?

Brian.

Could be with the age of our cars. Check the voltage being supplied through the ignition switch compared to the battery while the engine is running.

One point of caution - I'm assuming you jumpered before the resistor. These old ignition system designs date back to the days when we all had 6 volt batteries. The resistor is designed to drop the 12 volts in our cars back down to something the coil and points could live with. If you jumper across the resistor, it would be like supercharging - lots more power with a dramatically shortened lifespan.

[olzed]

DJ. The resister was still in the circuit.

I cleaned and adjusted the points a few days ago, but will have another look if need be.

I think I might have found the problem. The spade connector on the wire that goes to the resister was quite loose, so i remedied that. Similar problem to yours Andy.

I just took her for a 10 minute spin and so far all is good.

More gas tomorrow and more driving. Fingures crossed.

Thanks guys.

Brian.

Edited December 16, 2012 by olzed

[djwarner]

Good, I hope you've found it. Intermittent electrical connections can be some of the most difficult things to trouble shoot. Some of my best war stories center around resolving this type of problem.