I searched through the forums and didn't see an answer that fixed my problem.

I can't seem to get cylinder 7 to fire on my 1974 302. I did a compression test, cylinder 7 was at 100 psi (cold), the other cylinders were between 95-105 psi. I pulled the plug and turned the engine over and it looked like it was getting strong spark. I ohm tested the wire, it was at 10k on the 2ft wire, within manufacture spec of ~10k per foot. Checked the timing, 12 degrees BTDC, when I pulled the plug it smelled like gas. I advanced and retarded the timing between 2 degrees and 15 degrees. With the engine running I tried leaning/enriching the air/fuel mixture.

Temps on headers are around 550 degrees with a warm engine idling, cylinder 7 is ~160 degrees, using Motorcraft ASF 32CA spark plugs. When I replaced the camshaft bearings a few months ago the piston rings and camshaft looked okay. I pulled both valve covers, and the valve springs on cylinder 7 seem to articulate at the same rate as the other 7 cylinders. I ended up replacing the spark plugs, cap + rotor, and plug wires. Firing order set on 15426378, non-HO motor.

Not sure if relevant but it has an aftermarket intake and Holley 4160 600 CFM. Thanks,

 

Interesting conundrum.

Seems you have compression, you mentioned plug smells like fuel so presumably there's air/fuel going in. That seems to be confirmed visually after you removed the valve cover and saw movement of the valves.

You're seeing spark with plug removed, the conditions inside the chamber are different than open atmosphere, but you do have spark.

Have you checked the header tube for #7? Is it possible something in there plugging it? If that were the case I think you would be seeing higher temps on the header tube near head though so probably not the case.

My other thought is the spark timing is off for that particular cylinder which could happen if the reluctor wheel inside the distributor was tweaked somehow. I know back in the day in some racing circuits, part of the tuning would involve bending the reluctor wheel ever so slightly to advance/retard timing for a specific cylinder. Maybe a nut or something fell into the distributor and ended up bending that part?

Only other thing I can think of doing is take actual measurements of the valve lift as you rotate the engine over by hand and see how #7 compares to the rest.

 

I would pull the plug and turn it over a few times or even start it. once it's blown out put a new plug in and try it again.

if that doesn't do it I'd try a different plug wire. always rule out the easy stuff first.

 

What he said.
Heck you can swap that plug into a different hole and see if it will fire in that hole or not?
Could do the same for the wire with a different hole too but only do 1 swap at a time.

If that dose not find the problem and #7 is still not firing I would look at a bad intake manifold gasket.
That hole is pulling in air only and not a air / fuel mix to fire.
Dave ----

 

#7 plug wire runs close to the brake tubing from the MC, start it up at night, look for sparks. A plug wire can test OK for resistance and still leak near adjacent wires or grounded items, like steel brake tubes. A damp foggy night heavy with humidity is really good.

Imagine that there is a spark traveling at the speed of light along this wire core, one that already jumped that one gap in the distributor cap from rotor to wire terminal .... and it's seeking ground, it will find one. The resistance to it in the cylinder full of tightly packed fuel / air mix is pretty high, so it sees a weak spot in the wire insulation, and leaps out to the steel there beside it.

 

Its got new long tube headers on, I checked for obstruction when I pulled the head yesterday. I tried the old trick of putting water in the valve recesses to check for leaks. Really interesting on the reluctor wheel, I haven't heard that before, I will check that tomorrow. I guess I will check the measurements when I'm in there as well.

 

I switched the #7 and #8 spark plugs with no change. I put a new set of plugs in with a hotter spark, went from a 32 to a 22 motorcraft. I figured there might be an issue with the wire so I just dropped in a whole new set. Its got new Felpro gaskets.

 

Fuzzface mentioning the intake gasket leaking made me think of something. You might check the intake runner for #7 and see if there are any cracks.

If you have new intake gaskets and the surfaces were prepped properly that should be okay, but it's possible there was a small tear in the new gaskets or the mating surfaces weren't completely clean when you installed the new gasket set. Stranger things have happened.

I had an issue with oil pouring out the side of the block on a fresh install of a 2.8li in an S10 I had years ago. It was leaking where the oil filter adapter bolted to the block. I had overlooked a piece of old gasket maybe the size of a BB during my cleaning. It was enough to cause a steady stream of oil to pour out when I fired it up the first time.

 

Do a leakdown test. You might have a valve that is bent or burnt and it will temporarily allow the cylinder to develop compression but then bleed down enough to prevent combustion. The compression tester just measures the maximum amount of compression on each stroke and doesn’t decrease until you push the pressure relief valve on the gauge.

 

What works better as it is not always damp / foggy / humidity so use a spray water bottle to spray down the wires at night.

I cant see it having good compression but not firing?
Even if it was low it would still fire just not give any power like the other holes do.
Do you think this hole with the best compression would leak down to 0 and not help make power?
If the air in that hole did not have fuel it could not make power.
Dave ----

 

Swapped in an HEI distributor today, still nothing on #7. At this point it's been 3 different sets of plug wires. Will have to do the leak down test at some point.

 

I would temporarily plug any large vacuum ports on the carb and manifold to see if it makes any difference...
has the after-market intake/carb been on there for a while, or recently added?

 

It might not be zero but something like 30 or 40 which isn’t going to be sufficient for complete and/or consistent combustion. Leakdown tests are easy. #7 might be a little more difficult because of the location but still shouldn’t take more than 20-30 minutes start to finish. Pop the valve cover, get #7 to TDC or disconnect both rocker arms, remove the plug, install the tool, apply compressed air at 100psi, done. If it’s bad enough to be creating a lack of combustion you’ll hear it immediately. You’ll hear hissing through the exhaust or intake manifold.

The OP isn’t getting one of the three necessities. He says he has spark, he says he has fuel, adequate compression and exhaustion of the charge is the only thing that’s left that needs to be verified. Maybe it’s great, and hopefully it is, but isn’t a quick test worth the peace of mind.

 

Intake is new to this engine, I pulled it off a different running 302. Compression test is next, waiting on parts from Amazon

 

Are you sure that the exhaust valve spring has the same resistance as the intake spring or the others?