Help! Got a misfire

[billyvette]

Hey guys, I have a 78 with a mallory unilite distributor (comp 9000 part number 9148201) with mallory wires and headers. This is an older distributor as I bought the car in 1998 and it was in the car then.

I have cleaned all the distributor cap contacts and rotor cap contacts in an effort to try to fix the miss (it worked one time before). The car ran good for about 15 minutes of driving and then started missing just as before I cleaned the contacts. A few weeks back the top blew off the battery as I attempted to start the car so I go and get a new battery, car starts perfect and runs fine, but I do not drive the car when I installed the new battery. Yesterday I drive the car and get this misfire and start to work on fixing it.

How can you check plug wires with a common electronic tester?

I have a tester with the red and black leads requiring a 9 volt battery and can tell you ohms, dc volts and ac volts- one with the rotary dial to change from ohms to some other setting or to turn off.

Plugs are the shorty header type plugs with very low miles and I have not pulled any of them as I thought they were ok due to low mileage on them.


Any input much appreciated.

 

[73shark]

You can use the ohms section of your VOM to check the plug wires. If they are original type carbon wires, they will probably be in the 10K to 30K range. If the newer spiral wound center conductor, then should be down in the 100s of ohms.

Probably should check the plugs just to be sure they're OK.

 

[billyvette]

thanks for the reply, i could not remember if it was ohms or another part of the meter, the wires are aftermarket taylor and look to be the same vintage as the distributor.
checking the plug wires will be my next activity.

 

[Vette79]

Hey guys, I have a 78 with a mallory unilite distributor (comp 9000 part number 9148201) with mallory wires and headers. This is an older distributor as I bought the car in 1998 and it was in the car then.

I have cleaned all the distributor cap contacts and rotor cap contacts in an effort to try to fix the miss (it worked one time before). The car ran good for about 15 minutes of driving and then started missing just as before I cleaned the contacts. A few weeks back the top blew off the battery as I attempted to start the car so I go and get a new battery, car starts perfect and runs fine, but I do not drive the car when I installed the new battery. Yesterday I drive the car and get this misfire and start to work on fixing it.

How can you check plug wires with a common electronic tester?

I have a tester with the red and black leads requiring a 9 volt battery and can tell you ohms, dc volts and ac volts- one with the rotary dial to change from ohms to some other setting or to turn off.

Plugs are the shorty header type plugs with very low miles and I have not pulled any of them as I thought they were ok due to low mileage on them.


Any input much appreciated.

I'm thinking cap and rotor replacement if you cleaned the contacts and it worked fine for 15 minutes and then went back to missing. Cap and rotor is relatively inexpensive and it may be time to replace it anyway.

 

[billyvette]

Tried the ohm test on the wires tonight.
I had the tester set on 200 ohms (looks like a horseshoe symbol) and ech wire was between a .80 to a 1.0. Does this sound right? I am going to get my electrician brother to look at the tester to see if I had the setting correct. Anyway all wires were with that close range with no great variation that I was expecting. It ran good for about 15 minutes then started missing, if it were wires I thought the miss would be all the time. They are aftermarket wires taylor 8 mm silicone, visually they look good and are very flexible- but I know that does not really mean they are good.

Definitely replacing dist. cap and rotor cap, probably wires as well.

 

[78 Shark]

Just a shot in the dark, if it runs good cold then maybe it is getting a vacuum leak through the Temperature Vacuum Switch, which only allows vacuum at operating temp, check the vacuum lines for cracks. Also maybe coil heating up or even the distributor module heating up. The module needs special thermal grease to cool and age may have degraded the grease. Just some ideas.. 78 Shark

 

[billyvette]

Where is the temp. vaccum switch and how can i check it?
Are you talking about dielectric grease? or something else?
I don't think i have any vaccum issues, I can't hear any sucking or whistling noises.

 

[78 Shark]

Where is the temp. vaccum switch and how can i check it?
Are you talking about dielectric grease? or something else?
I don't think i have any vaccum issues, I can't hear any sucking or whistling noises.

One vacuum port is on the water neck and one is on the intake next to it. Disconnect the vacuum line that comes out of the carb or intake and goes into the vacuum switch and plug the open vacuum line and see if it makes a difference. Yes dielectric grease is what I use.