351 m build - Ford Bronco Forum
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post #1 of 8 (permalink) Old 05-16-2019, 08:51 PM Thread Starter
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351 m build

My grandpa rebuilt a 351m and put it in a 83 ford f150 4x4. New cam, rings and bearings and worked the heads. It has the motor craft 2 barrel carb and a hei distributor. When starting it the carb has to be about half open and will only run about 1500 rpms up. When you let off the throttle it stumbles and dies. You can get it to run slower but have to keep pumping the gas pedal to keep it going. The carb has been rebuilt then he took it apart again just to double check everything. I checked the timing today it is fine as is the firing order. When you let off the gas and it slows down and sounds like a large cam lope for just a few seconds then dies. Any ideas why we cant get it to idle. Also sprayed around the intake and head for vacuum leak and nothing was found have checked all the vacuum hoses also.
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post #2 of 8 (permalink) Old 05-16-2019, 09:20 PM
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Is the choke opening like it should? Loping idle usually means too much fuel...try propping the choke open with a pencil. If the choke is not shut then disregard that.

I have been known to sometimes sing “The Hero of Canton” song from the tv show ‘Firefly’ while making breakfast on a weekend morning....
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post #3 of 8 (permalink) Old 05-16-2019, 09:27 PM Thread Starter
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It was all the way open. I am staring to wonder if maybe he did not put some piece in the carb. He is 83.
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post #4 of 8 (permalink) Old 05-16-2019, 10:10 PM
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With all gentle-ness to our Elders, (and great respect), you might take this opportunity to learn about carburetors yourself. He probably did a good job with what he had to work with. Nothing about putting down his work or good intentions, but a good soak in a very stinky carb vat followed by a careful check of all the tiny passages with an air compressor powered nozzle will make a huge difference. Sounds to me like either the float level is too high or the passages to the primary metering tubes are plugged. Don’t overlook the big hose that your truck may have going to the egr plate under the carb, they rot out and cause a vacuum leak.

I have been known to sometimes sing “The Hero of Canton” song from the tv show ‘Firefly’ while making breakfast on a weekend morning....
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post #5 of 8 (permalink) Old 05-16-2019, 10:15 PM
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Don’t know off the top of my head which carb that truck has, but a ruptured accelerator pump diaphragm will cause the same symptom, running very rich at idle...show a picture of the carb and the id tag if it still has one. Please.

I have been known to sometimes sing “The Hero of Canton” song from the tv show ‘Firefly’ while making breakfast on a weekend morning....
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post #6 of 8 (permalink) Old 05-17-2019, 09:34 PM
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Originally Posted by 79broncoowner View Post
It was all the way open. I am staring to wonder if maybe he did not put some piece in the carb. He is 83.
If it's anything like the Motorcraft 2100 I have on my F100 then it's a very very simple carb internally. 4 bbl carbs have a lot more parts inside that could get forgotten or lost during a rebuild.

I say open it up and see what's going on. Maybe he took out the jets and didn't screw them all the way in or something. You never know.
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77 F100 Custom SWB - 302/C4 2wd ~ My Daily Driver
78 Bronco Custom T/S - 351M/NP205/NP435 4wd ~ For Fun
75 F150 Custom LWB - 460/C6 2wd ~ Deceased
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post #7 of 8 (permalink) Old 05-17-2019, 10:03 PM Thread Starter
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He came by the house this evening. He said he ordered a new two barrel carb. I hope this fixes the problem he has a lot of money in this truck and is going to sell it once he gets it complete
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post #8 of 8 (permalink) Old 05-17-2019, 10:24 PM
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He came by the house this evening. He said he ordered a new two barrel carb. I hope this fixes the problem he has a lot of money in this truck and is going to sell it once he gets it complete
Man I sure hope it’s an exact replacement because if not getting al the linkage to fit can eat up a bunch of time. If it is a rebuilt old carb you have about a 50 percent chance of it working right straight out of the box. Tomco’s were the worst, and often all you could get...the joke about them among career mechanics was that the first thing you do before taking it out of the box was you need to position it upright and drop it about two feet onto the floor...That would jiggle the needle and seat and Maybe it wouldn’t flood as soon as you install it... put in many hours (for free of course), repairing and adjusting those. Keep your old carb at least a while until the new one checks out ok...

I have been known to sometimes sing “The Hero of Canton” song from the tv show ‘Firefly’ while making breakfast on a weekend morning....
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