Hi, I'm new to the forum and just recently got a 1979 Ranchero. I've had fords in the past, most notably my 1968 Torino convertible that I had to part ways with unfortunately, so I am fairly familiar with the SBF platform. The ranchero has a 351 Windsor motor swapped into it, when it originally had a modified motor and I am getting a dead misfire on cylinder number 6. Under accel and decel the car has no power and will pop from the driver side bank that has the misfire (the car has headers with dual exhaust). When I pull the sparkplug wire from the motor at idle nothing changes, and the exhaust temperature from the runner is multiple hundreds of degrees colder than the rest. I have put another plug on the wire and grounded it to check for spark, and I am getting a consistent spark at all rpm ranges. I have also done a compression test on the cylinder and got a solid 150 psi. The motor is carbureted with a national 2 barrel on it with a dual plane intake. I even pulled the valve covers and looked at the valves to rule out lifters/wiped cam lobe and everything seemed fine and nothing was loose. I inferred that I had a vacuum leak on that runner from the intake gasket leaking, so I pulled the intake and resealed it properly. The old gasket wasn't in good shape but it still didn't fix my issue. I have switched plugs around, tried plugging all vacuum ports, and done everything else I can think of. I am really at a loss on this one and i'm just looking for some ideas on what could be causing a total misfire of my cylinder 6.. Thanks to anyone that pitches in -Pelican of 301 Swim

 

What does the inside of the dist cap look like ?
Could there be a crack in there or moisture ?

 

The distributor cap looks good, a little dusty at the bottom but no corrosion or moisture or anything

 

If,,,, you carefully,,,, with engine running, remove the oil fill cap and shoot a little brake parts cleaner into the fill hole , what does the engine do ?

 

Just tried the brake fluid technique. Doesn’t do anything when I spray a little, though my pcv is set up differently with the valve on the driver side built into a breather/fill cap, and the passenger side has just a 45 degree barb for the time being. I shook the valve and it’s not stuck or anything.

 

What does the spark plug for that cyl look like after you run it a minute ? Is it wet or dry?

 

The spark plug for cyl 6 looks maybe a little white but not bad. Definitely a new plug and I have swapped it and tested spark. Definitely no fouling or excessive carbon or cracks.

 

I never did get an answer of if fuel was getting to that cyl, but if you have spark and fuel to that cyl and spark coming at the right time, then there must be a problem with the intake manifold runner going to that cyl. Look a little deeper at that section.

 

It’s pretty clear to me that my problem is most likely fueling related, as spark, compression, and valve movement all seem good. The only problem is that I can’t figure out why that is possibly the case. The intake is freshly resealed, which I thought was the problem. I went over it in a wash tank and cleaned it up and found no cracks or possible leak spots. The intake doesn’t seem to have any vacuum ports on that runner, except maybe the front carburetor vacuum port that is close to it which goes straight to the pcv valve on the drivers side front.

 

Have you tried to close off that hose with the PCV valve removed ?

Which barrel of the carb supplies cyl #6 ?

Are we sure there is no restriction in the header for #6 cyl ?

Did you verify the correct firing order for the engine ? I believe it is,,,,,,,,,1-3-7-2-6-5-4-8 notice plug wires 6 &5 are close in firing order, could they be reversed ?

Could you have a lifter for that cyl that might be hanging up keeping a valve for that cyl open ?
Did you verify correct timing?