WOT High RPM Miss / Studder

I was doing good up until a few days ago, now this is what I am troubleshooting...

440, 800cfm Eddy Thunder, Holley SD intake, 1 7/8 headers
Hughes Solid Lifter cam 3842 (238 / 242) degreed in.
Pertronix III ignition, Flamethrower III coil, ballast bypassed
New Firecore wires, cap, rotor, newer NGK plugs
10.4:1 cr, aluminum heads, Holley Black electric pump.
Timing: 21 initial, 35 all in at about 2700rpm. I am using vacuum advance (manifold) so the timing is set with it unhooked. Idle vacuum is around 14 hg

Going WOT the car pulls great up until 5200 to 5500 rpms then I can see the the tach needle flutter and feel the engine lose power. It does this until the car shifts at around 5700 rpms. Part throttle, off idle response is great. No hesitation, no miss and it revs fast and hard, will scorch the tires off the rims.

So my first thought was valve float. The cam specs a heavy double spring and that is what I am using (155# on the seat and 415# at lift) Pulled valve covers, checked lash and springs, keepers, etc. Not seeing an issue although there were a few that needed an adjustment.

Pulled some random plugs and the gap was .035" and the plugs were not fouled but looked tan/white = normal to me.

Next, I swapped out the Pertronix III for a stock points setup. Installed a fuel psi gauge.

Took it out for a spin and the stumble is still there and this time I got some popping when I matted the gas pedal. And while driving around in town I heard some pinging. So I added 5 gallons of 110 octane race gas (mix with 5 gal 91 octane) and backed the timing down to 18 / 32. Went WOT on the on ramp and it pulled to 5000 rpms and popped and stumbled. Fuel stayed at a rock steady 6 psi. All of these electrical / engine parts were bought new.

Soooo, besides trying out some fresh plugs what do you all think? It idles fine and runs great below 5k rps so I doubt I have a misfire with them or the wires. Distributor has minimal side to side play and mechanical advance works fine. Vacuum advance holds vacuum.

My train of thought has me theorizing that the cam may be on the way out. I was skeptical of the fast ramp / high spring pressure. Would some rounded off lobes cause a high rpm stumble but have no issues below 5k rpms? My next step is to pull the covers again and look at the valvetrain while turning the motor over.

Mark ... Any excessive glitter in the oil?

Had a very similar problem with my 1st 6 Pak years ago. After chasing ignition for a long time switched the end carb jet plates to a stock set, replacing the ones I had drilled per Direct Connection recommendations. Turned out the drilled jet plates had TINY swirl marks from the drill bit. OEM end carb plates and problem solved, quickest ET's. Maybe your issue is something in the carb. Just say'in.

Take your timing up a little and see if its still there.

dryheat said:

Mark ... Any excessive glitter in the oil?

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I'll check in a few minutes.

66Satellite47 said:

Had a very similar problem with my 1st 6 Pak years ago. After chasing ignition for a long time switched the end carb jet plates to a stock set, replacing the ones I had drilled per Direct Connection recommendations. Turned out the drilled jet plates had TINY swirl marks from the drill bit. OEM end carb plates and problem solved, quickest ET's. Maybe your issue is something in the carb. Just say'in.

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I've had the top off the carb, checked the floats, messed with the jets and rods and didn't see any difference except more fuel smell at idle. So I switched back to stock primary jets / rods and left the secondary jets at +12%

khryslerkid said:

Take your timing up a little and see if its still there.

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Hmmmmm. I'll give it a shot but I will have to modify the mechanical to limit my all in. There seems to be a bit more vacuum to be had around 23 - 24 degrees.

funny when you mentioned valve float. that was my first thought. springs fatigue, I had the same thing in a car I used for track days, it would fall apart at the same rpm , I could make it happen at will. new springs no problem

dan juhasz said:

funny when you mentioned valve float. that was my first thought. springs fatigue, I had the same thing in a car I used for track days, it would fall apart at the same rpm , I could make it happen at will. new springs no problem

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That is what it feels like because it seems to take forever to reach 5800 rpms after I hit 5000 rpms. I cannot imagine a block in the airflow (carb, intake, heads, headers) enough to shut it down before 6000 rpms. The springs have less than 900 miles on them. 155# on the seat / 415# at .60" lift. Double springs as well, none broken, all look fine. They were all set at 1.88" instaled height.

All in at 32 seems low. Take the timing up till she pings and then back it off. All in at 34-36. Also an o2 sensor would make things easier for ya.

You need more timing....36-38....CAMS can be tricky and you cannot rely just on standard numbers that everyone swears by....do trial and error and see what the engine wants....

Is it too lean?

That carburetor comes from the factory jetted for sea level. What is Yuma, 165' elevation? You've got the secondaries 12% rich? Maybe you should rethink that.

texas69bee said:

All in at 32 seems low. Take the timing up till she pings and then back it off. All in at 34-36. Also an o2 sensor would make things easier for ya.

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I just checked, all in is 34. 20 initial, and 14 mechanical. So I "should" be able to go as high as 24 initial with 38 being the all in total? And I have an o2 sensor sitting on my workbench, haven't installed it yet.

Bigal75 said:

Is it too lean?

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I don't think so. These are a few plugs I pulled to inspect. They are all about the same. I thought the same thing which is why I was messing with jetting but the result was burning eyes lol.

IQ52 said:

That carburetor comes from the factory jetted for sea level. What is Yuma, 165' elevation? You've got the secondaries 12% rich? Maybe you should rethink that.

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I will take your recommendation and swap them down for the +4% or factory jets?


And here are a few pics of my cam lobes. You can see the lifter swipe over the entire top lobe of the cam. The cam is beveled and should only show wear on one side, correct? And yes, I do detect a sparkly, copper look to the oil on the cam.

I agree on the timing, try 36-38, 32 is too low for an RB. Second, check your plug wires for a pin hole. It won't show at low RPM's but will arc at higher RPM's.

I had that issue, burned through 4 plug boots. Have ya checked those?

5.7 hemi said:

I had that issue, burned through 4 plug boots. Have ya checked those?

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I have and all of the plugs wires still look new and not close to the headers. Firecore wires and they are tight on the plugs and dist cap.

threewood said:

I have and all of the plugs wires still look new and not close to the headers. Firecore wires and they are tight on the plugs and dist cap.

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Get out a spray bottle of water. Throttle up to around 2500-3000 and spray the wires. Doing it in a garage will help see the arc.

Re-installed the Pertronix and it fired right up (so much better running than points!) and adjusted timing out to 25 initial / 39 total. Unhooked and plugged the vacuum advance. Took it out and ran it up a few times. It seems like the hesitation is pretty much gone although I did see the tach flutter ever so slightly at around 5600 rpms the first time. But I got on it several times and it pulled to shift point.

Now this is with a blend of race gas so I don't know if it has a bearing on the outcome. It is quite possible the winter blend gas is just shitty because I rarely keep the gas matted through shift points. Worse case is I manually shift it at 5200 rpms.

I still want to swap secondaries tomorrow but I have another question. Best to leave the vacuum advance hooked or unhooked? Pros and cons? It adds about 8 degrees at idle which would be 33.

Thanks for the help so far.

Ironbuilt said:

Get out a spray bottle of water. Throttle up to around 2500-3000 and spray the wires. Doing it in a garage will help see the arc.

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I did this and couldn't find an arc off the wires. A few are hard to see though.

Have you adjusted the rev limiter? Pertronix III's come factory set at 5500

Bad gas sure is a possibility. I wouldn't expect timing to cause this if you're in the 35* range w/ alum heads.