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Discussion Starter #1
So here's the skinny. 85 Bronco with a 300/np435. 270 duration cam, 4bbl 390 holley with clifford intake and headers. Was converted to a duraspark2 distributor. All this was done by the PO. I believe this engine was rebuilt 25'000 miles ago (in 2002) and it does have .040 oversized pistons. Has both the mechanical fuel pump and a small Carter electrical pump.

Since then I've added a bigger fuel filter and -6AN lines from the electric pump to the mechanical pump and to the carb. I just put new motorcraft copper core plugs in because the old ones were showing signs of running lean as a temporary fix. Realized I didn't have a plug gapper since the fire, so I used a set of calipers to check the gaps and they were all .042- .044.

Lately it's been running worse and worse with the increasing humidity and heat. It acts like it's running lean, where as last summer I had no problems. The carb is still tuned to Colorado elevation and I'm at 1000ft above sea level, but like I said, I've had this Bronco over a year and not had a problem. It has been popping out the exhaust erraticly for a while when I'm not on the throttle but not idling. Today it was still doing this and at lunch it tried dying on me. Idle was very low for what I normally see. 400-700 compared to the normal 1200-1500. Pumping the brakes at a stop in neutral would raise the rpms back up to normal range.

Then it died on me after work and barely wanted to start back up. Going down the highway was fine for a bit but then rpms started dropping and I had very little power. I'd let off the throttle a bit and some power would come back but not all. Then it would get back up to rpm and power on. It did this repeatedly. Once I hit the dirt roads it didnt do this again.

I'm pretty lost because I've never tuned a carb (ya ya i know I'm a youngin) and I feel that this may be more than that. I won't be able to work on it till Saturday morning so any help would be greatly appreciated. I don't know what jets or anything the carb has but it's a 4160 model. I was planning on tinkering with the carb this weekend already so it's not a huge deal. I do have another vehicle so downtime isn't an issue either. I don't have a fuel pressure gauge yet, but will be getting one.

Help!:whiteflag
 

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I had some issues with my 84 300 running bad and I thought it was the carb too. Similar symptoms to what you describe. Turned out to be the coil.

My set up is 2150 carb, converted to Duraspark II, dual exhaust manifold from a 4.9, stock cam and running both mechanical and electrical fuel pumps.
 

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Might have a brake booster starting to leak or maybe a faulty booster check valve. I would also check for other vacuum leaks before blaming the carb.
 

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Discussion Starter #4
The coil is old and I was planning on replacing it with the cap and rotor and plug wires. I'll check it out.

I visually checked very quickly for any unhooked vacuum lines and didn't see any, but a more detailed inspection is in order. Is there any way to test the brake booster check valve?

Thanks for the help guys
 

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You can cap the tubing going to the booster and see what it does. If you drive it like that just don't take it very far or fast because it won't brake very well at all. If you have a vacuum pump you could adapt it to the brake booster port. Pull it down to 18" and see if it holds.
 

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Scoop beat me to it. It sounds like your coil. Those are the symptoms I had. :D

Too often, we turn our attention directly to the carb when there's an issue and it usually isn't the culprit. Holleys don't have springs and metering rods like an Edelbrock so it's very difficult for them to go out of tune since very little but the butterfly plates actually move. That is to say that you don't have something junked up and plugged in side of it, but that shouldn't be the case if you've had a good filter on it.

For the brake booster, do what 460bronc suggested and just disconnect the vacuum line and cap it off. Your brakes will still work (just don't forget to push harder than normal). Although I don't recommend it if you have any sort of heavy traffic, I've driven on the interstate without issues with the brake booster disconnected. However, our interstate is pretty near dead at the time of day that I went.


Otherwise, you can pull the carb apart and clean it. You might have some junk in it somewhere.
For the main jets, you generally drop 1 jet size for every 2000 feet over sea level. So, you are probably only 1 or 2 jets leaner than ideal, which isn't much. I often make that sort of jet change when I just want to lean it out a tad for gas mileage, so you're not hurting there. :)
 

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Discussion Starter #7
You can cap the tubing going to the booster and see what it does. If you drive it like that just don't take it very far or fast because it won't brake very well at all. If you have a vacuum pump you could adapt it to the brake booster port. Pull it down to 18" and see if it holds.
I do have a vacuum pump and will try that, thanks. Im in the middle of nowhere so traffic isn't a concern and I can downshift if I need to. Drove my Stang with no brakes 25 miles through the busy part if town and down the highway. Just gotta control rpms and pay attention.
 

