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1985 Ford F150 engine misses

I am stumped. I’ve had my mechanic work on this issue (the engine misses), and for the past few months thought the issue was resolved… Truck is 27 years old and has only 75K actual miles. Here’s what I have done.

Installed rebuilt carburetor - 2009
New plugs, wires, distributor cap and rotor - 2010
Adjusted carburetor, timing, and choke - Feb 2012

I normally drive it every couple days, yet during last winter it set up for a couple weeks with 10% ethanol fuel. In early spring, the engine started missing while driving (about 1/3 of the time). After taking to the shop a couple times, my mechanic basically gave up. I believe it’s time for a new mechanic. I may also give up and just get a newer truck, but not quite ready yet.

I suspected water in the tank, knowing that ethanol can attract moisture, so i ran the tank dry then bought 5 gallons ethanol free fuel (i live in the country where we can still find it). Engine started, ran great for a few months, and i thought i had solved the problem…until yesterday, when the engine started doing the same thing again just as it did when the problem originally started.

I would like to have some thoughts before I take it to another mechanic, so please send them my way

Has anyone checked for a stretched timing chain?


Is it the 6 cylinder or 8 cylinder engine? The Winsor V-8s are notorious for jumping time while the 6 has a gear and rarely skips. Both engines used a mixture control solenoid on the carburetor that could cause problems. When the engine is warmed up and idling partially close the choke and listen for the engine to either increase or decrease rpms. If it increases the carburetor is leaning out. If if stalls down the carb may not be the problem.

These engines do not have tioming chains…intake manifold or EGR leaks were very common…check that.

@kawasaga - the v8s sure do:

With a poeblem that comes & goes as yours does I’d not be inclined to look at cam timing first, since once a chain wears it stays worn and if it slips it stays slipped.

Have you checked your fuel line pressure? This sounds to me like a fuel problem, either metering, pump, filter, or line. If this is a 27 year old pump…well…perhaps…

I was talking about the 6 cyl really…sorry.

Ohh…the V-8 carbs never had closed loop control…only the 300-6 did…except for a few in Conada.,…never did the V-8 trucks have O2 feedback.

It is straight line 6

Excellent. Lots of room under the hood to sork in.
Let us know how that fuel pressure test works out.

holersl writes to say it is a straight line 6 cyl. 300 Pretty standard.

Is the problem a real dead miss on one cylinder that can be isolated, or is the engine running rough at idle?

The engine runs rough on the road and also at idle. The miss is not in sinc with any cylinder misfiring but is intermittent.

Grab the top of the carburetor and try to twist it. The throttle plate screws may have worked loose.

  • Coil. Have the shop put it on their ignition analyzer. Verify correct spark amplitude.
  • Gummed up fuel system. Drain tank, flush w/fresh gasoline, change fuel filter, maybe try some carb cleaner.
  • Vacuum leak. Have the shop test all vacuum hoses and each vacuum operated device for diaphram leaks.
  • Stuck valve, non-sealing ring, bad head gasket, etc. Have the shop do a compression or leak-down test.
  • Carb problem. Ask the shop to put another carb on it, borrow it from another truck if necessary, then drive it for a week as a test to see if the problem goes away.

Remove the distributor cap. Put a socket with a breaker bar on the crankshaft bolt. While someone watches the rotor in the distributor and rotate the engine in one direction and stop. Now rotate the engine in opposite direction. If the engine can be rotated in the opposite direction more than 5 degrees before the rotor in the distributer begins to rotate the timing chain is stretched.


I just don’t see this as a likely timing chain problem. It could be that of course, and the symptoms are somewhat consistent with a timing chain problem. But the truck only has 75K on it. Wouldn’t it be unusual for a timing chain to go bad at such low mileage?

The 6 cylinder has timing gears… No chain.

You guys are right. The 300 CID had timing gears not a chain. However, if this engine has the original timing gear set the cam gear has plastic teeth. These plastic teeth have been known to shred off the cam gear throwing the ignition and valve timing off. So you can use the rock the engine back and forth method to see if the gear is worn.

One of the other ways to determine if the gear is shredding it’s teeth is to drain the oil. And if small bits of plastic are found in the drained oil the cam gear is shredding it’s plastic teeth.


The 300-6 NEVER has head gasket trouble. It does, however have a PITA two -iece intake / exhuast that tend to leak (vacuum and exhaust)… If you put gaskets on the exhaust, it will not last long so I’d beware of that…Spray carb clean around the EGR and intake and I thnk yu’ll find a bit of RPM rise. :slight_smile: