Help me diagnose my miss over 4500rpm.

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I have had the valve covers off to change gaskets. Nothing obvious. As far as I know, it's stock under there, but I don't know for sure. Springs may have been upgraded with the cam. Damn I hope I find it to be an ignition problem.

Cley
 
Hey Cley,
Just reading the comments here and I was wondering what you were doing to get the revs to over 6000rpm with a car running 3.23 rear gears? I'm not being critical, just trying to understand what happened. Seems to me that at that RPM with those gears , you were moving at a pretty high rate of speed and not in a 1/4 mile environment either. If the rpms were sustained for an extended period of time, you may have valve train damage. If not, my bet's still on an electrical problem.
 
I held my 1/2 shift in a burn out at the track a little too long. With wet tires, It flashed up there quicker than I expected. Not sustained by any means. I sure hope electrical too. I did check the push rods for straightness when the valve covers were off and they all checked out good. I appreciate any and all input as I am new to this. I have always beat on my vehicles but never had anything with too much power so nothing really broke.

Cley
 
I held my 1/2 shift in a burn out at the track a little too long. With wet tires, It flashed up there quicker than I expected. Not sustained by any means. I sure hope electrical too. I did check the push rods for straightness when the valve covers were off and they all checked out good. I appreciate any and all input as I am new to this. I have always beat on my vehicles but never had anything with too much power so nothing really broke.

Cley

Cool. For what it costs and for how easy it is to do, try a new ignition box. The upgrade can't hurt anyway and it could eliminate your problem. Sure beats Disassembling the top of the engine which would cost almost as much factoring in gaskets,coolant etc..
 
I'm gonna go through my ignition before I get too crazy worried. But yes, that's one of the next steps if necessary.

Cley
 
I'm gonna go through my ignition before I get too crazy worried. But yes, that's one of the next steps if necessary.

Cley
You already did the ignition; what did I miss? Oh the looking good plugs? Mine are about 100,000 miles old,installed in 1999.They look good too.

You said bit of a cam and 100psi. I checked Martensville elevation@ 2300ft.
For the heck of it, I threw a 268 cam into an 8/1 360, at the Wallace calculator,and it got 116psi.
Either your cam is way bigger than a 268, or it's a slow-azz 268, or the engine ain't an 8/1, or you are at higher than 2300ft.
Or the engine is hemorrhaging pressure .

If I drop the Scr to 7.8, then
Static compression ratio of 7.8:1.
Effective stroke is 2.61 inches.
Your dynamic compression ratio is 5.96:1 ............................... 5.96
Your dynamic cranking pressure is 99.00 PSI.

V/P (Volume to Pressure Index) is 78....................................... 78

The low pressure is not cause of your miss, but whatever is causing it could be... like bent or sticking valves, or more specifically worn out or broken valve springs. Good luck.
 
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time for a leakdown test

Trying to learn something here so I gotta ask..

How will a leakdown test help find a miss that only occurs at high rpm? Cley says the engine runs fine up to 4500 rpm. If there's a bent or burned valve, bad rings etc.. wouldn't the miss occur throughout the rpm range?
 
Trying to learn something here so I gotta ask..

