[cody220501]

I'm new to the forum world so let me know if I'm not doing it right. I've recently rebuilt a 350 .040 over, Dart Iron Eagle heads 180cc, air gap intake, new MSD distributor, wires and coil, Holley 600 4 barrel electric chock. All new parts except for the crank and block. It's only been driven maybe 3 miles. Engine might have 3 hours total run time. I initially had some cooling issues that I got resolved, temp never got over 205 F.
It was running fine until one day it just decided to start cutting out consistently and exactly at 3300 rpm, kinda like it has a rev limiter. I've replaced the NEW spark plugs (they were very black), readjusted the NEW spark plug wires after noticing one night they were arcing in a few places, adjusted timing all over the place in small increments, replaced the NEW MSD Blaster coil, replaced the NEW fuel pump and inline fuel filter.
Engine is properly grounded, haven't checked valve springs yet because it runs perfect up to 3300 rpm. I would think a crack in a cylinder would show itself prior to 3300 rpm? I've read where it might me overloading with fuel but I wouldn't think that would be so consistent. Any advise, opinions or suggestions about what problem I might have or what I should look into next would be greatly appreciated.

[cobalt327]

Put a vacuum gauge on it and see what the idle vacuum is, and how the needle behaves at idle and at 3300 RPM.

Look for a broken valve spring or springs, including the inside damper. For some reason, even though the damper itself doesn't contribute squat to the overall spring pressure, when one breaks it sometimes causes that valve to float or bounce excessively.

[cody220501]

Ok a little more background, I've got a comp cam XE 268H cam in it. Initially adjusted rocker arm lash to 1/2 turn past zero lash.

I've readjusted rocker arm 1/4 turn more (3/4 total) past zero lash, inspected valve and damper springs and found nothing unusual.

Vacuum gauge readings at the manifold:
12 @ idle 800 rpm
21 @ 2000 rpm
21 @ 3000 rpm
19 @ 3300 rpm
<19 but >16 @ 3300 rpm and gas pedal to the floor...sounds horrible and vacuum needle very erratic

[406 bug]

plugged cat. convtr.????

[KMatch]

My money is all in on the ignition. Solid 12V to the coil and distributor? MSD itself properly grounded? My Mallory has a separate ground wire. Can you scope the ignition? If not, open up the gap on a spark plug and put it on the end of one of the plug wires as a spark tester in place of 1 cylinder and I'll bet you see the spark get erratic at 3300. All else fails, pick on the distributor/module.

[cody220501]

No cat...it's in a 67 Camaro with true dual exhaust. I'm going to play with the distributor and make sure it's working properly next I think. I'm fearing it may be the cam but I'm not pulling it until I try everything else first.

[oldbogie]

Originally Posted by cody220501

Ok a little more background, I've got a comp cam XE 268H cam in it. Initially adjusted rocker arm lash to 1/2 turn past zero lash.

I've readjusted rocker arm 1/4 turn more (3/4 total) past zero lash, inspected valve and damper springs and found nothing unusual.

Vacuum gauge readings at the manifold:
12 @ idle 800 rpm
21 @ 2000 rpm
21 @ 3000 rpm
19 @ 3300 rpm
<19 but >16 @ 3300 rpm and gas pedal to the floor...sounds horrible and vacuum needle very erratic

I'd say from the vacuum reading at 3000 and WOT that there is one or more "sticking" valves. This can be actual sticking in their guide bore, or could be a spring or several are loosing control or are broken.

Question is whether these springs are from a XE268 kit or someplace else. Checking them would be a pressure against height test.

This situation can also be a binding or bending push rod or a lifter that looses plunger position as the revs come up.

The black plugs are either the carb is too rich, this can include the choke doesn't come off, the ignition fails to fire consistently, the plug heat range is too cold, the engine is running too cold, the intake is unheated and fuel is separating from the mixture to run into the cylinders as a liquid, the compression is too low for the cam timing.

Bogie

[cody220501]

Ok. Can't remember the spring rate but I matched them to the cam with a Dart rep. The heads and springs and stainless valves are all brand new along with the roller tip rockers. I agree that it could be a valve issue but why did it run perfectly one day and have this problem the next? Is it obvious when a spring or damper is broken? Talked to buddy who suggested it might be a wiped cam lobe, is this a reasonable possibility?

