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Drivability Issue - Autolite/Motorcraft 2100 Carburetor?

nosaj2k

Full Member
Joined
Dec 19, 2008
Messages
226
Loc.
Green River, WY
Alright fellas,

I realize diagnosing drivability issues via the web is difficult at best, especially given the numerous variables potentially involved. But I’ll go against conventional thought here anyway. I have a 1970 Bronco w/a relatively stock 302. I say relatively stock because I suspect the PO had the engine rebuilt/installed with a slightly larger than stock camshaft. Additionally, I am running a Pertronix distributor. I am guessing the engine has roughly between 10,000 to 40,000 miles. Carburetion is handled by a recently rebuilt Autolite/Motorcraft two-barrel 1.08 venturi 2100 carburetor. Gearing is 3.50:1, and I’m running 33” tires (I know). The issue I am having occurs under normal, low key street driving conditions. Just off-idle, the Bronco tends to stumble, or hold back for a second before beginning to accelerate. This also occurs upon acceleration immediately after shifting (manual trans). This issue was present prior to the carb rebuild - In fact I have had this same problem with two other 2100 carbs (so you're telling me it's not the carb?). I am currently running 46 jets (6200’ in elevation), and have moved the accelerator pump rod to the top hole in the over travel arm. This problem tends to get better as the engine warms, but even at full operating temperature it's still very apparent. Aside from this annoyance, the thing runs EXCELLENT. Oh, and no vacuum leaks to speak of.

Any thoughts? Thank you in advance.
 
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phred

Contributor
Bronco Guru
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Aug 25, 2006
Messages
3,432
Loc.
Earth
Check and make sure the plug wires are good, plugs are not fouled, dizzy cap is good and not cracked. I has similer issue with an old ford truck and after rebuilding the carb 3 times I changed plug wires and the issue was gone.
 
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nosaj2k

nosaj2k

Full Member
Joined
Dec 19, 2008
Messages
226
Loc.
Green River, WY
Check and make sure the plug wires are good, plugs are not fouled, dizzy cap is good and not cracked. I has similer issue with an old ford truck and after rebuilding the carb 3 times I changed plug wires and the issue was gone.

Thanks for the info phred - I just installed new Taylor 8mm plug wires, Bosch spark plugs (gapped to accomodate electronic ignition), and a new cap. The problem persisted prior to these modifications :(
 
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ryansbronco

Bronco Guru
Joined
Mar 6, 2002
Messages
1,160
Loc.
The place of bronco
couple thoughts

try putting the old wires back on

check for vacuum leaks

check the vacuum advance on the dizzy

check to see if the dizzy is loose

check the trimming

pull the #1 spark plug and see if it is discolored or looking rich
 
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nosaj2k

nosaj2k

Full Member
Joined
Dec 19, 2008
Messages
226
Loc.
Green River, WY
couple thoughts

try putting the old wires back on

check for vacuum leaks

check the vacuum advance on the dizzy

check to see if the dizzy is loose

check the trimming

pull the #1 spark plug and see if it is discolored or looking rich

Ahh - The distributor vacuum advance. You might be on to something? How can I check this? I did recurve the distributor prior to installation with the idea of achieving full timing advance sooner. I believe I set the initial timing at 12, with full timing at around 38 degrees coming in at 2000 rpm (if my memory serves me correctly)? I wonder if the vacuum advance isn't providing the needed (if any) advance prior to the centrifigal advance?
 
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red hot71

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Joined
Nov 26, 2009
Messages
648
Loc.
kent wash.
I'd change the power valve,at 6200 ft. I think you drop it 2 points. The reason I say this is, Ford when they sold cars in lets say Denver the only difference was the power valve. Ford didn't mark there valves with a # but color coded,I think 12 was stock and a 10 was higher altitudes. I don't think the rebuild kits don't take this in to account and if they come with a power valve it may be a sea leval thing.

Holley PVs fit but I think you have to modify the carb were they screw in. Or these guys https://buy.walkerproducts.com/carburetor/components/ford/ have them.

Something to look into.
 
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nosaj2k

nosaj2k

Full Member
Joined
Dec 19, 2008
Messages
226
Loc.
Green River, WY
Thanks for all of the input guys.

ryansbronco - It looks like the vacuum advance is working properly at the distributor - I will verify vacuum at the carburetor port.

red hot71 – Very good suggestion. You are probably correct about the power valves included in the carb kits – they probably are for sea level. I’ll definitely look into it.

Jms5580 – I am pulling 16.7 Hg of vacuum at idle.
 
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nosaj2k

nosaj2k

Full Member
Joined
Dec 19, 2008
Messages
226
Loc.
Green River, WY
long shot
are you running an electric choke? stock manual choke? do you have a heat riser? or no choke at all?

I opted to replace the stock style heat choke with an aftermarket electric version. I did this because I no longer had the heat connection from the exhaust manifold to make the stock heat choke work correctly. The alternator stator post supplies the power for the electric choke.
 

ryansbronco

Bronco Guru
Joined
Mar 6, 2002
Messages
1,160
Loc.
The place of bronco
it may not be adjusted correctly, or could be sticking. the choke plate is a tight fit sometimes. Wire the choke flap open and test drive.
try when its warmer outside. with the choke wired open they tend to take longer to get to idle when its cold out side. I run no choke all year round. I too have headers. this kit can be used to run the factory choke. http://www.dormanproducts.com/p-649-03840.aspx
Im too lazy to install it. ;D
 

red hot71

Sr. Member
Joined
Nov 26, 2009
Messages
648
Loc.
kent wash.
The after market electric choke is that a ford replacement or is it a holley style? Fords take 6-7V from your stator,but the holley uses a full 12V switched ingition.

The ford electric chokes are a heat assisted, you still need stove heat. The holley are stand alone.
 

72_EB

Contributor
66to77
Joined
Feb 4, 2003
Messages
4,962
You usually won't get a solid 12v off the stator terminal on the stock alternator.

Put the choke power aside for now and just wire open you choke plate. See if that solves it.

TI don't believe it's a Holley unit, but maybe it does require 12 volts?
 
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