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Stumbling on acceleration

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Old 09-22-2018, 05:26 PM
Metalleg1984
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Stumbling on acceleration

Need some help on my 1984 F150 2WD 351 HO. I picked up this survivor a couple of weeks ago. has 132k miles and is in all original condition. It starts right up, idles great, sounds great, has excellent oil pressure, however, I noticed that on acceleration, it hesitates and stumbles. I have replaced plugs, wires, cap and rotor , but it still stumbles. I've read that it is probably the carburetor. It has the original Holley 4180 and I am considering replacing with an Edelbrock 1406. Would this work, and what issues should I look for when I tackle this swap?
Thanks in advance for the help.
 
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Old 09-22-2018, 09:58 PM
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Originally Posted by Metalleg1984 View Post
Need some help on my 1984 F150 2WD 351 HO. I picked up this survivor a couple of weeks ago. has 132k miles and is in all original condition. It starts right up, idles great, sounds great, has excellent oil pressure, however, I noticed that on acceleration, it hesitates and stumbles. I have replaced plugs, wires, cap and rotor , but it still stumbles. I've read that it is probably the carburetor. It has the original Holley 4180 and I am considering replacing with an Edelbrock 1406. Would this work, and what issues should I look for when I tackle this swap?
Thanks in advance for the help.
If you are going to swap the carb to something else avoid the Edelbrock if you have an auto you need to buy the adapter kit separate, reroute the fuel line etc etc. Either opt for another Holley or one of the Summit M2008 carb's.
Also swapping the carb to a non emissions style carb will mean modifying or removing some of the emissions equipment.
The carb is very likely due for a rebuild so you also have the option of rebuilding the original.
 
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Old 09-23-2018, 07:13 AM
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If you are not up to rebuilding a carb, a new one is probably your best option. But as was mentioned, a aftermarket carb will need a few modifications to make it fit, even a another Holley. Everyone has their opinions on carbs, I gravitate toward the holley's myself since that is what I am more familiar with, but if you want to run a Edlebrock, it would work fine also. Most important thing, make sure you put a good fuel filter on it. Your stock carb probably has the filter screwed into the carb, the aftermarket carbs usually require a external filter in the fuel line, so that is part of the minor modifications you need to make.
 
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Old 09-23-2018, 07:55 AM
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Sounds like power valve issues, timing advance, or as others have said, just an old carb that is in need of rebuild. Itís up to you, but when I got my truck (86 351W HO F150), it was in much of the same shape as you describe yours with all the emission vacuum hoses, smog ports, all that stuff just in disarray in the engine compartment and it was just laying there, not hooked or working at all. I took the time to replace the stock intake with an Edelbrock intake (no egr), got rid of the smog pump and capped the thermoactor holes in the heads, fixed about a half a dozen of vacuum leaks, and replaced the original carb with a 4160 Holley Street Warrior (600cfm). Iím usually an Edelbrock guy, but I didnít want to worry with rerouting fuel lines and linkage. Just my two cents, but when I did that the engine woke up quite a bit.
 
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Old 09-23-2018, 08:33 AM
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What about an EGR issue? The valve is supposed to be closed during acceleration. If it stays open (sticky valve or lack of command to close), that could cause your symptom.

The EGR valve is also supposed to be closed at idle. However, maybe the idle speed and mixture got jacked around to help mask that.

Iím not sure of the best way to troubleshoot an EGR system. Maybe somebody else can chime in. But itís something to consider before loading a new carburetor on the parts catapult.
 
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Old 09-23-2018, 10:30 AM
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Thanks to all for your input. Sounds like I have some work to do. I did pull the vacuum line off the EGR , capped the port and plugged the line. Truck ran exactly the same as when connected. Probably start with a new EGR. I'm pretty sure that the carb is original and could use a rebuild. Thanks again for the help. I'm sure you'll hear from me again soon. I'm mechanically inclined, but I ain't a mechanic.
 
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Old 09-23-2018, 12:17 PM
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Originally Posted by Metalleg1984 View Post
I did pull the vacuum line off the EGR , capped the port and plugged the line. Truck ran exactly the same as when connected.
Maybe hold off until we hear back from some others. My memory is fuzzy, but I could have sworn the EGR test is a little more involved. Don't quote me on that, however. Something about needing a certain amount of exhaust pressure to seat the valve, but now I'm doubting myself.

I'd also suggest checking fuel pressure and volume. A problem there could cause a stumble .
 
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Old 09-23-2018, 12:17 PM
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Originally Posted by Metalleg1984 View Post
Thanks to all for your input. Sounds like I have some work to do. I did pull the vacuum line off the EGR , capped the port and plugged the line. Truck ran exactly the same as when connected. Probably start with a new EGR. I'm pretty sure that the carb is original and could use a rebuild. Thanks again for the help. I'm sure you'll hear from me again soon. I'm mechanically inclined, but I ain't a mechanic.
The cheapest and easiest thing to do would be to simply rebuild your existing carburetor.

