Shudder and stumble off idle

[jstone4646]

What would the symptoms be of a failing ignition coil?  My truck is still sputtering off idle and I am curious if a weak coil might be causing that at all.  Idle is fantastic, but drop her into gear and its stutter, backfire, hesitate, and power loss central.

I pulled my spark plugs which were only put in a few days ago and found them completely covered in soot.  I know that would suggest possibly weak spark...or running too rich.  Just looking to see where I can start diagnosing this shudder, power loss, and backfire I am having only when off idle.

Battery, plugs, wires, voltage regulator, dist cap, and rotor are all new.  I also bought a timing light and set her right on the money at 10BTDC.  So I am fairly confident timing can be tossed out of the equation now.  To bring vacuum into the equation, all my vacuum lines are new also....here is my setup on that just to put that out there as well:

Holley 4150:
timed vacuum port connected to distributor
PCV port connected to PCV valve
Power brake port is plugged (brakes are running off manifold splitter)

No other vacuum ports are really necessary on my truck as I understand it since I do not have A/C.  If it is not connected to the carb it is plugged, except the one line that runs down to the transmission which comes off the manifold.  Vacuum on my truck is pretty steady around 20 at idle.

[grumpin]

I've never had a coil do that. Not saying it won't.

Some things that pop in mind are EGR valve and the accelerator pump or power valve, I think Holley calls it.

[jstone4646]

I was hoping I could stay away from the carb but all the research I have done actually points to the accelerator pump that you are talking about.  My truck does not have an EGR system at all so I can rule that out.

[Gary Lewis]

I think the accelerator pump is suspect.  You should be able to see two strong streams of gas shoot into the carb when you crawl up over it, engine off, and open the throttle quickly.

[jstone4646]

I will take a look at that tomorrow, thank you Gary.

[jstone4646]

I did a late night check on the truck, the fuel stream coming out of the jets (or whatever the term is...sorry I am still learning carbs) is very noticeable, I can hear the streams when I pump/manually push the throttle cable and see a decent amount of fuel being pushed out.  When the motor is warm I hear a nice little swish when I open the throttle (engine off) followed by some steam/smoke coming out of the top of the carb.

[grumpin]

I couldn't find the ohms range in my 1986 manual for the coil, but I found this link.

https://itstillruns.com/test-ford-ignition-coil-5941137.html

I had a hard start problem on my 92 Bronco when cold and I found one side out of range and the new coil fixed it.

Another thing is a vacuum leak. Could be around the intake manifold also. I believe you checked this though. Couldn't hurt to check all around with some brake cleaner or starting fluid.

And I would still be suspicious of the carb..

[jstone4646]

I have never rebuilt a carb, but I am game to give it a shot.  The rebuild kits are cheap, and I have the manual for my specific carb with all the necessary specifications.  The only thing holding me back is not knowing exactly what is in my motor.  I know she has a 302 cam, but I do not know if it is a direct factory cam or a mild upgrade.  Based on sound alone if she is any type of upgraded cam, it is extremely mild.

Would not knowing this information change how the carb would be rebuilt as far as jetting and adjustments?  Or would I be safe to rebuild to factory specs?

[grumpin]

I would go with original specs.. Make sure the jets are clean.

[Gary Lewis]

I agree. Rebuild stock.

[jstone4646]

Thanks for that link Grumpin, based on what I read in that article and what my coil shows, I believe we can rule out a bad coil as it falls within the specs....I am going to go ahead and order the correct Holley rebuild kit.

[grumpin]

You're welcome!

That's great! One more item eliminated. Good job!

I like the meter! Never owned a Fluke, but used many, the last job I had, we had three in the shop.

[FuzzFace2]

jstone4646 wrote

I did a late night check on the truck, the fuel stream coming out of the jets (or whatever the term is...sorry I am still learning carbs) is very noticeable, I can hear the streams when I pump/manually push the throttle cable and see a decent amount of fuel being pushed out.  When the motor is warm I hear a nice little swish when I open the throttle (engine off) followed by some steam/smoke coming out of the top of the carb.

Even if you heard the squirt you will find most of the time you will when you work the throttle by hand.
With day light open the choke and look down inside and move the throttle just a little it should squirt. Most of the time I find they do not. The Holley you can adjust this and should squirt with any throttle movement.
When you rebuild it you will see all the adjustments the Holley carb has. I have never had good luck on the pump cam adjustment but had with going with bigger squiters, where the fuel comes out. As a guess it is a 28 and I found 30 or 31 works for me most of the time. I also have 1 or 2 Holley's with a 50cc pump on it, they come with 30cc pumps.

When you do rebuild it with pen & paper make note of the squirter(s) number, main jet numbers, power valve number you install from the kit (6.5 as a guess) and what hole the pump screw is in.
This way you know what is in it to start and if you need to make changes up or down you know with out pulling the carb apart to check.
Dave ----