[oldbogie]

Originally Posted by cody220501

Ok. Can't remember the spring rate but I matched them to the cam with a Dart rep. The heads and springs and stainless valves are all brand new along with the roller tip rockers. I agree that it could be a valve issue but why did it run perfectly one day and have this problem the next? Is it obvious when a spring or damper is broken? Talked to buddy who suggested it might be a wiped cam lobe, is this a reasonable possibility?

Could be a lobe and tappet. Springs are funny things which is why I always go with the cam manufacturer's recommendations, there's more to it than spring rates which is where most guys stop and assume that for given equal rates the springs therefore react the same, this isn't true.

Bogie

[KMatch]

My thinking: I won't rule out cam/lobes/springs and such, but... If it's consistent at 3300, black plugs, and simply won't rev beyond that, I'd pick on the distributor hard and heavy first. Even with a few sticking valves/broken springs, it should be able to blow up at WOT on as few as 5 or 6 cylinders. The ignition system is the only thing so consistent - even when burning up, they can be consistently "wrong". I just thought I'd toss my thoughts out there before anyone goes pulling a cam or heads.

Matter of fact, it just popped in my head: many years ago I had a Camaro do exactly as you state in the shop. It's the one with the distributor behind the water pump. It ran like a spotted ape, but peaked around 5500 rpms or so - consistently, and you wouldn't think it was even a problem if you didn't know how it was supposed to run. I found it had a bad coil wire then it would bury the tach with that one part. Belonged to a cop... You did mention arcing plug wires.

[cody220501]

I'm going to give it another night run tonight and see if I'm still arcing anywhere. How much arcing is too much? It was initially arcing at about 4 places very small and faint but definitely arcing. One wire would arc wherever I was touching it, I could move my finger along the wire and the arc would follow but it wasn't a full voltage arc obviously or I would have only done it once.

[oldbogie]

Originally Posted by cody220501

I'm going to give it another night run tonight and see if I'm still arcing anywhere. How much arcing is too much? It was initially arcing at about 4 places very small and faint but definitely arcing. One wire would arc wherever I was touching it, I could move my finger along the wire and the arc would follow but it wasn't a full voltage arc obviously or I would have only done it once.

Any arcing is too much, it needs to be tight. This will also cause the vacuum gauge needle to swing around as cylinders miss fire, this will also blacken the plugs making it look like the mixture is rich. Once plugs are blackened you might as well throw them away, the carbon forms a conductive path down the insulator that can bleed the high voltage off faster than it can jump the gap.


bogie

[KMatch]

What Bogie said. Another test is water in a spray bottle with a touch of salt mixed in and take a grounded test light and just slide the metal end along the wires. If it arcs, it's no good. Rerouting them might get you home but you'll be last at the track.

I'm betting the arcing you're seeing is at idle. Watch it at 3300 and I'll bet it lights up like a Christmas tree.

After 30 years in the field I won't do the "touch test" on plug wires. Once you get bit with 50-75,000 volts, I'm betting you'll move on as well. If an old boss of mine had one hand under the hood and wanted to be friendly and shake hands with you, you'd best just move along...

[cody220501]

Ok I just finished running it in total darkness and I still have one or two sporadic arcs but nothing like it was the first time I checked and still have the same issues, can't get over 3300 rpm.
Next chance I get I'm planning to pull all the new MSD components and replace it with a mallory ignition setup and new plugs (this will be my 4th set of plugs). If it's not the ignition or valves I plan to go to the fuel and air delivery components. Thanks for the help so far and I'll update and I work it out.

[cobalt327]

Originally Posted by cody220501

Ok I just finished running it in total darkness and I still have one or two sporadic arcs but nothing like it was the first time I checked and still have the same issues, can't get over 3300 rpm.
Next chance I get I'm planning to pull all the new MSD components and replace it with a mallory ignition setup and new plugs (this will be my 4th set of plugs). If it's not the ignition or valves I plan to go to the fuel and air delivery components. Thanks for the help so far and I'll update and I work it out.

If you have an air cleaner on it, remove it to be sure the choke isn't trying to close. That will keep the engine from revving and will blacken hell outta the plugs, too- all at the same time.