How will a leakdown test help find a miss that only occurs at high rpm? Cley says the engine runs fine up to 4500 rpm. If there's a bent or burned valve, bad rings etc.. wouldn't the miss occur throughout the rpm range?
If the springs are just a lil weak, the engine doesn't care at low rpms. But as the Rs rise they begin to bounce around and the lifters jump into action and take up the slack. Now the valves are not closing any more.And if it's intakes, the heavy valves, that are floating then exhaust can get up into the intake. But worse is the pistons begin to pump the freshly inducted mixture back into the intake where it mixes with the exhaust from other cylinders. And of course when ignition occurs it lights the fire and some of the now burning gasses get up into the intake through the non closing intakes.
The result of all this is a lotta popping in the intake, and the engine just flat quits revving.
But if it's the exhaust valves that are not sealing, then on the intake stroke, which immediately follows the exhaust stroke, now the falling piston sucks back some of the exhaust, as it is pulling in the intake charge. So now the engine suddenly goes lean and possibly the hot exhaust pre-ignites what's in there, and the popping goes out the exhaust, and again the engine just quits revving. This is the most-likely scenario.
Depending on how loose the springs are and how far the lifters have pumped up, and how well the lifters are doing their job; When the revs fall below this floaty business, then recovery time can be anywhere from several/many seconds to a few minutes. If you shut the engine off with pumped up lifters, it can take forever for the lifters to bleed, cuz only 4 of them are under pressure at any one crank position.
And yes with .135 lifter travel, on certain builds, the pumped up lifters can drive valves into the pistons, resulting in bent or broken valves, and trashed engines, or just very low cylinder pressures.
I always do an LD test whenever an engine quits revving, it's just the safest thing to do. If I suspect bent valves, I just back off the rocker gear and put any old amount of air into the chamber. The air pressure will normally push the piston to the bottom, and then start leaking out one of the valves. I don't need to know what percent is leaking cuz it will be obvious. With the piston at the bottom, and the valve gear off, I can tap the valves with a hammer/mallet and see if the valve pops back onto the seat. I know the sound they are supposed to make when the valve hits the seat and seals the chamber. If I don't hear it and the valve is hissing, there is no point in going any further, I just pull the heads.

I'm not saying this is OPs problem but checking is cheap compared to throwing parts at it and hoping for the best.
 
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If the springs are just a lil weak, the engine doesn't care at low rpms. But as the Rs rise they begin to bounce around and the lifters jump into action and take up the slack. Now the valves are not closing any more.And if it's intakes, the heavy valves, that are floating then exhaust can get up into the intake. But worse is the pistons begin to pump the freshly inducted mixture back into the intake where it mixes with the exhaust from other cylinders. And of course when ignition occurs it lights the fire and some of the now burning gasses get up into the intake through the non closing intakes.
The result of all this is a lotta popping in the intake, and the engine just flat quits revving.
But if it's the exhaust valves that are not sealing, then on the intake stroke, which immediately follows the exhaust stroke, now the falling piston sucks back some of the exhaust, as it is pulling in the intake charge. So now the engine suddenly goes lean and possibly the hot exhaust pre-ignites what's in there, and the popping goes out the exhaust, and again the engine just quits revving. This is the most-likely scenario.
Depending on how loose the springs are and how far the lifters have pumped up, and how well the lifters are doing their job; When the revs fall below this floaty business, then recovery time can be anywhere from several/many seconds to a few minutes. If you shut the engine off with pumped up lifters, it can take forever for the lifters to bleed, cuz only 4 of them are under pressure at any one crank position.
And yes with .135 lifter travel, on certain builds, the pumped up lifters can drive valves into the pistons, resulting in bent or broken valves, and trashed engines, or just very low cylinder pressures.
I always do an LD test whenever an engine quits revving, it's just the safest thing to do. If I suspect bent valves, I just back off the rocker gear and put any old amount of air into the chamber. The air pressure will normally push the piston to the bottom, and then start leaking out one of the valves. I don't need to know what percent is leaking cuz it will be obvious. With the piston at the bottom, and the valve gear off, I can tap the valves with a hammer/mallet and see if the valve pops back onto the seat. I know the sound they are supposed to make when the valve hits the seat and seals the chamber. If I don't hear it and the valve is hissing, there is no point in going any further, I just pull the heads.

I'm not saying this is OPs problem but checking is cheap compared to throwing parts at it and hoping for the best.


Wow, great explanation and makes a lot of sense. Also seems to be a straight forward procedure that costs relatively nothing to do. Cley did your miss start occurring before or after the high rpm buzz? As well AJ/FormS , should you also see discoloration of the sparkplugs in the affected cylinders ?
 
