Motorcraft 4350 help
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Freshman User
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Motorcraft 4350 help
Hopefully I can get a little help from a carb guru here. I received a free 1977 F250 camper special last year and have been slowly getting her going. I think my final issue is the carb and can't quite get it figured out. The carb is a 4350 motorcraft spreadbore which I rebuilt and adjusted to specs,tanks cleaned,new fuel pump,wires,plugs/rotor and cap. Pulls 18" of vaccum which I believe is good. After some time messing with the carb I have seen improvement but it is not 100%. She idles awesome and drives fairly well. Let me explain the problem! The issue I have is only under load driving down the road. When I try to get in the throttle some,maybe half throttle or more she is choking out like someone is putting thier hand over the top of the carb. If I let off some it is fine and drives normal but as soon as I give her a little more like if I was trying to go up a hill it chokes out.
I am trying to avoid switching carbs if possible but I am a bit at my wits endafter so many tries and taking it apart numerous tmes. I can't tell if it seems like too much fuel or not enough or if something else may be a problem. Any help from the smart carb guys would be helpful.
Yesterday, 05:57 PM
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I am trying to avoid switching carbs if possible but I am a bit at my wits endafter so many tries and taking it apart numerous tmes. I can't tell if it seems like too much fuel or not enough or if something else may be a problem. Any help from the smart carb guys would be helpful.
Yesterday, 05:57 PM
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Laughing Gas
Joined: Sep 2003
Posts: 968
Likes: 5
From: Corbett, Oregon
Sounds like an accelerator pump issue or possibly an enrichment circuit (think power valve) / main jet issue.
An accelerator pump not giving a full shot, or plugged accelerator pump nozzle(s) would act that way.
A late opening enrichment circuit coupled with main jets a couple sizes lean will also cause similar problems.
Another possible cause could be fuel delivery. Do you have a new fuel filter on the rig? What is the fuel pump pressure and volume like? I have had a bad fuel pump cause the problems you describe.
Roger Carter
An accelerator pump not giving a full shot, or plugged accelerator pump nozzle(s) would act that way.
A late opening enrichment circuit coupled with main jets a couple sizes lean will also cause similar problems.
Another possible cause could be fuel delivery. Do you have a new fuel filter on the rig? What is the fuel pump pressure and volume like? I have had a bad fuel pump cause the problems you describe.
Roger Carter
Junior User
Joined: Sep 2013
Posts: 82
Likes: 0
Sounds like an accelerator pump issue or possibly an enrichment circuit (think power valve) / main jet issue.
An accelerator pump not giving a full shot, or plugged accelerator pump nozzle(s) would act that way.
A late opening enrichment circuit coupled with main jets a couple sizes lean will also cause similar problems.
Another possible cause could be fuel delivery. Do you have a new fuel filter on the rig? What is the fuel pump pressure and volume like? I have had a bad fuel pump cause the problems you describe.
Roger Carter
An accelerator pump not giving a full shot, or plugged accelerator pump nozzle(s) would act that way.
A late opening enrichment circuit coupled with main jets a couple sizes lean will also cause similar problems.
Another possible cause could be fuel delivery. Do you have a new fuel filter on the rig? What is the fuel pump pressure and volume like? I have had a bad fuel pump cause the problems you describe.
Roger Carter
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Don't worry I am still here just working swingshift! I have been thinking along the same lines as you guys. But I have a new fuel pump and I also know that the bowl is getting plenty of fuel as after I ran it again this last weekend I took it back apart without dumping any fuel to check. I have dipped and sprayed through passages and spray bar including the accelerator circuit.
This coming weekend I am going to start giving the accelerator pump some adjustment. I did that last weekend but I was going the wrong way and tightend it down instead of loosening which from what I have read was incorrect. This will probably be my last shot with the 4350! It stiffels me that it seems so good at idle and I mean so gooooood. It has taken some time to get there but I can see the light at the end of the tunnel
I also wanted to add that I am getting very quick at taking the carb off and taking it apart!!!!
