70 Ford 391 cid misfire

boberjagel · Oct 1, 2019

Have an old grain truck that has a misfire only at low idle sporadically. As soon as you crack the throttle it goes away. I've put electronic ignition in, new plugs ,wires, cap ,rotor, new carb, new spacer between carb and manifold, changed distributer, checked every possible place for vacuum leaks. Cant figure this thing out. What else can I be missing to fix this.

casecollectorsc · Oct 1, 2019

Broken valve spring??

Sprint 6 · Oct 1, 2019

Probably has a valve sealing issue. May also have a lobe going down on the cam. Time for a compression check, both static and running.

ohiojim · Oct 1, 2019

that was my first thought, something in the valves.

cole in mo. · Oct 1, 2019

A lot of those old ford FE and FH engines would have the intake gasket suck in on the intake ports, would seem like a miss at idle but come out of it at throttle, but you have to pull the valve covers to see, with the covers off at idle, spray a little starting fluid on those areas to see.

ss55 · Oct 1, 2019

Could dirt or other restrictions in the carburetor idle circuit be causing lean idle fuel mixture? Does opening the idle screw(s) help?

Steve@Advance · Oct 1, 2019

Don't think it would have an EGR, but if it does, it should not be heating up at idle.

Does a partial choke help? If so, the idle is lean.

A test for intake gasket leak, pull out the PCV valve, block the hole in the valve cover, block the vent, if it draws vacuum (provided the rings are good)in the crankcase, there is an intake leak.

Run a compression test, engine warm, throttle open, look for differential in pressure between cylinders.

A vacuum gauge can be helpful in diagnosing such problems.

jimg.allentown · Oct 1, 2019

Could also be a throttle shaft getting loose from a worn bushing. Even though you say you put on a NEW carb, was it really new? Or maybe a rebuilt? Or just new to this engine?

Just a thought.

mmfan55 · Oct 1, 2019

Weak value spring can cause that

Bruce(OR) · Oct 1, 2019

Well, pop the hood tomorrow night and look for a light show. If you see any loose spark jumping around the plugs or the wires, you found your problem. I remember a few years ago, Okay, MANY years ago. . . raising the hood on a car and seeing the plug wires "glowing" with all the
cracks in the insulation.

Hobo,NC · Oct 2, 2019

I would grab my Propane enrichment tool and see if it made the condition better are worst... Are put another way somehow manually change the fuel ratio... You can also do this somewhat with the choke flapper on the carb..

Manually changing the ratio will not affect a mechanical condition are a spark issue....

I would think a valve spring are cam issue would give ya a little pop out the carb throat ?

A little info on how the idle mixture screws affect idle would also help ?

Cover all the basics ignition timing and such...

Destroked 450 · Oct 2, 2019

Those rebuilt governed Holley carbs are pretty pricy, haven't seen a new one for several years, same with the distributors that have the internal governor system.
1st thing I'd do is figure out which cylinder is miss firing then swap that plug and wire to another cylinder to see if the miss fire moves with them.

Rebuilt a engine some years ago that had a miss fire on start up, thought it was a vacuum leak, change carbs, resealed the intake twice, changed spacers.
Turned out one of the NEW spark plugs wasn't any good.

70 Ford 391 cid misfire

boberjagel

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casecollectorsc

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Sprint 6

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ohiojim

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cole in mo.

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ss55

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Steve@Advance

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jimg.allentown

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mmfan55

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Bruce(OR)

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Hobo,NC

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Destroked 450

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