rough idle
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Senior User
Joined: Jan 2011
Posts: 145
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From: Dothan, Alabama
rough idle
I jumped in my truck this morning and drove it around and had no issues. A little while later I got back in and the first light I came to it idled very rough. I could put in neutral and it would get better but its been fine prior to today. I had the carb rebuilt this past winter. 351m....I even adjusted the fuel mixture screws because it had more of a gassy smell from the exhaust thinking it was too rich but it didn't do anything...so I returned them to their original setting and had to idle the truck up to get it to run better but its high. Its not misfiring and I hear no vacuum leaks under the hood. I checked the distributor and its not loose so I don't think the timing changed..what could it be? I am assuming there is something wrong with the carb?
Elder User
Joined: Dec 2007
Posts: 813
Likes: 6
From: Up Nort Minnesota
How many turns out are your idle mixture screws? 5 to 7 seems normal in my experience with these 351M/400 engines.
If it seams to be loading up at idle, I guess I'd suspect the power valve. Power valve is hidden under a square cap retained by 4 screws on the front bottom side of the carb. Power valve is a vacuum operated diaphragm valve that opens the flow of fuel from the float bowl to the carb throat when the engine is under heavy load. There is a tiny vacuum port in the carb body that controls this diaphragm. The diaphragm becomes cracked over time and then gas gravity feeds through the diaphragm and through that tiny vacuum port which feeds it directly into your engine. It's like having a 3rd idle mixture port open feeding extra fuel into the engine.
If the crack gets bad enough it will drain your float bowl into the engine while sitting and it will also fail to open the power valve on acceleration, you might experience a "lean pop" backfire with your foot in the gas and the truck under a load.
If it seams to be loading up at idle, I guess I'd suspect the power valve. Power valve is hidden under a square cap retained by 4 screws on the front bottom side of the carb. Power valve is a vacuum operated diaphragm valve that opens the flow of fuel from the float bowl to the carb throat when the engine is under heavy load. There is a tiny vacuum port in the carb body that controls this diaphragm. The diaphragm becomes cracked over time and then gas gravity feeds through the diaphragm and through that tiny vacuum port which feeds it directly into your engine. It's like having a 3rd idle mixture port open feeding extra fuel into the engine.
If the crack gets bad enough it will drain your float bowl into the engine while sitting and it will also fail to open the power valve on acceleration, you might experience a "lean pop" backfire with your foot in the gas and the truck under a load.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
If you have to have your screws out that far; you've got a vacuum leak. Should be no more than 2 turns.
Moderator
Joined: Nov 2007
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From: Denver Metro Area, CO
I've got a 400 M-block, I think they're at 3/4 turns. Five to seven == something is wrong there.
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Elder User
Joined: Dec 2007
Posts: 813
Likes: 6
From: Up Nort Minnesota
They must work different in TX 
My 77 F250 351M manual trans, 6 turns each. My 77 F150 351M C6, 6.25 turns.
A previous truck, 78 F150 400/C6... complete rebuild, fresh as a daisy, 7 turns each.
Those all wear motorcraft 2V original carbs.
My 70 F250 with 360/4spd is at 1 3/4 turns (holley 600 cfm sq bore 4 barrel, vac secondaries.)
My 77 F250 351M manual trans, 6 turns each. My 77 F150 351M C6, 6.25 turns.
A previous truck, 78 F150 400/C6... complete rebuild, fresh as a daisy, 7 turns each.
Those all wear motorcraft 2V original carbs.
My 70 F250 with 360/4spd is at 1 3/4 turns (holley 600 cfm sq bore 4 barrel, vac secondaries.)
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Senior User
Joined: Jun 2012
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From: Southern Ohio
I think it really depends on what carb is being used to determine how many turns of the idle/air screw. With a 2bbl on a V8 I had 4 half turns and a 4 bbl I had 2 half turns. My 300 has a 1bbl and I have it a little more than 2 half turns. It also depends on your initial timing an Idle. More timing means you need more fuel to keep the engine from runnin lean. The more the timing is advanced you may have more troubles with rough idle and over heating.
Elder User
Joined: Dec 2007
Posts: 813
Likes: 6
From: Up Nort Minnesota
My edelbrock and holley 4 barrels on these same engines run great at 1.5-2.25 turns average, I don't know why I have to spin my mixture screws out so far on my 2100's... I just figured it was the needle profile being "sharper", less blunt.
I know I don't have vauum leaks.
I know I don't have vauum leaks.
Thread Starter
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Senior User
Joined: Jan 2011
Posts: 145
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From: Dothan, Alabama
Ok so I don't know enough to diagnosis all this stuff myself so I took it to a local auto shop...they had to replace my power valve and they found two vacuum leaks..but its still not driving quite right..its A LOT better don't get me wrong but there something missing...
It stills seems like the idle is off..almost like the idleidle changes while sitting at a light..that and when I start from a dead start it hesitates to move like the tranny hasn't engaged but hasn't doesn't this before and doesn't do it every time...is this a completely different issue or could one have exacerbated the other?
Btw I haven't paid for services rendered yet at shop...he wanted me to drive around and see it how it went
It stills seems like the idle is off..almost like the idleidle changes while sitting at a light..that and when I start from a dead start it hesitates to move like the tranny hasn't engaged but hasn't doesn't this before and doesn't do it every time...is this a completely different issue or could one have exacerbated the other?
Btw I haven't paid for services rendered yet at shop...he wanted me to drive around and see it how it went
Moderator
Joined: Nov 2007
Posts: 22,415
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From: Denver Metro Area, CO
I wonder if the idle speed is too low, and/or the transmission needs fluid.
That's a good shop, I like them! See if you can drive around with one of their guys in the truck with you so you can SHOW them what you're seeing. Trying to describe stuff like this in written format can be really hard to do, so, show them....
That's a good shop, I like them! See if you can drive around with one of their guys in the truck with you so you can SHOW them what you're seeing. Trying to describe stuff like this in written format can be really hard to do, so, show them....