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71 F250 w/ 360ci motor, w/ rebuilt Motorcraft 2100 2bbl off idle bog;

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1967 - 1972 F-100 & Larger F-Series Trucks Discuss the Bumpsides Ford Truck

71 F250 w/ 360ci motor, w/ rebuilt Motorcraft 2100 2bbl off idle bog;

 
  #1  
Old 11-20-2018, 03:58 PM
KTM520EXC
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71 F250 w/ 360ci motor, w/ rebuilt Motorcraft 2100 2bbl off idle bog;

Not the first time I have had issue with this type of carburetor, so here it goes. Truck starts well both hot and cold, runs fine

in mid range and top end, my only issue is transitioning the fuel circuit from and idle and until I can get to ¼ throttle. There is a

bog which feels as if it is fuel starved. I tried to unhook the vacuum advance and it was worse. I do not believe I have any vacuum

leaks, but not ruling it out either.

With truck in (P)ark, I was able to get 20.5 on the vacuum gauge, so what’s next, try to adjust

the accelerator rod, if so which direction ??? BTW, the more I advanced the timing the better it

ran, but there was a point it started to run like crap with the initial advance, so found a sweet spot

and stayed there.

Any help appreciated !!!

Michael

 
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Old 11-20-2018, 04:15 PM
mrpotatohead
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I've experienced this as well, long story short, I had to increase the main jet size by 2 or 3 numbers. I have a long research study that explains why todays fuel has way less BTU content than that of the fuel in 1971. It totally corrected my off idle hesitation and cruising lean surge.
 
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Old 11-20-2018, 06:32 PM
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Originally Posted by KTM520EXC View Post
Not the first time I have had issue with this type of carburetor, so here it goes. Truck starts well both hot and cold, runs fine in mid range and top end, my only issue is transitioning the fuel circuit from and idle and until I can get to ¼ throttle. There is a bog which feels as if it is fuel starved. I tried to unhook the vacuum advance and it was worse. I do not believe I have any vacuum leaks ... With truck in (P)ark, I was able to get 20.5 on the vacuum gauge, so what’s next, try to adjust the accelerator rod, if so which direction ???
If you're pulling 20" of vacuum at idle then the timing is plenty enough advanced, that can be ruled out. In fact, that might be a little too far advanced. Be careful with setting the idle mixture screws. They have a big effect on how the off idle acceleration works. What's called the transition slot and idle circuit is in play for a lot of driving so it has to be just right. Too rich can cause an off idle stumble as well as too lean.

Carburetors are very sensitive to float level. Before making any other adjustments make sure this is correct. What you're actually doing by adjusting the float, is setting the fuel level height in the carb bowl. The whole carb is calibrated by the manufacturer on this being correct at all times under all conditions, and stay there regardless of engine demand. The initial float level adjustment is done on the bench during rebuild, but the fuel level needs checked after installation on the engine. Maybe you've done this already, but it's important to mention. The easiest way to check is let it idle for a few minutes on level ground. Engine should be warmed up. Shut the engine off. Then remove the carburetor horn (4 screws) and move it to the side.

The fuel level in the carb bowl is called "wet height" in the manual, if you can't find this information for your truck try the width of a US quarter or 29/32", from the surface of the fuel to the machined flat surface of the carb. Adjust the tang on the float up or down as necessary to achieve this. It's probably the single most important adjustment on any carburetor.

The accelerator pump rod holes - the top hole (#4) is for temperatures below 40° F. second from top (#3) is for temps 40° to 80°, and third from top (#2) is temps above 80°. 2100s also have inboard and outboard hole positions on the pump lever, the inboard one is a bit more juice. This is where the Shop Manual comes in handy, because while the 2100 carburetor was used on everything, they came in many different Venturi sizes and had several different settings, jet sizes, power valves, etc depending on the specific application.
 
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Old 11-20-2018, 08:45 PM
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Tedster & Mr. Potato Head,

Many Thanks for the responses, OK, so I am speculating that given it is on #2 rod, it is designed to be

a bit leaner for summer and #4 is designed to be a bit richer for winter ? Talk about float bowl 101,

nice tip. This is also applicable for motorcycles, which I have a lot more experience than vehicles and

yes I agree that float bowl level is extremely important, why Ford did not adapt the float bowl adjustment

from Holley, kind of escapes me, much easier and you can access the fuel screws with the Holley.

I just speculated the Company that did the rebuild, set the float height correctly, so probably should have

a look in case moving up to #4 does not work.

Took at look at the BTU ratings on current fuel, 10% ethanol is an eye opener as I am pretty sure my

Sam's Club has it, where I purchase my fuel.

https://en.wikipedia.org/wiki/Gasoli...lon_equivalent

Michael
 
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Old 11-21-2018, 06:34 AM
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Yeah Holley really hit one out of the park with the ability to adjust float height on the fly like that. What's nice about the 2100 design is there's no gaskets for the bowl to leak, and if one is careful, the float height/fuel height can be observed while the engine idles. Another important thing to beware of is the fuel pump pressure, a lot of the parts store replacements sold these days are way off the beam (too high).

