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Intermitent Electrical misfire on #6
#1308773 Sun Apr 28 2019 02:11 AM
Joined: Feb 2013
Posts: 257
On my '48 GMC with a later model 228 engine. I have one of those visual spark testers where the plug wire goes onto one side of the tester and the other side of the tester goes onto to the plug. Using that, I found an intermittent weak spark on #6, the engine stumbling each time the spark showed yellow in color. I switched #6 wire with one of the other wires but the miss remained on #6. I compared the resistance of each of the spark plugs and #6 was much higher than the others. I also ohmed the wires and while they seemed OK I had a fresh set on the shelf so the truck got the new wires and also a fresh set of plugs.
The new wires and plugs have helped but the stumble is still there from time to time. Am I right in thinking that I should be looking at my distributor now? What am I looking for? Wobble in the shaft? Worn cam lobe? Other usual suspects?
Help is appreciated! Thanks, Jim

Re: Intermitent Electrical misfire on #6
Eureka Jim #1308778 Sun Apr 28 2019 02:38 AM
Joined: Apr 2005
Posts: 9,760
Grease Monkey, Moderator General Truck Talk & Greasy Spoon
Look inside the distributor cap for carbon arcing. If #6 you will find a line from the middle contact to the #6 tower. Good luck.🛠

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Re: Intermitent Electrical misfire on #6
Eureka Jim #1308784 Sun Apr 28 2019 05:23 AM
Joined: Feb 2013
Posts: 257
Thanks for the suggestion Martin. I didn't find any evidence of carbon arcing. However, the metal tab on the top of the rotor that contacts the carbon was getting kind of rusty---not a surprise here in Hawaii where it seems everything rusts---and the brass part was partially discolored black on its top side and rough to the touch as if it was burned. It also was kind of a loose fit on the distributor shaft. I dug up a new rotor and it snugged down nicely when I put it in. Could a sloppy fitting rotor cause a miss?
I started the engine up and it's firing on all 6 cylinders! Since the problem was intermittent I can't declare it fixed but everything I'm doing is helping. I won't have a chance to drive it until Monday but I'll report back after I put a few miles on it.
After all the things I've had to fix---$$$---it would be nice if it was just the rotor and not a worn out distributor!

Re: Intermitent Electrical misfire on #6
Eureka Jim #1308785 Sun Apr 28 2019 05:55 AM
Joined: Mar 2019
Posts: 197
I had the same problem recently. Checked the usual suspects for a missing cylinder...plug, wire, cap. I found a lot of play in the rotor, and the distributor is worn. Replacing the rotor, and adjusting the points resolved most of the problem. It still misses occasionally at idle, so I plan on replacing the distributor too. I'm guessing the accumulation of factors caused the miss. Why it occurs on #6, I can't say.


Last edited by Shakey61; Sun Apr 28 2019 05:56 AM.

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Re: Intermitent Electrical misfire on #6
Eureka Jim #1308797 Sun Apr 28 2019 02:39 PM
Joined: Jun 2004
Posts: 8,371
Check the point gap. Sometimes there is just enough runout in the dist shaft or cam to effect only one cyl. if the points are too close together.

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Re: Intermitent Electrical misfire on #6
Eureka Jim #1308869 Mon Apr 29 2019 02:19 AM
Joined: Mar 2010
Posts: 9,654
Does that distributor have a grease cup on the side of it?
If it does, take off the cup and make sure that the spring and friction disc which rides on the distributor shaft are all there.
This assembly of parts is designed to put a specific amount of lateral pressure on the distributor shaft to eliminate shaft wobble, thus eliminating inconsistent points performance.

Last edited by 52Carl; Mon Apr 29 2019 02:24 AM.
Re: Intermitent Electrical misfire on #6
52Carl #1308885 Mon Apr 29 2019 04:21 AM
Joined: Feb 2013
Posts: 257

Carl, unfortunately there is no grease cup.

Re: Intermitent Electrical misfire on #6
Eureka Jim #1308969 Mon Apr 29 2019 10:34 PM
Joined: Mar 2010
Posts: 9,654
It should have a grease cup. Replacement distributors commonly don't come with one, but there should be a threaded plug where one belongs.

Re: Intermitent Electrical misfire on #6
Eureka Jim #1309210 Wed May 01 2019 07:41 PM
Joined: Feb 2004
Posts: 22,368
Even without a grease cup, there needs to be a spring and a phenolic disc riding against the shaft inside that hole. You can install a 1/8" pipe thread grease fitting and give it a very small shot of grease at every tuneup if you desire.

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