Engine starting/running rough

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Jul 1, 2015 | 04:45 AM

benbuilder

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Fleet Mechanic

Joined: Sep 2012

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Engine starting/running rough

My daughters 75 f100. 302. Auto. headers. Truck ran great until yesterday.
Yesterday she started it and heard a big backfire. I restarted it, it ran rough for a bit and then it seemed fine.
I took it out of the garage and parked it. I started it again about an hour later and went out for a ride. It stalled going down the road. I restarted it and heard another loud backfire. I am not sure if it stalled or not, but I coasted into a lot. It would not start so I had it towed home.
When I got home I put an inline spark plug tester to see if I was getting spark and it started rough. I took off the tester and restarted it but it ran rough.
This morning I started it as normal and it ran rough again then stalled. I tried to immediately restart it but it would not start.
Where should I start looking? There is no way the timing chain could have jumped is there?

Jul 1, 2015 | 04:55 AM

gatorfor88

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Joined: Jan 2013

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From: Panama City

Timing could have jumped. Turn the engine over to TDC #1 and pull distributor cap to see if it is close to #1. Back firing is usually ignition/timing related.

Jul 1, 2015 | 05:10 AM

benbuilder

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Fleet Mechanic

Joined: Sep 2012

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I will check the timing, but how/why would timing jump? 75,000 legit miles on truck. Worn chain at 75,000 miles?

I am no expert, just asking questions and learning.

Jul 1, 2015 | 08:07 AM

bkaul

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Joined: Aug 2010

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From: Knoxville, TN

More likely than the timing chain (especially with that low mileage), it's possible the pin holding the gear on the distributor shaft could have sheared or fallen out and allowed the gear to slip a little.

Backfiring can also be a sign of overly lean mixture, but usually I'd expect to encounter that at idle/startup.

Jul 1, 2015 | 11:38 AM

jackburtonme

Tuned

Joined: Jun 2015

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From: Southern OR

I'm not sure I've ever seen one "jump time", Your problem is probably inside your distributor.

Jul 1, 2015 | 05:32 PM

bigdan1979f-250

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Joined: Apr 2006

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From: mid-MO

Engine starting/running rough

Duraspark or points? With the Duraspark, the reluctor spins on the shaft driven by the drive from the oil pump and each time a tooth on it passes the magnetic pickup it fires. Point is, there is a small roll pin that locates the reluctor on the dizzy shaft. I had one work it's way out and it behaved similar to what you describe. Check that and then as mentioned roll engine to top dead to see if dizzy rotor is even close to #1 position.

Jul 2, 2015 | 04:31 AM

benbuilder

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Fleet Mechanic

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Thanks for the help. It is a duraspark. Yesterday I replaced the fuel filter on a suggestion from my uncle. I am pleased to say that it MAY have been the problem. I let it idle for 10 minutes and drove it around a bit. It ran perfect. Time will tell. At least I know it is not a timing issue.