“Misfire” at 2000+ rpm

[JackBEEQuick]

Hi all, I have been chasing a “misfire” for a month or so now, here’s the scenario:
I have a 69 coronet 440 engine for the street/strip
It Developed a misfire after a weekend of racing, made 14 passes total and started to get the miss on my 12th pass IIRC. That’s progressively gotten worse. Wasn’t do it until around 4800-5000rpm at first now it’s showing itself around 2000rpm.
Once got back home I realized I discretely broke a motor mount, thinking that issue was maybe tugging on a wire or messing with something in that regard I thought that could be the issue, replaced that, still there. I have new plugs, replaced cap, rotor, wires , it’s still there.
I then put in an all new MSD 6AL Box, coil, wires, the whole setup. The “miss is still there.
Thinking it’s fuel related now I then pulled the 6pack carb and intake and put on 750 Holley, that did not fix it either. I’m running out of thoughts on this… could it be an internal engine, torque converter (brand new Coan), or transmission issue? To me I feel like the “miss” or whatever you want to call it at this point , would be there at startup/idle if it were any of those things. Anybody ever encountered a similar gremlin?
To add I have the motor grounded from the head to the firewall and the front intake bolt to the negative battery post.

 

[steve340]

It could have a broken valve spring. Remove the valve covers and check.

 

[JackBEEQuick]

Thanks that’s a good idea,
I checked that out when I readjusted the lash. Running solid mechanical lifters. Did not notice any broken springs, is difficult to see the inner ones though, I’m going todo a compression test next and rule that out

 

[steve340]

Typically it is the larger outer one that breaks.
I did see one once that was fractured and quite hard to spot.
Good you checked in any case.

 

[1967coronet]

If it is now having a miss fire at 2000 rpm.
Start pulling plug wires one at a time and replacing to see what cyl is going dead on you.

 

[JackBEEQuick]

After the new ignition install, would there be anyother electrical components that could cause a cough like that ? I tried 2 different voltage regulators (used) but those did not change anything either

 

[1967coronet]

Just a thought , but check your fuel pressure at idle and were its starting to miss fire. You should have at least 8lbs or a hair more under load. Idle at least 5

 

[JackBEEQuick]

If it is now having a miss fire at 2000 rpm.
Start pulling plug wires one at a time and replacing to see what cyl is going dead on you.

Thanks 67, will try that. Even though it starts and idles well, do you think that will reveal it? Or are you saying pull a wire and rev to 2000 one cylinder out at a time.
I should add it only has the miss when in gear and under load at 2000ish.. In park I don’t have the miss

 

[1967coronet]

Thanks 67, will try that. Even though it starts and idles well, do you think that will reveal it? Or are you saying pull a wire and rev to 2000 one cylinder out at a time.
I should add it only has the miss when in gear and under load at 2000ish.. In park I don’t have the miss

Well if it won't miss fire sitting check your fuel pressure.
But yes if you do check the cylinders sitting just pull one and replace at a time and listen for the no change cyl.
Also not trying to sound stupid but you may be sucking crap into your filter or just have some bad fuel , water ect.

 

[steve340]

Based on your current description it could be a spark plug or ignition lead problem.

 

[1967coronet]

If nothing else slap another 12 volt coil on it.

 

[RJRENTON]

It could have a broken valve spring. Remove the valve covers and check.

I concur.....either a broken spring or a burnt valve or a bent valve stem or bent push rod or your starting to wipe a cam lobe....as suggested pull the valve covers and look with a high intensity light....do you have a rev limiter? You could do a leak down test to determine if there is any internal damage or pull the spark plugs and use a bore scope to see if there is any piston damage.....just my opinion and thoughts.....
BOB RENTON

 

[Kern Dog]

Just a thought , but check your fuel pressure at idle and were its starting to miss fire. You should have at least 8lbs or a hair more under load. Idle at least 5

No offense meant but mine idles around 3 1/2 to 4 psi but it jumps right as soon as I crack the throttle.

 

[1967coronet]

No offense meant but mine idles around 3 1/2 to 4 psi but it jumps right as soon as I crack the throttle.

View attachment 1869813

I hear you , 4 to 5 , I think is fine prob my cheap gauge lol .
Then the jump at throttle hit under load should for sure show a problem or not with fuel , just what the Op was describing made me think of a dying pump.
Possible to plumb in a gauge he can see and take it for a short spin.

 

[MWbirdLOVER70]

I hear you , 4 to 5 , I think is fine prob my cheap gauge lol .
Then the jump at throttle hit under load should for sure show a problem or not with fuel , just what the Op was describing made me think of a dying pump.
Possible to plumb in a gauge he can see and take it for a short spin.

The fabled fuel pump push rod maybe? To short and worn?

 

[1967coronet]

The fabled fuel pump push rod maybe? To short and worn?

Mabey ? Was not sure yet if he has a mechanical or electric pump.
Seems like each time I chase a fuel problem it ends up electrical/ ignition or if I chase a ignition problem it ends up fuel .
Lol.
I sure hope for his sake it's not internal,
Just the day at the track slow progression down the rpm range made me think fuel supply.

 

[gtxrt]

could be as simple as a bad spark plug.

 

[RJRENTON]

The fabled fuel pump push rod maybe? To short and worn?

IMO....as noted previously, this not going to be a quick and dirty or easy fix.....such as loose wires, bad fuel pump, low fuel pressure, vacuum leak, bad spark plug. Pull the valve covers and look.....b4 something catastrophic occurs.....like drop a valve while running....and grenade the engine........
BOB RENTON

 

[PRND21]

How much slop is in your distributor shaft? Does the advance plate move nice and smooth? Just a couple more items to check.

 

[1967coronet]

IMO....as noted previously, this not going to be a quick and dirty or easy fix.....such as loose wires, bad fuel pump, low fuel pressure, vacuum leak, bad spark plug. Pull the valve covers and look.....b4 something catastrophic occurs.....like drop a valve while running....and grenade the engine........
BOB RENTON

Bob, post 3 he did pull the covers and check springs and adjustment of the solids.