Here is a link to my old thread
https://www.ford-trucks.com/forums/1...r-special.html
This coming weekend I am going to start giving the accelerator pump some adjustment. I did that last weekend but I was going the wrong way and tightend it down instead of loosening which from what I have read was incorrect. This will probably be my last shot with the 4350! It stiffels me that it seems so good at idle and I mean so gooooood. It has taken some time to get there but I can see the light at the end of the tunnel
I also wanted to add that I am getting very quick at taking the carb off and taking it apart!!!!Here is a link to my old thread
https://www.ford-trucks.com/forums/1...r-special.html
Junior User
Joined: Sep 2013
Posts: 82
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Well hopefully adjusting the pump will fix it right up for you, if not at least we have checked something off the list. Have you rebuilt the carb completely, new seals, etc.? If not I would suggest that as a next step as minor leaks or worn parts can cause havoc with carb adjustment. You said you had installed a new fuel pump as well, but have you checked the fuel filter? I have had case where the system worked great at idle, but under high demand sediment (read rust or dirt from tank) would be stirred up and clog the filter. Easy check to see if that's the case... Might wanna try that as well before dragging the carb off again.
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Cargo Master
Joined: May 2009
Posts: 2,587
Likes: 47
From: Eagle River, AK
What kit and parts have you put into your 4350. I have one on my 76 F250, but fully planned to swap it over to a standard square bore carb due to all the inherant problems with the 4350.
If I remember correctly, ALL the information I read stated that the 4350, even brand new, had "tip in" problems at various spots in the throttle. With not a single one stating that they found and had the perfect set-up for daily driving and a heavy foot every now and then.
The 4350 is still on my 76, depending whether or not it gets sold next summer, I might give rebuilding it a shot or just swap it over with the adapter and square bore carb I have sitting in the shed.
If I remember correctly, ALL the information I read stated that the 4350, even brand new, had "tip in" problems at various spots in the throttle. With not a single one stating that they found and had the perfect set-up for daily driving and a heavy foot every now and then.
The 4350 is still on my 76, depending whether or not it gets sold next summer, I might give rebuilding it a shot or just swap it over with the adapter and square bore carb I have sitting in the shed.
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Junior User
Joined: Jan 2008
Posts: 76
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From: SK, Canada
This may not be the answer you are looking for but...
About a year ago, I finally gave up on the 4350 on top of my 460. I had 2 original carbs and I could never get either one to function well even after multiple rebuilds and lots of time.
I put a Edelbrock 600 CFM (1406) carb on with a spreadbore / squarebore adapter and my truck never ran better with the new carb right out of the box! I have since tuned it and richened it up to clear up the lean cruise problem I was having.
The point of my reply was not to say the Edelbrock I installed is the world's best carburetor but to point out that it was an inexpensive option that is 1000% better than the 4350 and has been completely reliable even in the winter!
Just my 2 cents...
About a year ago, I finally gave up on the 4350 on top of my 460. I had 2 original carbs and I could never get either one to function well even after multiple rebuilds and lots of time.
I put a Edelbrock 600 CFM (1406) carb on with a spreadbore / squarebore adapter and my truck never ran better with the new carb right out of the box! I have since tuned it and richened it up to clear up the lean cruise problem I was having.
The point of my reply was not to say the Edelbrock I installed is the world's best carburetor but to point out that it was an inexpensive option that is 1000% better than the 4350 and has been completely reliable even in the winter!
Just my 2 cents...
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A little better
Well after I did my regular maintenance on my daily driver I had a chance to mess with this carb again for the umteenth time. I started cranking on the accelerator pump adjuster and going for the two block run. Took about 8 runs but finally it is better and didn't have the pop and choke. Since I have never really driven the truck till now I am not sure if it is 100% right but it is lots better. I did notice that the top end of the running range feels a bit soft but everywhere else feels really good.
I am going to try and get a trip permit later this week,drive it around some then see if I can get it to and through emissions. One reason I don't want to switch carbs is for emissions as I don't know if a 1406 would clear emissions.
I am going to try and get a trip permit later this week,drive it around some then see if I can get it to and through emissions. One reason I don't want to switch carbs is for emissions as I don't know if a 1406 would clear emissions.
Junior User
Joined: Sep 2013
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Well after I did my regular maintenance on my daily driver I had a chance to mess with this carb again for the umteenth time. I started cranking on the accelerator pump adjuster and going for the two block run. Took about 8 runs but finally it is better and didn't have the pop and choke. Since I have never really driven the truck till now I am not sure if it is 100% right but it is lots better. I did notice that the top end of the running range feels a bit soft but everywhere else feels really good.
I am going to try and get a trip permit later this week,drive it around some then see if I can get it to and through emissions. One reason I don't want to switch carbs is for emissions as I don't know if a 1406 would clear emissions.
I am going to try and get a trip permit later this week,drive it around some then see if I can get it to and through emissions. One reason I don't want to switch carbs is for emissions as I don't know if a 1406 would clear emissions.
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