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Discussion Starter #8
AB, I think the PO had issue tuning it to begin with, that's why it has the electric pump. Its always ran lean, just not enough for me to tear into it in a hurry since I'm a carb noob.

The old fuel filter was one of those cute little see thru things right before the carb with a plastic element. Now it's got a 7" by 2"dia r2c filter on it with a huge 100micron stainless element. Is 100microns too big? It's made for dirt track racing so I assumed it was a good choice for my super dusty application
 

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Are you sure you only have one fuel filter? My '81 had two of them and I didn't know about the second one for a while. It's only a few feet from the fuel tank.

Otherwise, you need to find out what your carburetor is currently installed with. If you don't know what's currently in it, you won't know what's off or what to change. You'll need to pull the primary fuel bowl off and see what size of jets are currently in it, as well as what the current power valve is.

Also, a quick check you can do right now is to take the air filter off and look down the primary throat while it's running. If you start moving the throttle, do you get an IMMEDIATE squirt of fuel? There should be absolutely no delay whatsoever. If you do, your accelerator pump/cam isn't set up right and you'll have lean surges on acceleration. (However, that wouldn't affect cruise or idle.)

I also just noticed that you said 1000 - 1250 RPMs was normal and 500 - 700 is abnormal. That should be the other way around. You should be idling around 650 - 750.
 

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Discussion Starter #10
Are you sure you only have one fuel filter? My '81 had two of them and I didn't know about the second one for a while. It's only a few feet from the fuel tank.

Otherwise, you need to find out what your carburetor is currently installed with. If you don't know what's currently in it, you won't know what's off or what to change. You'll need to pull the primary fuel bowl off and see what size of jets are currently in it, as well as what the current power valve is.

Also, a quick check you can do right now is to take the air filter off and look down the primary throat while it's running. If you start moving the throttle, do you get an IMMEDIATE squirt of fuel? There should be absolutely no delay whatsoever. If you do, your accelerator pump/cam isn't set up right and you'll have lean surges on acceleration. (However, that wouldn't affect cruise or idle.)

I also just noticed that you said 1000 - 1250 RPMs was normal and 500 - 700 is abnormal. That should be the other way around. You should be idling around 650 - 750.
You are correct, I have no clue how it's set up so I can't buy any new stuff without tearing into it first. I have read about the accelerator pump test and will do it.

Yes what I stated for idle is what I'm experiencing. Once warm it idles around 1000 but not below ( before I started having issues). I know it should idle at 750ish. That's how I always knew something was wrong. It ran great, but got horrible milage, maybe 8. Is this just adjusted on the carb? I need to get a timing light and check that as well.

I checked the lines on the frame when I installed the filter and new lines from the electric pump forward. It's about midway up the frame. I didn't see the usual canister type fuel filter that I'm used to seeing on fords. I'll look harder and replace it if I find it. This Bronco had dirt caked in the rear cross members and frame and I've pulled two dry cacti from the undercarriage so I'm sure that filter would be toast.

Thanks again, I'm gonna dig into it in the morning.
 

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Hah, yeah, it sounds like you probably would have found the filter under there if that was the case. :D

For the idle, that is set on the carburetor. The screw is where the accelerator cable connects to the carb:



If you CAN'T get the idle to come down, you may have a vacuum leak. Another way to test this is to turn your idle mixture screws on the metering block all the way in. It should easily kill the engine.

But, you should be able to drop it down pretty easily by turning the screw outward. Then, set your idle mixture screws on the metering block with a vacuum gauge connected to the engine (direct manifold vacuum). You want to turn the mixture screws in slowly until the vacuum just barely starts to drop, and then back the screw out slowly until the vacuum peaks. Once you've adjusted both screws, count how many turns it takes to screw them all the way in and then average them. So, if one is 9/16 of a turn out and the other is 1 15/16, set them to 1 1/4 turns each. Readjust slightly as needed.