Have you changed the cap and rotor and have you checked the bushings in your distributor. Just a thought and the cap and rotor are cheap. I had a cap with a hair line crack in it and it would do all sorts of funny things and it was a brand new cap
 
Wow, great explanation and makes a lot of sense. Also seems to be a straight forward procedure that costs relatively nothing to do. Cley did your miss start occurring before or after the high rpm buzz? As well AJ/FormS , should you also see discoloration of the sparkplugs in the affected cylinders ?
That's an interesting question. IDK the answer. I became a motorcycle mechanic in 94, after I made my decision for Christ. God gave me a new vocation. So I haven't worked on a whole lotta autos after that. Motorcycles are mostly DOHCs since IDK seems like forever now. And they rev 10,000 to 14,000 and don't have hydraulic lifters, for the most part.
But I have experienced lifter pump up with teeners, and SBCs, and other pushrod hydro-engines. Just never thought to check the plugs,lol. In fact I had a 340 once that would float the valves at 5500; shocked the chit outta me the first time it happened. Springs solved that. My teeners used to do it all the time, so I just starting shimming the springs or swapping to stronger ones. I guess I couldda shifted sooner, but that ain't my style. I threw a few bent valves away before I started shimming the springs.
But never looked at the plugs.
 
Sounds like classic fuel starvation to me. No way in hell are you ever going to have spark knock problems with 100 PSI cranking pressure. You could probably run off 79 octane. Tape a fuel gauge to the windshield and tee it into the fuel line at the carburetor and drive it down the road. I bet the bottom drops out of the fuel pressure. Unless you actually diagnose it, you may never find out what's wrong. The fuel gauge taped to the windshield will at least confirm or or eliminate a fuel problem and is very simple to do.
 
Not sure from your description exactly what it's doing, but hopefully this might help.
I chased this through the whole ignition and fuel system for a LONG time.
It turned out to be simple, the fuel was getting overheated.

When I finally ran a vacuum/fuel pressure gauge out through my scoop into the cabin, I watched my fuel pressure drop to zero.
I heat sheathed the 3/8" supply line by the headers and ran a standard 5/16" fuel line for a return.
One or both fixed it.

I have been hearing a lot lately about people having problems with fuel boiling/evaporating/vapor locking, getting too hot in the lines from proximity to the headers/exhaust, and possibly in the pump as well. New fuel SUCKS! It cannot take any heat
Good luck!

You can watch the tach hit 4k and then drop. After I lifted it came right back and cruised at 3k (70mph) no problem.





This is on the same stretch of road, after the fix.

 
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So, is it bucking, or falling flat on it's face? Is it breaking up staccato, or cycling in and out of pulling?
 
If I had to describe it, it is breaking up, then when I shift and the rpm drops, it's perfect again till it hits that 4500 rpm again.

Cley
 
I can tell you this-
I was just mapping my timing, and my headers go from hot to REAL HOT by 3000rpm.

Run it with a fuel pressure gauge on it.
 
If I had to describe it, it is breaking up, then when I shift and the rpm drops, it's perfect again till it hits that 4500 rpm again.

Cley
I had a 340 once that did that when the plugs "overheated" A new set always fixed her right up.The root cause of that was too-lean running.
My 367 does that all the time......... if I forget to reset the rev-limiter,lol.
 
I had a 340 once that did that when the plugs "overheated" A new set always fixed her right up.The root cause of that was too-lean running.
My 367 does that all the time......... if I forget to reset the rev-limiter,lol.

Back when I was racing, if I left plugs in over the winter, no matter how old they were, it would have a high rpm miss in the spring. New plugs cred it.
 
Well it took me.long enough to get this done but a new ignition control module fixed it. I went with the Hi rev 7500 from Rick Ehrenberg on Ebay. I sure hope it lasts.

