pulled the plugs on my new to me 75 1.8 with carbs today. 1,2,&4 look good grey in the center (3 electrode plugs) black around the sides. # 3 was totally black. it was idling rough so I pulled one plug wire at a time to see the difference it made. All but #3 made a difference. When I pulled #3 there was no difference at all. It has an electronic fired dizzy in it as well. Cap and rotor are new. What ya think?

Was it oily and black or sooty and black? Got a picture?

Could be a bad plug wire. If you have an ohm meter you can check the spark plug wire to make sure that resistance is similar to the others that are working.

sooty black. dry as a bone. Sorry I cleaned it a put it back in. Ordered new plug wires and a set of NGK RBP5ES plugs.

A quick and simple test to see if your plug wires are bad is get a water spray bottle and mist the wires gently, no need to go over board. If you have a bad wire blowing fire, you'll find it quick.

If misting doesnt tell you anything, install a spark checker on both a know good cylinder wire and the bad one. Look at the difference. A good ignition system should blow big blue 20kv spark.



Just hook the end of the wire from the spark plug to the tester. That clamp on the tester, ground it to the engine.
Click to view attachment

The spark tester gap is ajustable, 20kv is like a 20mm gap.

If all these these come back good, I have even more stuff to check. Keep us posted.

20mm gap? 7/8 of an inch? Did you mean 2mm?

Zach

The tester has markings on it to test a 20kv gap. Its probably closer to 10mm here is a pic of mine...

Click to view attachment

OK. So at this point you can decide to do a compression check (which I would recommend) or just work on the carb. For the carb, start by verifying everything in the idle circuit (idle jet is the correct size, properly seated, and the volume screw isn't too far off). You should also make sure you don't see any drips of fuel from that barrel when the car is running (or even stopped if it's bad).

Doesn't hurt to make sure all the basics are done too... Ignition working (see above) and valves adjusted properly.

https://www.amazon.com/dp/B0002STSBM/

Yeah I will do a compression check this weekend along with a valve adjustment I'll keep you posted. I ordered that cool tool as well. My son is having similar probs with his wild man Jeep so we will share it.

What are the proper idle jet sizes? These are 40 IDF's

I'm a Dellorto guy, but I seem to remember 55-60. I think it would be telling if you compare barrel #3 to ANY of the other three and see what's different.

I will do that. Thanks.