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401 motor running rough


Guest Robert Ricci

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Guest Robert Ricci

My 1963 401 motor is running rough. Did a tune up , rebuilt carb, and checked for vacum leaks. Did a compression test and found one at a low of 155 and a high of 175. Any suggestions or common problems I should check for? Thanks Bob

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I am assuming the reason ya did the tune up is because it runs rough. Does it run rough cold or hot? What was replaced for the tune up? Plugs, cap etc. My first guess is it could just be old gas. If its running rough all the time pop the hood while she's running, kill the lights in the garage at night and see if ya have any stray sparks. I found a bad coil this way. I am not that concerned on your compression reading, yet as they are often not conclusive. Just back track a bit so we can help ya out. Also, when you changed the plugs, did they all look alike, quite often they can tell you which cyl is causing problems.

Edited by brh (see edit history)
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How many miles are on this engine? Was it running smoothly before this happened? I had a '63 Electra once with about 24,000 actual miles, 401 engine. It had a miss when idling. Turned out to be a cracked piston and broken piston rings on that one piston. Probably broke when someone forcefully turned the engine over after sitting for many, many years without being run. Replaced that piston, honed out that cylinder, and it was fine after that.

"BRH" is right about the spark plugs--what condition were they in when you took them out? Was one of them black and oily and not firing?

Pete Phillips, BCA #7338

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Is the roughness more pronounced at idle and in gear? Does it seem to smooth out by the time you get to highway cruise speeds? Does it do fine on flat land, but takes more throttle (than it used to) to go up a hill at highway speed?

Other than "bad gas" or spark plug wires, it could be a burnt valve (which is what I described in the questios above). If it's not too bad, it'll show up mainly at lower rpm and under load, but smooth out at higher rpms (as the other cylinders are producing enough power to "cover for it"). Burnt valves usually are caused by valve guide wear, which can also accelerate the wear on the valve seals, letting too much oil down through the guide, which can then be sucked into a cylinder with the normal air/fuel mixture (if it's an intake valve).

Is it "roughness" or "a miss"?

What about the distributor points? Were they changed and the new ones installed with "point grease" on the rubbing block (where it slides across the breaker cam, to open and close the points)? What point gap and dwell readings did you obtain? Were the spark plugs gapped to the recommended specs prior to installation?

Just some thoughts . . . about provided information.

NTX5467

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this is just a stupid question are the spark plug wires on correctly, cause been there done that!

LOL or is one off, because ya wanted to check timing, forgot and slammed the hood shut, with the wire laying on the manifold. Them v-8s run rough on 7. Been there did that.

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Guest Robert Ricci

Car is missing the plate between the carb and intake. I ordered one and will install. It is rough at low idle. I did a complete tune up wires, plugs, cap rotor, and car ran the same.

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Hopefully, this plate will do the trick!

Hope so too!! Also, check your heat riser, make sure its not rusted closed.

Did you balance the jets on the carb? If ya got one lean and the other rich its gonna run rough.

Edited by brh
One more question (see edit history)
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If this is the thin stainless steel plate, it will go against the bottom of the carb "bare", with the composition gasket going between the plate and the intake manifold. Be SURE to make sure the contours of the manifold match the gasket pkg and the carb base.

The stainless steel plate will keep raw exhaust gases, via the "heat track" in the carb mounting flange of the intake manifold, from actually contacting the base of the carb itself. It's all there for colder climates in the cool, damp winters, to help prevent "carburetor icing".

Just some thoughts,

NTX5467

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