misfiring when warm

[rja]

hello folks-don't post here much, except when I have a question or problem. '74 1800 sport 37k miles. pretty much stock; still running dual point. car doesn't get driven much; fortunate to have lotsa choices. anyhow, last two times I decided to warm up the spider and go for a spin, it warms up just fine and when it comes to temp, it sputters, tach jumps around a lot, and it will barely idle. if it does die, it doesn't want to restart. don't believe it's a fuel problem, filter is full and can hear the pump running. I'm thinking condenser bad when up to temp, but what do I know? would like some thoughts, if possible, before I start throwing parts at it. any comments much appreciated.

[MattVAS]

Your Tach is run off your ignition coil. If the tach is jumping all over the place the the ignition coil is most likely bad.

Bad ignition coils are a common problem on these cars as they are rarely replaced. And when a bad unit occurs it can often show it's worst problems when hot which occurs normally once the car reaches temp. Then they don't like to restart a car until they have fully cooled off.

[rja]

thank you sir. any tests you can do when hot to determine if this is the cause of the problem?

[spider2081]

thank you sir. any tests you can do when hot to determine if this is the cause of the problem?

I like to monitor the voltage on the + side of the coil. It should be the same when car is running good and bad. The ignition switch side of the coil should not be much less that battery voltage when the engine is not running and should be close to the same as the alternator output voltage with the engine running. If the primary voltage to the coil drops the coil can not function properly. Next measure the voltage on the points side of the coil. It should be less than the ignition side with the engine running. If the points don't close or are bad the voltage rises. If the condenser shorts the voltage drops.
Note: the wire diagram I looked at does not show a ballast resistor between the coil and the ignition switch.

[rja]

thank you for that detailed descriptive. I will put it to good use. I believe it to be the original coil on this vehicle, also.

[rja]

well, the pretty blue bosch coil didn't solve the misfire problem. guess I delve further into points/condenser/ignition voltage and see if I can't solve this. did the ignition switch rebuild (pull apart, file on the contacts, reassemble w/those little bitty bolts and nuts) a coupla years ago and haven't had problems with it since. the headlights are relayed, also; so I'm thinking points/condenser/maybe advance springs and weights while I'm there. tach still bounces some, just doesn't have any acceleration when you push the go pedal. but will idle very slowly. alas, so many cars and so little time....

[RRoller123]

Arcing spark plug cables would do that too. Try watching the engine in complete darkness, and see if you can spot any arcing.

[92spi]

thanx; will look for that at my next opportunity.

[Roger1200]

Hi, I would not have suspected a coil at 37k miles......however there are a number of other things lurking around Marelli Distributors to trip you up. Gummed up advance return springs, earth leakage through low tension insulator, bad condensor or high points resistance. All able to be repaired for a couple of buck and in a few minutes ..... Good luck with it.

[baltobernie]

Next time, try jiggling the ignition key while it acts up. You may have refurb'd the switch, but they do seem to ultimately fail, even after fixing.