my68barracuda
Well-Known Member
I had the new motor in the 68 Barracuda up and running last November. I had basically carried over the MegaSquirt setting from the prior slant six motor. The new motor should be 100+ HP more, has 10.3 to 1 compression, a much racier cam, fully ported head, long rods, 2.2 L pistons, but I am using the same fuel system, injectors, rail and the same ignition components, MSD box and coil and a lean burn distributor. The car started and idled ok and I had driven it around the block but did not lean on it at all. Then it sat till a week ago.
Got plates and insurance in effect and took the car for a short drive. With the car setting still it would easily rev to 4500, however with the car pulling its own weight, it would accelerate to about 3000 RPM then mis fire like crazy. not backfiring out the exhaust or snorting through the intake, it was like it would just cut off then back on.
My first though was to adjust the AF and VE tables, that may have helped a little, backing the ignition timing down and closing the spark plug gaps down to .030 also helped a little. But it certainly was not fixed.
I did a lot of internet searching and found an incredible number of posts where folks had issues with MSD boxes mis firing above 3000 RPM. Fixes included swapping boxes and coils and working to eliminate EMI interference and current jumping spark plug wires. Also found several threads where folks also had MegaSquirt issues, again the fixes were eliminating EMI interference, getting the rotor phasing right, getting a stable timing signal, switching trigger polarity from high to low to low to high. I found one post where a person had mentioned that they resolved the miss above 3000 RPM by reducing the coil duty cycle from 50% to 25%. The Megasquirt manual recommends that when running a MSD box to set coil duty cycle to 'fixed' that is easy to do with the drop down menu. But they do not mention adjusting the coil duty cycle, which the default appears to be 50%. I changed the coil duty cycle from 50 to 25% with the engine running. With a MegaSquirt making that change was as easy as changing numbers in a text box on a computer screen. Immediately the idle smoothed out. I took the car for a short ride and the engine was absolutely singing. The mis fire above 3000 RPM is fixed.
I contacted the MSD tech line to get information on how reducing the coil duty cycle could resolve this problem. The Tech said it sound as if the coil was not getting enough time to fully charge, and by reducing the coil 'on' time the coil had more time to charge.
Passing this on as this simple fix got my motor running great.
Got plates and insurance in effect and took the car for a short drive. With the car setting still it would easily rev to 4500, however with the car pulling its own weight, it would accelerate to about 3000 RPM then mis fire like crazy. not backfiring out the exhaust or snorting through the intake, it was like it would just cut off then back on.
My first though was to adjust the AF and VE tables, that may have helped a little, backing the ignition timing down and closing the spark plug gaps down to .030 also helped a little. But it certainly was not fixed.
I did a lot of internet searching and found an incredible number of posts where folks had issues with MSD boxes mis firing above 3000 RPM. Fixes included swapping boxes and coils and working to eliminate EMI interference and current jumping spark plug wires. Also found several threads where folks also had MegaSquirt issues, again the fixes were eliminating EMI interference, getting the rotor phasing right, getting a stable timing signal, switching trigger polarity from high to low to low to high. I found one post where a person had mentioned that they resolved the miss above 3000 RPM by reducing the coil duty cycle from 50% to 25%. The Megasquirt manual recommends that when running a MSD box to set coil duty cycle to 'fixed' that is easy to do with the drop down menu. But they do not mention adjusting the coil duty cycle, which the default appears to be 50%. I changed the coil duty cycle from 50 to 25% with the engine running. With a MegaSquirt making that change was as easy as changing numbers in a text box on a computer screen. Immediately the idle smoothed out. I took the car for a short ride and the engine was absolutely singing. The mis fire above 3000 RPM is fixed.
I contacted the MSD tech line to get information on how reducing the coil duty cycle could resolve this problem. The Tech said it sound as if the coil was not getting enough time to fully charge, and by reducing the coil 'on' time the coil had more time to charge.
Passing this on as this simple fix got my motor running great.
Last edited:
