I have a 1985 Corvette with a Miniram, 58mm throttle body, AFR 195 Eliminator heads, 1.6 roller rockers, flat tappet cam (224/*224* @ 0.050", 0.528"/0.528" valve lift with 1.6 RR, and 112* LSA), MSD 8.5mm ignition wires, 1 3/4" Hooker long tube headers, Corsa full true dual 2 1/2" exhaust, and some other smaller things.

I'm currently using NGK UR5 spark plugs gapped at 0.045".

My engine was running very smooth last week. Yesterday I noticed it was running a little rough. Today I adjusted the valves because I heard cylinder 1 exhaust valve chattering a little. I used a stethoscope to hear it.

Still ran a little rough and the exhaust tone sounded different despite me adjusting the valves. Gave it a little gas and it sputtered a little and had a few small backfires out the drivers side exhaust. At this point I knew there was something wrong.

I took out the spark plugs. Cylinders 5 and 7 are very dark gray while the others are okay.

I did a leak down test on Bank 1 in which all the cylinders checked out okay.

I did a compression test on cylinders 5 and 7. 5 was 200 PSI, 7 was 190 PSI.

I figured that the only possibilities for fouling 2 plugs at the same time (unless coincidental) were either crossed spark plug wires or carbon tracking in the distributor cap.

The plug wires were in the right spot and the distributor cap looked pretty good, no signs of carbon tracking. Right now I am using my factory distributor because my D.U.I. distributor is being exchanged/repaired under warranty.

I ohmed out the plug wires and they were good. Looked them over for any cracks or tears and could not find anything. I did however, find some brown discoloration on the angle boot that goes near the header. Must have gotten a little hot, but not enough to crack or distort the rubber angle boot.

I inspected the plugs for cracks, they looked okay. I did a fuel pressure leak down test (I guess that's what you could call it?) and I have pressure dropping pretty quickly. I clamped off the return hose and it does that. I think that it's going back down the feed line in to the tank because I did not smell any gas when I sniffed the MAF sensor with the throttle plates opened. Also, I put a napkin in both cylinders 5 and 7 and there was no gas on the napkins. I think it's the check valve in the fuel unit.

So, I'm not really sure where to go from here. Any other ideas on what would cause 2 neighboring cylinders to foul plugs?

Any help is appreciated.

Thanks.

 

I went to change the cap and rotor and noticed that the rotor looked a little slanted when I put it on. I removed the rotor and placed a level on the center of the distributor shaft and also on the manifold in comparison. This is what I got. Has anyone ever seen this before? I don't think it's supposed to be this way. I did have this particular distributor in a box with old parts so maybe it got bent some how.

 

Did some more research on the tilted distributor thing and it appears it is designed like that by GM.

So what else can cause a misfire on 2 cylinders right next to each other?

 

How old are the wires? Looking and doing an Ohm test tells very little. If there is even one strand of wire intact, the reading would say "good". I'd garage the car and close the door on it almost all the way leaving enough for exhaust. Run the engine and see what you get if you spray a fine mist AROUND, not at, AROUND the plug wires.

\So now after adjusting the valves it gets worse? Can you reverse what you did? I would say you should get a scanner to see what the ECM is seeing.

 

No it runs the same as before the valve adjustment.

Someone I spoke with did mention a common problem with SBC misfiring on 5 and 7 because of the wires touching each other. Mine do happen to be touching each other. I am taking everything in to consideration. I will try your water mist test tomorrow.

Also these wires are from like May or June.

I imagine your test has to be done in the dark?

 

I don't know it is such a big deal since I have a few wires that do touch. Yours look relatively new but I would rule it out. And yes, the test must be done in the dark so you can see if there is ANY arcing because of a cracked boot or leaking wire.

 

Replace the burnt wire with a known good one
Bought a cheapie set from Oreillys just for this purpose

 

Neighboring cylinder issues are often an indication of a gasket leak or some other means of the air and/or fuel being exchanged between those 2 cylinders that are next to each other.

I hate to even mention it, but keep in mind that the most common location for a head gasket fail on these things is around #7....

Either the intake gasket where the 2 cylinders both get the same mismanaged air & fuel, or possibly a taste of coolant, OR worst case a head gasket where there is some form of pressure exchange...compression going TO the other cyl, or exhaust being forced into the neighboring cylinder. A crack that's forming could be allowing gasses to transfer between while under the pressure of combustion. Low pressure (intake stroke) in 1 cylinder and high pressure ( combustion stroke or compression) in the neighboring cyl invite transfer.
To do a more definitive test to help pin it down further, would require hooking up to a more sophisticated scanner that was capable of readying all the engine dynamics real time, such as exhaust pulse & composition, spark, o2 from each cylinder cycle, timing etc. Then, it might give you an idea if/where the problem is.

Is it possible to "clean up everything, reassemble and drive for a while to see if the situation/problem reappears?
That would be my next step to see if it was acute or something that's got real physical damage.

With your compression numbers @ 200 psi, which I'm guestimating to be in the area of 10.5:1 or a bit better, that's easy to squeeze a little past a weak gasket.

 

But I just installed new head gaskets about 2 - 3 weeks ago.

 

Also I have 10:1 compression, or if you want to get technical, 9.99:1. And the leak down test results for 5 and 7 were 15%. 106k miles on the factory bottom end.

 

I changed the plugs to AC Delco 41629. I had NGK UR5 plugs in before. They are the stock NGK plugs for a 85 Corvette. My AFR heads require 3/4" length of threads to expose the tip to the chamber, the UR5s were too short for that. Also upon further investigation it looks like the NGK plugs were overheating, not fouling. So I think the AC Delco plugs are a little bit colder. When I put the new plugs in, it sounded great, revved smoothly, felt like everything was better.

Then again, every time I change my plugs it runs smooth, then over time gets a little rough. I'm starting to think I'm seeing a pattern...

 

Why did you get short plugs? Didn't AFR give you plug specifications?

 

I got short plugs because I wanted to try the stock equivalent plugs with the set up. The AC Delco plugs that I just installed are recommended by AFR on the 195 cc page on their website.

 

here goes....


rule of thumb in shop work,

when a new problem appears post maint, or repair, always return to the last work performed to find the reason why.

2-3 weeks?

I hope NOT for your sake, but occams razor comes to mind at this point.

 

I think the plugs overheated from being too hot of a plug