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High End Miss...5/7 interference?

Frustration

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OK...been tracking a high end miss on FRUSTRATION's 426 MW engine. We run a MSD Pro Billet Distributor with an MSD 6AL. I run Autolite R72 plugs gapped to .035 and 38 degrees of timing that's locked in.

We've run this combo for more than 20 years with no real issues. Last year the car developed a miss at the top of each gear. It pops (embarrassingly) through the open headers. I've replaced the cap, rotor, wires, and coil. No changes.

During the off season, I took the heads to have a new valve job done and replaced the old valve springs. The old springs were checked, but were not completely shot. At the same time, we had the locks and retainers replaced with titanium units.

While cleaning bolts and brackets for paint and re-assembly, I had an idea. Not long ago, I made a small clip out of a wire coat hanger. I attached the clip to the lower rear bolt on the drivers-side valve cover to route the spark plug wires away from the headers. The clip holds the wires (5 and 7) together until they're above the header, then they split into separate directions.

I've heard about interference issues on the 5/7 spark. I know folks have run cams designed with a different firing order to eliminate this interference. I'm wondering if this clip may have caused more problems than it helped. As I think back, the miss started right around the time this clip was installed. I won't get to the track until spring.

Do you think this clip may have caused the problems described? I am not changing the combo as it's worked very well for years. There's obviously a gremlin in hiding! Thanks in advance.
wire clip.jpg
 
I'm no engine guru by any stretch, so take this with a grain of salt, but these two things come to question.

I run Autolite R72 plugs gapped to .035

Have you tried a wider gap, or always run at .035?

Do you think this clip may have caused the problems described?

Maybe, that one might be solved by a little experimentation.
 
One other quick thought is you may have developed an errant spark that you can't see with the naked eye.

You might try some water in a spray bottle set to mist, and at night (you want it dark) fire it up and spray the mist around, cap, wires, coil etc.

If there is an errant spark it should show itself.

It only takes a few minutes and costs nothing.
 
Thank you for the quick replies. I'll try the mist once the engine is put back together. That will be later this week. I agree that experimentation with the clip would give me the answer, but no track time until March or so in PA.
 
I would take a spray bottle with water and spray the clip and wires in question. With the engine running, of course, this may help reveal spark issues.
 
A miss at the top end sure does "sound" like a valve floating. For erratic spark, an easy test I've run is to simply run the engine in complete darkness & look for sparks jumping.
 
It may be a little lean also?
You can certainly experiment with greater insulation in the areas of concern.
Wrap the wires in a piece of inner tube and see if the problem persists.
You could modify your hanger with some dip-it to insulate it better.
Even if it's a little ugly for testing, just so you can find / rule things out.
 
Yes. Always ran the plugs at .035. Tried other gaps up to .055. Car liked .035 best. Yes. It happens at around 4500-5000. I tried removing the built in rev limiter chip. That did nothing. Once I put it back together, I'll try some of the suggestions above. I don't want to overthink it. The car ran great for years with this combo. Other than the valve float, interference from the clip, or something electrical going bad (box or distributor) I'm at a loss. I'll let you know what I find, but likely won't have an answer until spring. Thanks again.
 
Water mist while brake torquing in the dark. Or be brave and hold the wires. If you cross the wires (like a braid) they shouldnt pick up interference. That being said same rpm everytime I'd be looking at valve train/springs.
Doug
 
Seems to me if the miss reared it,s head after
you installed the clip that is possible your problem.
I'd reroute those two wires so they don't run
next to each other &see what happens.
Like one other person stated a lean miss fire would be the next thing to address
 
I did run the car without the rev-chip...no change.
 
Too lean will cause a miss at higher rpm. Low fuel pressure will also cause a miss at higher rpm. I've had both issues. Look at the fuel pressure gauge when it acts up...if the needle is bouncing you have an issue.
 
Too lean will cause a miss at higher rpm. Low fuel pressure will also cause a miss at higher rpm. I've had both issues. Look at the fuel pressure gauge when it acts up...if the needle is bouncing you have an issue.
A couple years ago I had a hi-end miss. A buddy of mine crawled under the dash and found a short in the Summit Tach Playback device. I removed it and everything was then fine.
I have since then bought a Tach with the playback feature right in the tach. I like Summit and Jegs but I won't buy anything electrical from them anymore.
 
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