512 missing on number 7 cylinder head scratcher

75Royal

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Engine 400 based 512 -11 to 1 compression about 5k miles since built 600hp 700tq at flywheel

Prosystems venom vx 850 carb

phantom 340 fuel pump

comp cams hydraulic roller

trick flow heads

msd 6al

mopar distributor no vacuum advance 8* initial 36* total



Engine has developed a miss on number 7 only by plug inspection. Miss is at idle all the way through RPM range. I've had a few odd misses over the 5k miles but mostly above 4500RPM where it would kind of fall on its face. A few of those times were due to the headers scorching a random wire. I've got the headers wrapped up now so hopefully that is behind me.


What I've done so far:

Electrical:

checked for spark on #7 looks good visually

Swapped plug and wire with number 5 wire and plug- #7 goes dirty again same miss.

Cap and rotor swap

pickup coil and reluctor swapped

Wire resistance check

swapped coils

Gonna see if my machinist has a spare distributor laying around I can try on Monday to rule out all electrical.


Mechanical:

Compression check all cylinders look good within 5psi of each other

Leak down 100psi input- all cylinders between 95-97 number 7 cylinder leaked to 97

valve lash looks good, all rockers tight, springs look good, cant collapse a lifter by hand. No interference or visual geometry issues.


I'm running out of ideas besides a distributor issue or a weak lifter.


I did have a spring clip come off the cap one day and the rotor whacked the cap hard. When I had the distributor out I ran it up with a drill and couldn't see a wobble by eye. Just seems weird one cylinder is just dead missing.


Any thoughts?
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CBODY67

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What intake manifold?

You also realize that #5 and #7 fire consequtively? On a single-plane intake, the runners pull from the same area of the plenum. Guess which cyl might get the lesser charge?

What does #7 plug look like? What plug gap and heat range? Any evidence of coolant loss, even marginally? Hydrocarbons in the coolant?

Using a bore scope (i.e., "Critter Cam or similar), how different does #7 look from #5? Using a non-comtact heat gun, any differences in the head temp between the #7 spark plug and the #5 spark plug?

Just curious,
CBODY67
 

75Royal

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Edlebrock rpm air gap for the intake. Running the motor with the wire off of number 7 the plug doesn’t even get wet with fuel. Header tube on 7 doesn’t even steam water when running. Coolant looks good. No bubbles in the radiator when I did the leak down. Gone over the firing order so many times it’s the only sequence of numbers I have memorized besides my phone number and social security number! Ha. I can pull that plug after running it with wire on and not even come close to burning my grimy fingers too. Don’t have a bore scope so maybe next thing to look for.
 

75Royal

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Also ran it with vacuum booster line disconnected and spraying carb spray directly into that runner to see if it came to life and no go. Was trying to solve it without pulling the intake first but I think I’m getting to that point now.
 

Davea Lux

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Try pulling the wire off of #7 and hooking it to a spare plug with a good ground. If the spare plug fires, you likely do not have an ignition issue. If you spray a small amount of fuel around the intake port for #7 and the miss goes away or the engine speeds up, you have a vacuum leak that is killing that cylinder.

Dave
 

mopar440

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This is a long shot, but maybe that cyl is out of round somewhere in the stroke? Uncle Tony ( Mopar YouTube guy) had a weird miss/power issue because someone had honed the ridge too much, causing a loss of compression at the top of the cylinder. I know it passed leak down, but where was the piston?
 

mopar440

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Also, you have access to a scope? It could tell you alot about what's going on, watching the spark.line
 

CBODY67

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Low compression by itself, even if it's lower than other cyls, should not cause a miss. Loss of potential power, but not a miss. Nor should it color the plugs differently, usually. It might cause something like an "uneven running" situation with one weak cyl and the rest doing what they should, I suspect. This might be interpreted as "a miss" when it is not.

A leak-down test has its place and purpose, but it only checks "sealing", not if the oil scraper rings were installed correctly to do their job as designed (in many cases, there is a "top" and "bottom" to some of those pieces). IF the plugs are being colored by oil getting past the rings.

In any event, we've got to await further information from the OP. It appears a lot has been done, so things ought to be getting closer to discovering the problem.

Enjoy!
CBODY67
 

75Royal

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Also ran it with vacuum booster line disconnected and spraying carb spray directly into that runner to see if it came to life and no go.
This is a long shot, but maybe that cyl is out of round somewhere in the stroke? Uncle Tony ( Mopar YouTube guy) had a weird miss/power issue because someone had honed the ridge too much, causing a loss of compression at the top of the cylinder. I know it passed leak down, but where was the piston?
Leak down was at tdc. Going fishing today. Definitely something to look at. Have a mini bore scope coming to take a peak in there before I crack open the motor. I'll keep the updates coming.
 

Outsider

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If you verify good spark to the cyl, then look for a vacuum leak at that intake port, irregularity in the int manifold port runner, or carb issue. Perhaps try a different manifold and/or carb.
 

halifaxhops

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Start with a scope or cylinder kill to eliminate the ignition either it is or is not then go more into the mechanica/vacuum side.

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Davea Lux

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Another thing that you might want to check. Comp Cams has had an issue with the rivets on the cross links that center their rollers breaking. When this happens the roller turns sideways and wipes the cam. If you pull the valve cover and one of the rockers for #7 is not moving properly, that is likely your problem.

Dave
 

furious70

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MSD boxes don't like getting hot, I've had them miss and break up when hot. Got a spare to try?
 

75Royal

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Well that distributor whack was the culprit when the clip came off. Intermediate shaft is worn out and bottom of the distributor gear. Put the cap 180 out and it moved to number 4 cylinder. Tower bushing and shaft and gear on the way.
 

75Royal

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Funny uncle Tony’s garage just had a video about this. Probably should’ve watched that earlier!
 

75Royal

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Now who has a good trick removing that. Thread it and use a slide hammer?
 
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