“Misfire” at 2000+ rpm

[Kern Dog]

Sometimes, a small issue can have consequential effects.
With my first 440 swap in 2001, I had Cherry bomb mufflers bolted directly to headers for the break in. I had to hammer the mufflers a LOT to keep from rattling them against the torsion bar crossmember.
The car idled fine but above 2000 rpms, mine wanted to flop around and flutter like a Halibut on a hot plate. Once I put full exhaust on it, it smoothed out and ran strong. I figured that the restricted exhaust surely screwed with the carburetor transition between idle and primary circuit.
Another car, I had cheap mufflers and blamed them for weak performance of the car so I swapped in a pair of Flowmasters. Little change. I then replaced the pathetic SP2P intake with a mild Weiand and the car flat out scrammed once the choked off intake was tossed aside.
Seriously....that crappy SP2P intake was crap....the ports were smaller than a 318 2 barrel intake. I should have known though...I had one of those crappy intakes on a Chevy 350 years ago and that car was a slug too.

 

[Geoff 2]

Since this happened while racing, & got progressively worse, I suspect:
- bent valve
- broken spring
- blown head gasket.

Compression test should be the next step.

 

[JackBEEQuick]

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

I Was running an Electric pump in series with the mechanical to feed that 6pack. For fear of it being the mechanical pump, the rod for it, or even the cam lobe for it, I bypassed it, running only the electric at 6 psi to the now 750. It didnt cure it either. Im sure its something stupid as the miss is consistently there with everything I replace. Im going to check compression on all the cylinders and check all the spark plugs again maybe the porcelain cracked on one or something

 

[1967coronet]

I Was running an Electric pump in series with the mechanical to feed that 6pack. For fear of it being the mechanical pump, the rod for it, or even the cam lobe for it, I bypassed it, running only the electric at 6 psi to the now 750. It didnt cure it either. Im sure its something stupid as the miss is consistently there with everything I replace. Im going to check compression on all the cylinders and check all the spark plugs again maybe the porcelain cracked on one or something

If the plug check and compression test goes good just for the heck of it try another coil.
Got my fingers crossed it's not anything bad.

 

[padam]

How much did your lash change?
Possible cam lobe going away.

How is the ignition timing? Has it changed?

 

[JackBEEQuick]

despite it being my first time checking solids, I never made any adjustments they all seemed fine. no timing was changed but just the fact that it has the exact same miss with my new MSD as it did with old stock ignition, pretty much rules out the ignition. same being said for the fuel and carburetor. by swapping an entirely different intake and carb and it still has the exact same miss... only thing that remained were plugs, so ill have to recheck those when I do compression test.

Has anyone ever had a torque convertor slip or break and give a misfire type of symptom like this?

 

[padam]

The reason I asked about timing, is because earlier this year I had a situation where my vacuum advance was adding so much timing it was causing the engine to miss, but it ran good at full throttle.

 

[JackBEEQuick]

upon doing the compression test when I pulled the no. 5 spark plug (worst one go figure) it was pretty squished. so now Ill have to figure out what squished it. Cylinder has good compression , I re-gapped the plug and ran it, all is good for now. A fellow racing buddy said its not unheard of for some carbon buildup to breakup and compact the plug. wont know until I get ahold of a scope. But I guess the silver lining is I learned how to install an MSD ignition, a fuel system , and an intake and 4 barrel carb lol the reason I didnt pull the plugs right away is because they were installed brand new when I put in the MSD, live and learn.

side note: my car really liked the 750 Holley on it, it has much better manners on the street, it laid some nice rubber, but It doesnt pull as hard down the stretch as the 6 pack, with the 6 pack you could feel the outboards open up and pull hard. I could be leaving some on the table yet with the new MSD ignition, I have 3 other bushing sizes to try to up my advance. For now I need to put some miles on and keep an eye on that cylinder.

 

[mopar 3 B]

Loss of fuel pressure in its self will not cause a miss. The engine will simply go flat. Check the spark plug wires. Straight runs are pretty but can cause arking between adjacent firing plugs. Meaning the original problem may have been fixed and a new one created.

 

[RJRENTON]

Loss of fuel pressure in its self will not cause a miss. The engine will simply go flat. Check the spark plug wires. Straight runs are pretty but can cause arking between adjacent firing plugs. Meaning the original problem may have been fixed and a new one created.

With today's current spark plug wires, complete with their inherent suppression construction and dielectric materials of construction INDUCED energy in adjacent parallel wires is non-existent......maybe in the old days, where the conductor could be influenced by adjacent induced wires....but not now.......
BOB RENTON

 

[1967coronet]

Loss of fuel pressure in its self will not cause a miss. The engine will simply go flat. Check the spark plug wires. Straight runs are pretty but can cause arking between adjacent firing plugs. Meaning the original problem may have been fixed and a new one created.

Just a question because there is much I do not know.
Would not fuel distribution in the intake after low bowl levels be affected by a low fuel pressure ?
Then resulting in a miss fire ?
Not saying that is the culprit this time but in my past I have ran into that.

 

[kiwigtx]

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.

Is it definitely a 'miss-fire' ....or is it possibly a 'back-fire' that you're hearing?

 

[RemCharger]

What were the actual results of the long awaited compression test.

 

[RJRENTON]

Just a question because there is much I do not know.
Would not fuel distribution in the intake after low bowl levels be affected by a low fuel pressure ?
Then resulting in a miss fire ?
Not saying that is the culprit this time but in my past I have ran into that.

The original problem of a "misfire" is a complex issue. Simple solutions or comments about low fuel level/fuel pressure affecting mixture distribution is not a solution. Low fuel level affects WHEN the booster venturi start delivering fuel to the air stream. Low pressure affects the carbs ability to provide the correct mixture as the delivery system cannot provide the volume of fuel required to insure the liquid fuel to fuel vapor transition is adequate to maintain correct mixture ratio to all cylinders......or leaning out at high demand conditions and power fall off.......just my opinion......
BOB RENTON

 

[mopar 3 B]

Just a question because there is much I do not know.
Would not fuel distribution in the intake after low bowl levels be affected by a low fuel pressure ?
Then resulting in a miss fire ?
Not saying that is the culprit this time but in my past I have ran into that.

Have you checked the fuel flow volume of the pump? Procedure is on the factory service manual.
After the needles and seats the carb really doesn't care what the fuel pressure is. Pressure creates flow. What the carb wants is volume of fuel. But they kind of go hand in hand. To many things can happen in the intake.

 

[Geoff 2]

Fuel pressure affects fuel level.....which affects the setting for the floats. The 'hole' in the needle & seat also affects fuel level. The fuel pump has to have enough flow volume to deliver the reqd fuel @ peak hp. A 300 hp engine using pump gas will require about 25 gph when the mixture is correct.
Below is a chart from Carter showing the affect that n/seeat size, pressure has on final fuel level. All carbs using n/seats work on the same principle.

 

[JackBEEQuick]

What were the actual results of the long awaited compression test.

180 give or take a point, 179 on the cylinder in question (5) and I didnt even have the tester completely tight as my headers make it very difficult.
Its still running good, whatever hit my spark plug was a "light tap" as it hit just enough to flatten the ground electrode and did no damage to the center electrode, just closed the gap to lose spark.

Im going to put a scope down it tonight and recheck everything again.

thinking ill index the plugs next, that might give me proper clearance