Why is it dying?

[jwshira]

My Dart 351W has developed a new trick. It happens when for instance I pull out of a driveway and see a red light ahead. I accelerate moderately hard in first gear, then just drop my foot off the gas to coast to the red light. As soon as I drop my foot off the gas, the engine dies. Just stops dead, like I turned the ignition key off. No stumble, just dead. If I put it in neutral, it starts right back up, but occasionally with a bit of a stumble on startup before it idles smoothly again. If I accelerate through the gears, slow or really pushing it, it doesn't die. Only under the sort of situation I described above. My first thought is some sort of ignition problem, but I'm not sure where to look. I have an MSD distributor, and the rotor and cap look good. MSD 6 box. I have a Crane coil (installed by the PO) but don't know if I should just replace that to see what happens, or keep looking elsewhere. I have noticed at cruise, that I think I feel an occasional stutter, like a very intermittent miss, but it might be nothing more than road bumps I'm feeling. If any of you have any suggestions, I'd sure appreciate hearing them. This is at the least embarrassing, and at the worst, potentially deadly.

[Tommy]

Has anything similar happened under hard braking? And do you have mechanical or vacuum secondaries? And what is your idle RPM?

[8litercobra]

I had something similar, engine might die when I let off the throttle. Then crank right back up as if nothing ever happened. I got lucky and spotted a small crack in the MSD mag pick up module in the distributor. The mounting of the sensor with caulk kept it together pretty well, I guess, and made the fault difficult to see. Odd why it seemed to happen on engine deceleration. Anyway, worth a quick look! MSD - may suddenly die

[jwshira]

Originally Posted by Tommy

Has anything similar happened under hard braking? And do you have mechanical or vacuum secondaries? And what is your idle RPM?

No, I had a hard braking situation a couple days ago, and no stalling issues. Carb is a Quickfuel 650 with mechanical secondaries, idle is right around 900.

[jwshira]

Originally Posted by 8litercobra

I had something similar, engine might die when I let off the throttle. Then crank right back up as if nothing ever happened. I got lucky and spotted a small crack in the MSD mag pick up module in the distributor. The mounting of the sensor with caulk kept it together pretty well, I guess, and made the fault difficult to see. Odd why it seemed to happen on engine deceleration. Anyway, worth a quick look! MSD - may suddenly die

I'll dive a little deeper into the distributor to see if I can see anything like that. he really odd part is that it only happens on decel after a quick hard accel. Normal deceleration, or winding back down through the gears produces no drama at all.

[Gaz64]

Sounds like a harness moving around.
With the engine idling, move your distributor pickup wiring all the way up towards your 6AL, and also and wiring that can move under g force.
I would also like to know if the car accelerates well continuously, at least for 3 gears.

[mrmustang]

While everyone else is focusing on spark, perhaps a look at your fuel pump/fuel system and a further search for a vacuum leak may be in order. Had a similar issue a year and a half ago, mysteriously went away, only to rear it's ugly head on a weekend cruise when the fuel pump finally gave out 125 miles from home. With Covid regulations in place, the car got a nice ride home on a flatbed, with wheel bonnets, we got stuck in an Uber

[Tommy]

My next move would be to reset the warm idle to 11-1200 RPM just for diagnosis. Repeat the action that caused the engine to die before and see what happens. If the engine dies again, I'd focus more on electrical causes. If it keeps running, I'd focus on the fuel system. .... My thinking is that the rapid acceleration/deccleration may inded be causing something electrical to move; but it is also possible that going from near wide open throttle to fully closed butterflies may be causing an air fuel ratio problem. ... For the record, I'm far from an expert on this topic. If someone better informed comes along, take their advice over mine.

[jwshira]

All good suggestions so far, and I'll give them a try one by one. I really appreciate the efforts to help, welcome any additional thoughts, and will report back with any updates and hopefully a resolution.

[jwshira]

Originally Posted by Gaz64

Sounds like a harness moving around.
With the engine idling, move your distributor pickup wiring all the way up towards your 6AL, and also and wiring that can move under g force.
I would also like to know if the car accelerates well continuously, at least for 3 gears.

I'll spend some time tomorrow with wiring to see if I can find anything obvious.
Running through the gears, it pulls hard...real hard...first through fifth. It's definitely not a fuel starvation due to fuel bowl slosh issue.

[jwshira]

Originally Posted by mrmustang

While everyone else is focusing on spark, perhaps a look at your fuel pump/fuel system and a further search for a vacuum leak may be in order. Had a similar issue a year and a half ago, mysteriously went away, only to rear it's ugly head on a weekend cruise when the fuel pump finally gave out 125 miles from home. With Covid regulations in place, the car got a nice ride home on a flatbed, with wheel bonnets, we got stuck in an Uber

Pressure gauge shows a steady 6.5 pounds of fuel pressure at idle, but it's under the hood so I have no idea if it drops under acceleration. I can run it up with a blip of the throttle lever and it stays steady, so I'm assuming at this point, that the pump is good. The filter however, is another idea I hadn't considered. PO put an inline filter in it also easily located under the hood, so I think I'll change that and see if it makes a difference.

[jwshira]

Originally Posted by Tommy

My next move would be to reset the warm idle to 11-1200 RPM just for diagnosis. Repeat the action that caused the engine to die before and see what happens.

Good idea...hadn't thought of that.

[Jerry Clayton]

Anybody near you that you can borrow another carb from??????

[Dirty Harry]

With the engine running, wiggle the wires on the back of the ignition switch. See if the engine cuts out. It's a long shot, but worth 90 seconds to try it.

[hauss]

Believe it or not I actually had a internal battery short that caused me all kinds of trouble Not saying this is your problem very unlikely but it does happen.

[jwshira]

Originally Posted by Jerry Clayton

Anybody near you that you can borrow another carb from??????

Actually, yes. I think I'll give that a try this weekend.

[jwshira]

Originally Posted by hauss

Believe it or not I actually had a internal battery short that caused me all kinds of trouble Not saying this is your problem very unlikely but it does happen.

Battery is an Optima Red Top and only about six months old, but Murphy's Law being what it is, it's certainly not impossible.

[Gaz64]

How about the rear float level a fraction high, and flooding the engine on decel?
Drop the level by 2 flats at a time, and retest.

[incoming]

Mine does the same thing. I can brake faster than the floats can keep up with. I just deal with it. I've never ran out of fuel on acceleration.

[jwshira]

Originally Posted by Gaz64

How about the rear float level a fraction high, and flooding the engine on decel?
Drop the level by 2 flats at a time, and retest.

Good idea. I'll try that before switching to the loaner carb.