Miss above 4000 rpm

[Texasdoc]

I have a 351w newly rebuilt (400 miles) that has a miss above 4k rpm under load (medium to heavy)

Details:
Holley 670 vacuum secondaries
Mallory Hyfire 6AL ignition
MSD 8478 distributor. Vacuum attached to manifold. Advance all in by 2500 (one silver, one blue spring)
MSD ignition wires
New E3 spark plugs
Typical 1 1/2 inch oval Cobra air cleaner
Initial timing set at 16
Manifold vacuum at 14 in Hg at idle
Base plus vacuum timing is around 20.
Mechanical advance 21 degrees (total 37)
Vacuum port adds 8 degrees starting at 11 in Hg

Idles at 1k rpm fine. It pulls strong up to 4k rpm then starts to miss and loses power. Sitting in the drive way, hitting the throttle causes the timing to go above 40 total advance. I noticed that even hitting the throttle i am gaining vacuum, not losing -vacuum goes up to 22 while doing this. I'm not sure of the total because my timing light shuts off at around 4k (???). I tried retarding the timing so the idle advance (base plus vacuum was around 18 making the total advance in the drive didn't go above 38, but it did the same thing.

I also noticed that the secondaries don't open. I'm afraid to put my foot to the floor but do "get on it" sometimes.

Any ideas?

[jhv48]

First of all, vacuum secondaries will not open when revving the engine under no load, so don't worry about that.

Usually a miss at that speed under load is a spark plug failure. That's the easiest place to start. Pull all 8 and read them. Report back with pics or descriptions of what you find. May be a lean condition or a spark delivery problem, but the plugs are the first culprit I would suspect before going any deeper.

[Texasdoc]

Thanks. I knew the secondaries won't open without a load. I put a clip on the secondary diaphragm shaft then drove for a while. The clip stayed at the top of the shaft.

What is the best way to test an individual plug and ensure that the wire is connected and the plugs is firing? I am assuming I cannot test continuity or resistance with a volt meter since the spark electrode is not electrically connected to ground.

These new E3 plugs don't have a gap to set. They all have probably less than 40 miles on them.

[Grubby]

Vac secondaries are not expected to open without load. I put a zip tie on the shaft of mine and could see that rotated on the shaft to indicate they opened.
See picture.

I would be thinking ignition problems for a high speed miss. Use a digital volt meter to check your plug wires for opens. They should show continuity.

John

[Texasdoc]

Thanks for the ideas. I will read the plugs, check the wire continuity, and try running it in the dark to make sure the wires aren't arcing.

[Rick Parker]

You may be pulling in too much advance and cauing some cross fire within the distributor cap (if it is small diameter).

[Texasdoc]

Here is one of my plugs. They all looked the same. I've checked the continuity of the wires. I made sure the wires were securely plugged in on both the cap and plug side. Firing order is correct. Wires for adjacent firing plugs are not run side by side in the looms to prevent crosstalk. No sparking when started in the dark. I wasn't able to check it tonight as I would like to keep my neighbors.



The plugs are so new, there isn't much color to them. Still have the gold color. The bases of the cathode tips/cage are slightly black. No tan color, no black soot, no wet oily appearance.

Originally Posted by Rick Parker

You may be pulling in too much advance and cauing some cross fire within the distributor cap (if it is small diameter).

I thought about this. This is why I retarded the ignition so I would only get 38 max with base, mechanical, and vacuum at 4000rpm. Maybe I should get one of the adjustable MSD rotors to make sure the cap is in phase.

I also found this thread clicky that discusses a similar issue. One fix was a new carb vs. carb tuning. Another thought was sparking wires (checked that by running the engine in the dark). The other thought was plugged fuel filter. I have two of those see-thru glass fuel filters: one between the tank and the electric pump, and the other between the pressure regulator and the carb. Both look clean.

I will try to drive it again tomorrow to see if playing with the plugs and wires "fixed" something.

[vatdevil]

If you have a mechanical fuel pump,check your pressure at high RPM.

[CowtownCobra]

Originally Posted by vatdevil

If you have a mechanical fuel pump,check your pressure at high RPM.

And check the fuel filter

[nealgs]

I had a similar problem on our 383 chevy (yeh, ok Ford fan boys - lol) but also running an MSD 6AL.

Check out the serial number on your 6AL (682013 is the one to look out for - fake) - my issue turned out to be a fake 6AL - would get a misfire/rev limit type condition at 4k rpm+

bought a new digital 6AL - no issues. Worth a try to see if you can borrow a 6AL or other MSD CD unit to at least narrow the issue area down a bit

Google search for Fake MSD 6AL

good luck

rgds
Gary

[STEVE-O]

Whats your rev limiter set for in the 6AL? I would hope it would be higher that 4K but worth checking if you havent already.

[601HP]

Originally Posted by STEVE-O

Whats your rev limiter set for in the 6AL? I would hope it would be higher that 4K but worth checking if you havent already.

That's a very good question and starting point.

David

[Texasdoc]

Originally Posted by STEVE-O

Whats your rev limiter set for in the 6AL? I would hope it would be higher that 4K but worth checking if you havent already.

Excellent point! I thought I was revving higher last month but at least it is something to check.

It is actually a Mallory Hyfire 6AL, so I doubt it is a fake. Thanks.

Fuel filters clean.

[CowtownCobra]

The carb secondaries may not have any fuel, isn't there a sight window to check that fuel bowl?

[Texasdoc]

No sight windows. Sight plugs. Fuel level is at the bottom of the holes when the plugs are removed.

[Dirty Harry]

I had a miss that was driving me nuts, trying to find. I, too, had the long oval air cleaner. The front of the base plate actually sat nearly on the plug wire where it pushes into the cap. The ah-HAH moment came when I noticed a burn mark on the underside of the air cleaner base. Spark was arcing to the air cleaner base rather than firing the plug. Maybe you've got spark going directly to ground somewhere. Also, you might try using your infrared temp gun to measure temps at each primary header tube. If one is markedly cooler, perhaps that cylinder is failing to fire consistently.

[Texasdoc]

Thanks for all the ideas. I haven't been able to test it and probably won't for the next few days. I will report back.

Tom

[Young1]

Wow this forum is so great. I have a 4000 rpm miss as well with my Effie. Vacuum secondaries also. Just to find the elusive time.

[STEVE-O]

The other thing to keep in mind. I have the 6AL and Smiths tach. For whatever reason which I havent determined yet, at 4K, the tach reading starts to drop. Almost like a missing tach signal or something.
The engine is still running fine but to look at the tach it appears like the engine is cutting out.
Granted, Im sorry to say, I dont have the engine at 4K or over very often or for very long so it isnt really an inconvenience.
May be the issue is the tach and not the engine.

[CHANMADD]

Look in the dizzy for the centre electrode. You may have to bend the tang on the rotor up some to engage the center electrode.