Cley
 
74 Plymouth Duster, 360, A-500, 8 ¼ w 3.23 sg. Running at 185 degrees (confirmed with good IR heat gun). 3/8” pick up in tank with -6 AN braided line between tank and fuel pump. New Carter HV fuel pump with Performance World regulator set to 6 psi. Fuel filter is perfectly clean. Holley 670 Street Avenger on an Edelbrock Performer 318/360 intake. Stock 1.88/160 J heads and unknown pistons (100PSI compression even on all cylinders when cold). Unknown cam, but definitely more duration than stock. Stock electronic ignition with cheap parts store ign. box with external ground jumper to case. I adjusted the reluctor gap to factory spec. Timing at 22 deg. btdc @ idle with total 34 deg. in at 2500 rpm. I messed with it for a quite a while and it runs best there. I just redid my intake gasket due to a bad vacuum leak and it seems perfect, no more vacuum leaks. ? I put new MSD wires on it, a new parts store cap and rotor, I will put new plugs in it as soon as I have time, but I did pull them when doing the compression test and they looked fine.

No hint of ping and runs smooth and sweet up to 4500 rpm then sputters badly (even free revving it in park). It recovers immediately when I shift. I used to shift at 5300 no problem, and accidentally let her spin up over 6000 once unintentionally.

The season is almost done here and I’ll be putting it in the garage for the winter with the intent of doing some work on it to be ready for spring. My question is this, where should I spend my time looking for this miss?

What spec does everyone use to set the reluctor gap to? Did my cheap shitty ignition box partially fail? I am leaning toward it being an ignition problem but I really don’t know.

Any thoughts would be greatly appreciated.


Cley

First thing I thot of was the coil, then valve springs --------
 
If I had to describe it, it is breaking up, then when I shift and the rpm drops, it's perfect again till it hits that 4500 rpm again.

Cley
that 'high rev' box from ehrenberg, looks EXACTLY like the chinese box they sell at oreilly's.
get a MOPAR not new box, or you will be right back where you were.
He sells 'fix-it' parts that are part, of a small parts kit for more than the kit. He's in it for the money unless you talk to him, and has vouched for an engine build that left one member robbed in value.

These are the exact problems i had building a previous 74 road runner where the new chinese parts failed after a few drives -only a few drives over 100 mph.

The ignition box failed multiple times and the voltage regulator once allowed over 16 volts which left me stranded with blown coils several times. The car drove for a while with the bad voltage regulator.

At any rate keep a spare box, coil, and starter in the trunk of your ride
 
74 Plymouth Duster, 360, A-500, 8 ¼ w 3.23 sg. Running at 185 degrees (confirmed with good IR heat gun). 3/8” pick up in tank with -6 AN braided line between tank and fuel pump. New Carter HV fuel pump with Performance World regulator set to 6 psi. Fuel filter is perfectly clean. Holley 670 Street Avenger on an Edelbrock Performer 318/360 intake. Stock 1.88/160 J heads and unknown pistons (100PSI compression even on all cylinders when cold). Unknown cam, but definitely more duration than stock. Stock electronic ignition with cheap parts store ign. box with external ground jumper to case. I adjusted the reluctor gap to factory spec. Timing at 22 deg. btdc @ idle with total 34 deg. in at 2500 rpm. I messed with it for a quite a while and it runs best there. I just redid my intake gasket due to a bad vacuum leak and it seems perfect, no more vacuum leaks. ? I put new MSD wires on it, a new parts store cap and rotor, I will put new plugs in it as soon as I have time, but I did pull them when doing the compression test and they looked fine.

No hint of ping and runs smooth and sweet up to 4500 rpm then sputters badly (even free revving it in park). It recovers immediately when I shift. I used to shift at 5300 no problem, and accidentally let her spin up over 6000 once unintentionally.

The season is almost done here and I’ll be putting it in the garage for the winter with the intent of doing some work on it to be ready for spring. My question is this, where should I spend my time looking for this miss?

What spec does everyone use to set the reluctor gap to? Did my cheap shitty ignition box partially fail? I am leaning toward it being an ignition problem but I really don’t know.

Any thoughts would be greatly appreciated.


Cley
 
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