The Motorcraft (made by Holley) 4180 you already have is a good carburetor, and better than an aftermarket Edelbrock carburetor, in my opinion. It is the same carburetor that the mighty Mustang GT 5.0 used from 1983 - 1985.
 
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Old 09-23-2018, 12:28 PM
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Could be as simple as a clogged fuel filter. Try replacing the fuel filter(s) before anything else.

The 4180 is a great carb. If it idles good and otherwise runs good it may not be the carb but rather fuel delivery; fuel filter(s) fuel lines, fuel pump.
 
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Old 09-23-2018, 12:29 PM
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Stumble on acceleration is most likely a accelerator pump problem in the carb. If it was a EGR problem, it would idle terribly if at all. It's very diffcult to mask a leaky EGR.

You can check to see if the accelerator pump is working properly and check it off the list of possibilities if it is working ok. All you have to do is pull the aircleaner off, and get a flashlight and crawl up in the engine compartment or get on a ladder and lean over the top of the carb. With the engine off, pull the choke door open if it's in your way, and shine the flashlight down the throat of the carb and push the throttle wide open. You should see two strong streams of gas pour into the throat of the carb with some force behind them. If they are weak and just dribbles, that is your problem. You can let the throttle back closed, and then push on it ever so slightly, and you should get squirting with the slightest movement.

You can mess with this about 3 times, but after that I would quit, since you will flood the engine moving the throttle too much without it running. No harm will be done, but it will make it hard to start afterward.
 
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Old 09-23-2018, 01:54 PM
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I should also add that the fuel circuit for the secondaries can get gunked up inside the carburetor if they never get opened up. I make it a point to push into the secondaries from time to time just to make sure the gas stays fresh.

If the secondaries are gunked up it up it would certainly fall down under hard acceleration.
 
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Old 09-23-2018, 02:19 PM
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Originally Posted by Brnfree View Post
I should also add that the fuel circuit for the secondaries can get gunked up inside the carburetor if they never get opened up. I make it a point to push into the secondaries from time to time just to make sure the gas stays fresh.

If the secondaries are gunked up it up it would certainly fall down under hard acceleration.
The secondarys should be moving fuel all the time, the secondary throttle plates should not be fully closed if adjusted properly and will move a bit of fuel even at idle.
 
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Old 09-23-2018, 03:01 PM
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Originally Posted by Franklin2 View Post
Stumble on acceleration is most likely a accelerator pump problem in the carb. If it was a EGR problem, it would idle terribly if at all. It's very diffcult to mask a leaky EGR.

You can check to see if the accelerator pump is working properly and check it off the list of possibilities if it is working ok. All you have to do is pull the aircleaner off, and get a flashlight and crawl up in the engine compartment or get on a ladder and lean over the top of the carb. With the engine off, pull the choke door open if it's in your way, and shine the flashlight down the throat of the carb and push the throttle wide open. You should see two strong streams of gas pour into the throat of the carb with some force behind them. If they are weak and just dribbles, that is your problem. You can let the throttle back closed, and then push on it ever so slightly, and you should get squirting with the slightest movement.

You can mess with this about 3 times, but after that I would quit, since you will flood the engine moving the throttle too much without it running. No harm will be done, but it will make it hard to start afterward.
This is the first thing you should do as this is a sure sign of a accelerator pump with a weak or no squirt.
If it is the cause I would rebuild your carb and go from there.
Also rebuilding yours there is no messing with vacuum lines that need to be plugged, other brackets to get the auto trany to shift right, fuel lines re-routed, etc.
Dave ----
 
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Old 09-23-2018, 03:38 PM
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Wow! Thanks everyone. Not only getting some great advice, getting educated. Once past the hesitation, she'll open up and run, pretty quick too I am hoping its just a carb rebuild. I'll give these suggestions a try tomorrow. Off to watch football now. Thanks again. I keep ya'll posted.
 
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Old 09-23-2018, 11:47 PM
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Originally Posted by matthewq4b View Post
The secondarys should be moving fuel all the time, the secondary throttle plates should not be fully closed if adjusted properly and will move a bit of fuel even at idle.
I agree that’s what the book says and I wish I could make that work but FWIW I couldn’t get a clean idle with the secondaries open. Maybe it’s because the later 4180s like mine only have two idle mixture adjustment screws? I don’t know. I can say the two that I rebuilt both had the secondaries at zero when I took them apart and that’s also where I found my tuning happy place.

Back to the original poster: when you say acceleration, do you mean “off the line” acceleration or do you mean “heavy throttle” as in sustained pulling a big load up a hill?

If it’s a stumble off the line then check the accelerator pump function as the others have already described. Also check the free play in the throttle linkage before it engages the accelerator pump lever arm. There should be a little bit of free play but if there is too much it can cause the accelerator shot to come late and cause a hesitation on throttle tip in.




 

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