The 2100 needs only 4 or 5 psi. The canister-filter type fuel pump I bought as a fresh replacement was running close to 10 consequently the float needed to be hung way down in the bowl to get the fuel height right. Excess pressure wastes gas and trashes the viton-tip needle and will probably flood sooner rather than later. So it's important to get all that stuff baselined and straight before thinking about power valve #s and jet size or pumprod holes & idle mixture settings, because the carb is calibrated by the factory on the assumption that adequate fuel is supplied in the bowl at all times under all conditions, not too high, not too low.

2100 is a really fun carb to mess with and the engine should run really sweet when dialed in. The shop manual carburetor specs are invaluable for setting them up because after several years lots of parts get inadvertantly or advertantly swapped, sometimes without much thought to the overall effect. Venturi booster clusters, missing check ball weights etc. Trucks and maybe other applications also included a primary float spring. The rebuild kits don't supply these for some reason. If Ford thought a .02c part was necessary and didn't leave it out I probably shouldn't either, is my thinking.
 
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Old 11-30-2018, 07:22 PM
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I am not discounting what Ted said by any means. I too think the float level should be checked. And yes, the idle screws also. The idle circuit in a carb is used a lot higher RPM and then transitions away than most people think.
But I think MPH hit it out of the park on the bigger jets. Especially since you mentioned "The company that did the rebuild". It probably doesn't have the jets your engine had originally. Let alone the gas being different now.
 
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Old 11-30-2018, 07:39 PM
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Sure, it could be jet size, but carburetor tuning must be done in a certain order and, fuel height in the bowl is numero uno, because the entire carburetor is calibrated around this. Ever wonder why the manual lists wet height being a fairly precise # like 29/32"? Why not just say 1 inch and call it good?

It's because 1/16" one way or another is equal to maybe one or two jet sizes difference, something like that. So don't worry about jet size until you're certain everything else is straight. Once the mechanic starts compensating for an error in one circuit by an additional tuning error the end result is it won't run as well as it should. Just take one thing at a time, in order is what I was trying to get across.
 
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Old 12-04-2018, 05:10 PM
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Originally Posted by Tedster9 View Post
Sure, it could be jet size, but carburetor tuning must be done in a certain order and, fuel height in the bowl is numero uno, because the entire carburetor is calibrated around this. Ever wonder why the manual lists wet height being a fairly precise # like 29/32"? Why not just say 1 inch and call it good?

It's because 1/16" one way or another is equal to maybe one or two jet sizes difference, something like that. So don't worry about jet size until you're certain everything else is straight. Once the mechanic starts compensating for an error in one circuit by an additional tuning error the end result is it won't run as well as it should. Just take one thing at a time, in order is what I was trying to get across.
Agreed. But while in there checking the float height I am suggesting he looks at the jets and sees which ones are in this REman carb. We all have run across reman carbs that weren't built with the original jets. A 302 carb and a 429 2V might have been soaked at the same time by a rebuilder.
 
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Old 05-15-2019, 05:53 PM
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OK bringing this thread back up, as I have been working on other projects and finally pulled the rebuilt carburetor and accessed the main jets.

So I have a pair of #55F main jets, so speculating F stands for Ford vs. Holley. There was a bunch of residual black stuff at the bottom and the fuel

smelled like bad gas, had a look see through the jets and they seemed fine, no obstructions. So is the 1st thing to try to do is go with #58's, #60's etc ...

Oh ya, the venturi marking on the side is 1.21, is there anything on the venturi's to confirm it is a 1.21 ???


BTW, did a compression check on engine to make sure everything was good and it is, just wanted to eliminate the obvious before getting in any deeper.

Best reading was 127psi, worst was 118psi and all the other cylinders fell within those parameters and average PSI was 123.

Michael
 
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Old 05-16-2019, 02:41 AM
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The compression levels sound good. A good rule of thumb is to increase the jet size in increments of 2. So that you don't go too far in one change. When you rebuild the carb, don't forget to change the acelleration valve.
 
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Old 05-16-2019, 07:02 AM
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Is this particular 1.21" venturi carb the stock OEM carb? My sense is "no", but, I don't know. Can make it work, but it may not be ideal. The most important carb "adjustment" is choose the right carburetor.

Have to look at your spark plugs carefully. Each carb circuit is tuned or adjusted individually. Off idle bog is a symptom of the idle circuit, accelerator pump circuit, transition slot, etc. Post clear close up picture of plugs. Make certain ignition system and distributor advance ignition timing is 100% before making carburetor changes.
 
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Old 05-16-2019, 07:55 AM
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Q- For KTM520 Where were you pulling vacume from ? I was going to check mine and saw no place to tie in a vacum gauage . Tried the hose to the distributer and got 0 untill I reved it up .
 
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Old 05-16-2019, 11:34 AM
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TenaciousT, at the carb, passenger side.

Michael
 
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Old 05-16-2019, 11:37 AM
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Tedster, no not the stock carb, rebuilt from a supplier off of RockAuto.

Michael
 
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Old 05-16-2019, 11:54 AM
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What size venturi did that originally ship with? Shop manual will have this info. That's not to say it can't be made to work with a 1.21" size carburetor, but in general too large of a carburetor will actually run lean.

Increasing jet size to compensate for this may get rid of a hesitation or bog just off idle, but then overall it will run pig rich. Set your jetting first on the highway, and then leave it alone, figure out what the acceleration pump shot needs to be. Truck engines designed to haul heavy loads always had smaller carburetors from the factory. Small venturi = high velocity air flow = better fuel atomization, very important for just off idle acceleration, where most street driving is done.
 

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