328 dist. rotor shorting to dist. shaft

I have had 3 rotors short to ground through the distributor shaft in the last 500 miles(same dist.). I have ohmed the wires and extensions all o.k.,new plugs.Am at a loss as to where the source of my high secondary resistance is.This started shortly after caps,rotors and plugs were replaced. Any advice would be greatly appreciated.

 

ck for "burn throughs" on the rotors...pretty common, actually, especially if old rotors...
TINY poin holes, look for dark pin hole spots...

 

Yeah these have been scorched badly,just tryin to figure out why.thanks for your help

 

What PN rotor?

 

Part# for rotor was126139,dist. cap was115518 ordered them from all ferrari parts and came in magnetti marrelli boxes, am wondering if the spring on the center (coil)contact is not giving enough pressure to the contact on rotor as the arcing is coming straight down the side of the rotor to the dist. shaft.Any ideas would be greatly appreciated. Thanks Sam

 

Use an ohmeter and check each plug wire and coil wire. Sometimes the screw holding the wires to the cap miss the wire center core, creating a small gap. Check extenders and plugs too.

 

My car had trouble with the carbon brush not making good contact. The car failed smog, but the rotor was okay. I replaced the brush with one from an Alpha Romeo. The proper brush was hard to find, but finally found an Alpha cap at Auto Zone and it had a 6mm Dia brush. The brush was a little too long so it was trimmed & radiused in a drill.
To have that many rotors fail it seems like the tracking could be coming from, or facilitated by, the cap somehow? You may want to try putting one of the old caps back on.

 

Did you check to see if the rotor is providing a proper gap to the cap? I had this problem many years ago with defective after-market rotors and caps one of my old Jaguar XKEs. The car was running fine, and I did a routine service on it, replacing the cap and rotor with an after-market version that was half the price of the Lucas OEM replacement. As soon as I started it up, the car started playing up.

It turned out that the rotor was not providing the proper gap with the posts for the spark plug leads. The gap was too big, by a wide margin. As a result, sometimes the spark would go to the plug, but other times it would find other paths to ground. Also, the metal piece on the rotor was not fully secured, so it was moving around a bit, causing other problems as well.

Even though the parts came in a Magnetti Marelli box, are you sure they are the right ones for that car? Did you compare them with the originals? In particular, measure the length of the rotor and check the gap, comparing the original parts to the new parts. If the gap between the rotor blade and the terminals inside the cap is too large, the spark will find the path of least resistance to ground.

 

Yes, there are some rotors out there, that are too short.
1981 is a Volvo.
1985 is a Fiat.

Both are close but too short,, wrong.

Edwardo

 

You also have to look at the rotor phasing in the cap. Because you can move the points plates in the cap, you can have the correct dwell and ignition timing, but the rotor phasing can be off, meaning that the rotor is very far away from the post for a particular cylinder when the spark happens.

Look at the end of the rotor, if most of the wear is concentrated on the corner of the rotor instead of in the middle, then the phasing is off. When I setup distributors on our machine, in addition to setting the dwell, advance curve and point synchronization, I also set the rotor phasing. We have an old cap that has holes machined in the top which allow me to look where the rotor position is when the spark occurs.

Anyway, you have an area in your ignition system causing KV of the spark to be too high, Work from the spark-plugs backwards and you will find it.

Brian Brown
Patrick Ottis Co.

 

Brian -- your mind is on 308, not 328.

I thought that the OP may have originally misspoke since he mentioned a "distributor shaft" and the distance from the end of the metal center post of the coupling mounting the rotor to the metal arm on the rotor carrying the spark is a lot larger on a 328 vs a 308 (hence the question to verify rotor PN). Sure wouldn't hurt to do a sanity check for rotor phasing and length as you guys suggest. Any possibility that a broken early "rubber" dist rotor coupling could misphase the rotor?

 

Thanks to all who have replied to my dilema. You have given me several avenues to check. As most of the emergency vehicles i work on are either diesels or distributorless gas engines I need to go back & recheck these things,will post results. Thanks Sam