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EFV-8 Club Forum / General Ford Discussion / Weird miss on V8

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rudy51custom
09-09-2019 @ 12:47 PM
Member
Posts: 4
Joined: Sep 2019
          
I have a 51 4 door Custom with a flathead 8. It's an older restoration (30 ish years), and I just bought it a few months ago. Original engine was rebuilt, and changed to 12v, and electronic ignition. About 10 years and 7000 miles since rebuild. Don't know too much about the details of the history, just kind of an overview.

So, it ran great for the first few months I've had it. Since the weather has been cooling off up here in Canada, I've noticed a strange miss during acceleration on 1 or 2 cylinders. It does seem to be the same cylinders every time, as the miss is evenly spaced. Doesn't need to be WOT, or half open, even just a little will do it. Will miss a little as you accelerate, but where it really starts to come in is when the RPM's get up a bit.

At this point, everyone is saying plugs and wires, possibly distributor cap. And I'm with you on that, as that was my first thoughts, but keep reading.

For example, when in 3rd gear going down the highway, it will start to miss around high 40's, up until about 55mph. Between 55 and 60 and up, it's smooth as ever. If you're cruising at 60, and start to go up a hill, it will still run smooth, unless the speed dips below 55.

This doesn't fit with plugs, wires and cap, at least as far as makes sense to me.

All of this is intermittent as well. I went out this morning, it did it on my way out, but not back. My first thoughts were that it was caused by temperature. I don't want to rule that out, but I'm beginning to think it's probably something else. I don't think it's a fuel flow issue. It has an electric pump with filter, and a new fuel tank. If it was, it wouldn't run well over 55....

So, any ideas?

TomO
09-09-2019 @ 1:17 PM
Senior
Posts: 5848
Joined: Oct 2009
          
First determine which cylinders are missing by shorting out one plug at a time and checking the engine speed as the plug is shorted. The plugs that don't make a big difference in engine speed are the failing cylinders.

Next pull the plugs and look for possible signs of the failure cause. Use this chart; http://www.verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm

If you cannot determine the cause of failure or the missing cylinder cannot be found, change back to a point ignition to see if the electronic ignition module is failing.

Let us know what you find.

Tom

carcrazy
09-09-2019 @ 2:22 PM
Senior
Posts: 684
Joined: Oct 2009
          
Does the miss occur once the engine is fully warmed up? If it happens when the engine is cold or during initial driveaways when the ambient temperatures are below 35 degrees F, it maybe due to an icing condition. If you have cold ambient temperatures and high humidity, icing could be occurring at either the venturis or the throttle plates. Do you have the exhaust heat passages at the bottom of the intake manifold open?

rudy51custom
09-09-2019 @ 7:02 PM
Member
Posts: 4
Joined: Sep 2019
          
I've pulled the plugs a couple times. They all look the same. They're a bit darker than the one shown in the chart as "normal". There's no deterioration on them at all, and they're Autolite 216's. I did run the gapper through, and they're all at 25, or close enough to be considered 25.

It happens cold or hot. Car has a 4 core rad in it, and electric fan that I don't use. I have a sneaking feeling there's no thermostats, as the temp gauge shows 100F unless you get stuck in traffic. Even in the heat of summer, sits at 100F. Yes, it works, and so does the other temp gauge the previous owner installed. It can't be icing with an ambient temp of 60F, can it? It's happened on an afternoon closer to 80F as well.

Also, forgot to say, has Offenhauser 2 carb intake with two Ford carbs.

carcrazy
09-09-2019 @ 8:03 PM
Senior
Posts: 684
Joined: Oct 2009
          
Does the car have overdrive? The kick down switch interrupts the ignition when the accelerator pedal is pushed down to the floor or near it. My car with overdrive will occasionally miss when the pedal is near the floor but not quite near enough to cause a downshift to occur.

Model B
09-09-2019 @ 8:35 PM
Member
Posts: 69
Joined: Oct 2009
          
Just a thought....When my '50 Tudor began to miss a while back, I tried to isolate where the miss was. Can be
frustrating for sure! I tried replacing some of the ignition parts i.e. plugs & wires etc. Found the problem to be
the vacuum advance had a bad diaphram. Replaced it and it has run great ever since. Good luck, Pat

rudy51custom
09-09-2019 @ 8:55 PM
Member
Posts: 4
Joined: Sep 2019
          
Had a quick look in the distributor tonight after kids were finally in bed. Cap does have something on the contacts where the rotor passes by. Doesn't look like corrosion, but I'd say more likely to be arc damage. Rotor contact has it too, and looks like previous owner has rather ham-fistedly cleaned the rotor already at least once. Could be the rotor-cap gap is too big. Distributor issues can be intermittent

I don't have overdrive, unfortunately.

Vacuum advance was going to be my next suspect. If things are a bit wonky, the shoe might fit.

pauls39coupe
09-10-2019 @ 4:56 PM
New Member
Posts: 124
Joined: Jul 2014
          
If you can eliminate the advance as the source of your problem, next I would look at the power valve in the base of the carb. The power valve should enrich the mixture under load ( 40-55 mph?).

TomO
09-11-2019 @ 8:51 AM
Senior
Posts: 5848
Joined: Oct 2009
          
Did you determine which cylinders are failing? Is it random cylinders?

Tom

rudy51custom
09-11-2019 @ 8:10 PM
Member
Posts: 4
Joined: Sep 2019
          
When it happens, it seems to be the same cylinder, but not every power stroke for that cylinder. Sometimes 2-3 in a row, sometimes only one, then not again for 10 - 20. Completely random, but feeling like it's one cylinder, maybe two. If it was one cylinder all the time, it would be easy to figure out.

I took it to town today, temp outside was low 30's celcius (around 90F), and it never skipped a beat at all.

I've ordered plugs, cap and rotor. Thought I might as well, since I was ordering other stuff anyway. Will see what happens when the parts come in.

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