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Rough idle only

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piper62j View Drop Down

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    Posted: 21-December-2023 at 11:34AM
My 76 Elite 351M has compression in the mid 170s.. Problem is it idles very rough.. Acceleration and cruise feels normal.. Idle vacuum  17 inches, timing 4 degrees btc and idle rpm per the oem sticker. 4 carburetors -3 professionally rebuilt and one new knock off.  New timing chain, plugs, wires, cap, rotor, rebuilt oem distributor, new ignition control box. 10w 40 dino oil.. New intake manifold gaskets. New fuel pump.  Cam and lifters appear smooth and shiny no wear indicated.. AIR pump and EGR valve connected as original. Automatic transmission.  Just can't get a smooth idle.. Any suggestions, hints, advice??
Thanks guys.. 
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72 RS 351 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 72 RS 351 Quote  Post ReplyReply Direct Link To This Post Posted: 21-December-2023 at 12:30PM
That's a hard problem to solve. I've had a few long term issues like that, late model and they ended up being a part I had replaced before the issue began, and in trouble shooting got around to those last.

I'd bet on the ignition, it's more complicated and easier for a simple bad connection to create such a miss. Be sure to check anything you did early on but haven't rechecked in a long while.

My last issue was a new coil(two years old when the miss began and less than 4000 miles). Two Accel coil packs, I liked them until one caused that miss, now I'll avoid the brand if I can. A new coil should not fail in normal stock use. I solved the issue with an old high mileage coil pack(all modular V8's used the same coil as the Explorer 302).

Edited by 72 RS 351 - 22-December-2023 at 3:33AM
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aquartlow View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote aquartlow Quote  Post ReplyReply Direct Link To This Post Posted: 21-December-2023 at 3:00PM
Try increasing the initial timing to 10-12 degrees BTDC to see if it idles better.  Depending on where idle RPM is set, a stock cam will generally yield 20-21 in hg at idle, 17 in hg indicates either a "small" aftermarket cam or possibly retarded cam timing, or maybe a combination of the two. Totally agree with Don about ignition anomalies, make sure ignition is well sorted before going any further. Hope this helps
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Rockatansky View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Rockatansky Quote  Post ReplyReply Direct Link To This Post Posted: 22-December-2023 at 5:41AM
i've seen new tune-up parts cause running issues that fishing the old parts out of the trash can fixed. 1 simple test to rule out fuel delivery / vacuum leaks is to manually hold the choke plate nearly closed to see if the engine smoothes out or stumbles. from normal operating temp it should stumble. test for lean by spraying carb cleaner or start fluid at the carb opening while at normal operating temp, if the engine picks up or smoothes out it's lean. could be only lean at idle, or a small vac leak that doesn't affect midrange rpm operation. 4 new carbs all doing the same thing, probably not. second bumping the timing up to 8* or 10* and see what happens.

did you thoroughly examine the EGR plate, remove the valve, decarbon and replace all gaskets with the correct type? the lower gasket is HD and exhaust temp rated not just a carb gasket
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