Loosing spark on number 3 plug

[Ken B]

I am helping a friend that has a strange ignition problem. The timing is correct. The point gap, plug gap, and rotor to body gaps are all fine. The engine runs great when the spark is advanced about half way down. When you retard the spark, we lose spark on plug 3. We have one of the clear distributor caps and we can clearly see that the spark goes away on plug 3. Another clue is that the ammeter goes negative when we lose spark on plug 3. The distributor is using the original style plates and points. Hopefully someone can suggest something to look for to fix this problem.

[Chuck Dempsey]

My only guess is that the distributor rotor is 'grounding' something at that particular spot. I'd try retiming the engine with the rotor turned 180 degrees and see if that changes the 'dead' plug....

Good luck!

[Rob Doe]

I have heard or read about the advance retard rod causing #3 to jump to the rod???
Turn the lights out and see if #3 is being shorted/grounded by the rod or the ignition cable???

I'm sorta pokin' in the dark here. LOL

How far negative on the ammeter? -4 amps? Are the points shorting against something?

[Patrick L.]

Yep, what Rob said. I usually slip a length of rubber tubing over that rod.

Why not check the #3 contacts in the cap with an ohmmeter to see what it has to say.

[nkaminar]

Distributor bodies, the part that has the pins and that the spark plug straps (wires) connect to have been found to be bad in the past. This is an infrequent problem. The problem is internal to the body.

Measure the gap between the rotor and cap. It should be about 0.020 all the way around although a smaller gap will give better performance. The gap is adjusted by bending the brass tab on the rotor. I have found housings were the gap was not uniform for all 4 pins.

If the ammeter is indicating discharge then the voltage is likely low which is why you are having issues at low rpm. Measure the voltage at the battery or at the terminal on the starter switch. If the battery is old the voltage can be low when the generator is not generating. You also could have issues in the wiring like a corroded connection that is causing lower voltage at the coil.

[Ken B]

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Thanks for all the replies. I will give them a try. Hopefully this weekend.

[Synchro909]

Our cars being RHD, we don't have that rod going close to the plug so I would never have thought of it as a reason for the issue. My guess is the fabled #3/#4 cross fire. I have had the situation where the distributor body was breaking down and spark would jump through the poor insulation from #4 lead that is built into the body to the #3 plug lead and vice versa. It was NASTY.

[jb-ob]

Ken,

'It was dark & moonless night'......engine idling in the dark, watch under the number three plug wire that it is not shorting to the distributor body.

Had the same issue with an original dist. body that had a small crack in the body under the #3 spark plug lead allowing it to short / misfire.

Or simply try another body.........

[Marshall V. Daut]

Move #3 spark plug to another hole and see if the misfire travels with it. If it does, the spark plug is defective, perhaps a crack in the porcelain or an internal short that can't handle the bump up in power demand. If the misfire stays at the #3 cylinder after swapping plug positions, the cause is probably a defective plastic body, as has already been suggested multiple times. There may be an internal crack or short near the #3 terminal inside the plastic body. If it is a body made in the 1990's, this is very likely, as there was a spate of defective plastic distributor bodies dumped on the market during this time. Usually the problem could be traced to the #3 electrode, especially in rainy or damp weather. Such a shiny, new distributor body nearly ruined a cross-country trip for me in the mid-1990's until I put on a local Model A owner's disgustingly grungy original cap. The engine instantly roared to life. That brand-new distributor body I had purchased before the trip was defective right out of the box, manifesting problems in rainy weather. It sounds to me as though you have the makings for the same scenario.
I would swap spark plug locations and go from there.
Marshall

[Ken B]

We did try swapping the plugs. The problem stayed on #3. It misfires when the spark advance is raised up about midway. It sparks again when we move the advance lever down. I am hoping my friend wants to get back to working on this soon.

[ndnchf]

What spark plugs are you running? If they are shorter than Champion 3X, the brass connector may be pulled to close to the spark advance rod and shorting to it. This happened to me using Autolite 3076 plugs.

[Bud]

I put heat shrinkable tubing over my bronze spark plug connectors. That fixed the spark jumping from the bronze connectors to the distributor body. Runs fine now.

[nkaminar]

I bought a new housing that was supposed to be high quality. It was loose on the distributor causing some pin to rotor gaps to be excessive. I could not adjust it (by bending the tab on the rotor) because then the other pins would hit. I went back to my old housing. That fixed things. But I did adjust the gap to the minimum I could and the car runs better that way.

[Ken B]

I finally had a chance to get back to help my friend with his model A missing spark on plug 3. We pulled the distributor out and removed the upper plate. The post that the points go on to wasn't tight and the connector at the end of the wire was rotated and closer to the body. I tightened the post and made sure the wire connector was more towards the inside of the plate. We put it all together and spark was back on #3.

[BillCNC]

Originally Posted by nkaminar

I have found housings were the gap was not uniform for all 4 pins.

I had that problem. My distributor body has the brass pins and one pin was shorter than the rest so I filed the other three to match. I blew the body off with air to make sure no stray metal filings were hanging out still.

Regards
Bill

[Y-Blockhead]

Ensure your distributor body is clean. Dirt and oil can cause carbon tracking which will ground the spark. I also use the longer spark plug connectors for a Model B to get the connector farther away from the distributor body and spark rod.

[nkaminar]

Ken, what you did was to improve the spark on all cylinders. #3 was marginal before but increasing the spark energy on all cylinders overcame whatever was causing #3 not to fire.