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Cadillac flathead 346 sputtering


Jeff Trahan

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My '47 Cadillac flathead spontaneously enters sputtering mode. As you can see/hear in the attached video, it idles fine at startup on the fast idle cam but then after several seconds it starts missing badly. After it starts sputtering, I manually rev it up a little and it's still at the same point of the fast idle cam, runs fine again for several seconds, then enters sputtering mode again. Any ideas what is going on? It also does the same thing when driving hot or cold. When it's hot, it sits and idles great at about 400 RPM without entering sputtering mode.

 

Jeff

 

https://youtu.be/wWFP92yaSgc

 

 

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Jeff,

    This could be either vapor lock (Aren't you in Phoenix with temps over 100 now?) or you may have an intake leak.  It is easy to check for a leak by spraying starting fluid or WD-40 on the intake flanges to see it the engine RPMs increase.

Good luck,

Mark Shaw

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Check for spark on all cylinders when it acts up, There are inline spark testers for this. Little neon gizmos that you put inline.

If its good, then maybe some junk floating around one main jet. (I don't know Caddy's so assuming its a two barrel)

It sounds like its dropping half of the cylinders, the same ones each time. Does it have dual points?

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Thanks for the suggestions. I'll try them out. The carburetor is a two-barrel and the distributor has only one set of points. I ordered some inline spark testers to see which ones may be dropping. If it is a fuel issue, it could be a problem with one side of the carburetor that feeds half the cylinders.

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I had a similar issue due to incorrect spring pressure on a new set of points I bought, although it would typically manifest at a higher rpm.  The points were correct by the book but when I pulled them back out and compared the spring rate to the original point set, the replacement was much softer than the originals.  I just cleaned up the original point contacts and solved the issue.  Just something to look at. You could also post in the CLC Forum.  Good luck.

Scott

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2 hours ago, Stude Light said:

I had a similar issue due to incorrect spring pressure on a new set of points I bought, although it would typically manifest at a higher rpm.  The points were correct by the book but when I pulled them back out and compared the spring rate to the original point set, the replacement was much softer than the originals.  I just cleaned up the original point contacts and solved the issue.  Just something to look at. You could also post in the CLC Forum.  Good luck.

Scott

 

I had the week spring problem once, Someone had mixed up the parts and was easily repaired.

 

Jeff, keep us informed as to what you find out.

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I didn't get the spark testers yet but couldn't wait to try it out so I pulled plug wires one at a time when my car entered sputtering mode. By the way, if anyone knows how to do this without getting zapped dozens of times, let me know. If I needed a pacemaker, I don't need one anymore with all the electricity coursing through my body. Anyway, it turns out that only four cylinders are sparking during sputtering mode. So I think we isolated this to an electrical issue, which my son suggested a few weeks ago. I thought it was a fuel issue. Damn kids know more than I do about these old cars. Tomorrow is Mother's Day, which means I am prohibited from working on cars. Any additional suggestions you have would be appreciated. I can try again next weekend. Thanks.

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If it’s just the same four cylinders then look for:

- Carbon tracks causing arcing in cap

- Bad wires

- Bad plugs

 

If it varies on which cylinders, it could be any electrical component:

- Points

- Condenser

- Cap

- Rotor (carbon track or gap)

- Coil or power to it

- Wires

- Plugs


I would suggest cost free testing first:

1) Running it in a pitch black environment and look for any arcing

2) Dwell meter and look for bouncing

3) Volt meter on power side of coil and ensure constant voltage source

 

If that fails then start going through each component and replacing it, starting with the cheapest first (condenser, points, rotor, cap, wires, plugs, coil...)

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So when you say only four cylinders are sparking I will assume that you are actually seeing no spark. like holding the wire close to ground and you see spark on some and not on others.

If this is the case then yes probably in the ignition.

I cant think of any reason that a simple points system would do this but I would start by installing all new ignition parts.

If you are just tracking down the miss by pulling the plug wires and they are always the same cylinders and assuming its spark without actually testing that wire for spark then it could still be fuel related.

I can see more reasons that the same cylinders would drop due to fuel than due to spark.

With your comments about getting shocked so often I might suggest that you may have plenty of spark.

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On 5/9/2020 at 4:24 PM, Jeff Trahan said:

By the way, if anyone knows how to do this without getting zapped dozens of times, let me know. If I needed a pacemaker, I don't need one anymore with all the electricity coursing through my body.

😮 I hate getting shocked by plug wires! Had a nerve conduction velocity test once - that was horrible. Constant zapping.  I’m like “Okay already, I’ll talk” .

 

One suggestion would be to use needle nose pliers with an insulated grip AND make up a ground wire that can be clipped to the pliers and the chassis. 

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Instead of getting zapped short one plug wire to ground at a time using a prick punch with an insulated handle to which a ground wire with an alligator clip has been added.  If a cylinder is running the engine will slow when you short it to ground, if not the engine will not slow.  The pointed tip on a prick punch can be pushed thru an insulated plug wire terminal boot without hurting it. 

Edited by Str8-8-Dave (see edit history)
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I'm pretty confident I figured it out. The point gap was practically zero. That's why four consecutive cylinders in the firing order were getting no spark when it entered sputtering mode. I should have thought of that earlier but I had the distributor professionally rebuilt less than ~250 miles ago and I just assumed it was still in top condition. I was wrong. After adjusting the gap and confirming the dwell at ~26 degrees, I took it for a ride and it was fine.

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