As for the coil, you can test it with a multimeter:

 

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Discussion Starter #12
Got a new coil and pcv valve today, but couldn't get a distributor cap or rotor. I took them both to advance and started with a 76 f150 model, and the rotor was too short horizontally to work. The 78 and up was very different looking. They didn't have any caps, but have one of each coming in this afternoon for me to look at and compare.

How do I tell what year my duraspark2 came out of??

BTW my cap terminals were toast and the rotor was pretty burnt up so I know this was not helping my situation

Edit: my distributor has a short bare wire in it that looks pretty dark in color and some reddish crumbs in it. It does have a pertronix ignitor in it too.
 

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Interesting, I didn't know there were any differences throughout the DSII years.

If you want to be absolutely positive, DSII distributors really aren't very expensive. I think I got one for about $50. The $80 came with the distributor, cap, and rotor at my local O'Reilly.

Otherwise, I'm not sure how to tell exactly which one you have. Where did you originally get it from?
 

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Discussion Starter #14
It was on the engine when I bought it. I don't recall seeing any info about it in the stack of paperwork that came with it.

I did get my carb torn apart. I've got no. 49 jets and the power valve has a 5 stamped on it and another 5 stamped 90° around the valve. So this means manifold vacuum should be around 10 or I'll have to change it out, correct?

The discharge nozzle has a 25 stamped on it. (In the main bbls, looks like a cylinder with two little feet) overall the carb was pretty clean inside and I saw almost no varnishing.
 

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The stock jet on the 390 is a 51. Being 2 sizes under isn't going to hurt much, and probably get you better gas mileage. Looks like you're at 1100 feet in Hoyt, so you'd probably be just fine with the stock jetting if you wanted to try it, but 49 is no where near off enough to cause any issues. I generally drop a few sizes anyway since Holleys run a tad rich stock.

For the power valve, that means it's a 5.5 and opens at 5.5hg of engine vacuum. Stock is a 6.5. Do you know what your vacuum is at idle? The smaller the number, the later it opens.

A discharge nozzle of 25 is usually too small for my liking, but it depends on the vehicle. If you notice a hesitation off the line, then you might want it a little bigger. I ran a 32 or 35 in mine, but I also have 3.00 gears (the lower the gears, the more gas it takes to get them moving).


Otherwise, none of those are really all that far off base, and shouldn't be causing you any serious drivability issues. The accelerator pump squirter can cause hesitations and stumbles from a stop, or when you get on the gas, but that's about it.
 

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Discussion Starter #16
It does like to hesitate off the line. I have a set of 51, 52, and 53 jets in my cart on summit. I don't know what my vacuum is, but have read the divide by two if it's under ten and if it's over ten use a 6.5. I've got both a fuel pressure and vacuum gauge on the way. I was curious about stock settings so thanks for that info.

I've noticed some of the discharge nozzles have little brass tubes to shoot the fuel and mine does not. Is this just a difference in sizes or something else? I did notice that the accelerator pump test did shoot immediately, so my cam and armature should be fine I assume.

I was lucky to find the blue gaskets inside the carb. What kind of rebuild kit should I get? They seem very generic but range from 40-120 bucks.
 

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Discussion Starter #17
Interesting, I didn't know there were any differences throughout the DSII years.

If you want to be absolutely positive, DSII distributors really aren't very expensive. I think I got one for about $50. The $80 came with the distributor, cap, and rotor at my local O'Reilly.

Otherwise, I'm not sure how to tell exactly which one you have. Where did you originally get it from?
You are correct, the two caps that came in were completely different. One matched mine 99.9%. It was from a 76 f150. The one that was supposed to be for a 78 looked like the cap from my 94 with the male/hei style terminals.

Looks like I'll be saying 76 f150 for dizzy parts from now on
 

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That's bizarre. From what I've known, 76 - 83 were all exactly the same when it came to the DSII. 84 and on, they went to the smaller cap size when they went computerized. Oh well, as long as it fits and works.

You'll notice that when you richen up your main jets that a lot of drivability issues will go away. Leaning them out is good for gas mileage, but you really have to fine tune everything to get it to work together right. Richening it all up just sorta blanket covers everything and makes it all cushy and easy. =P But... your gas mileage goes down a little. Since I put 5000 - 10000 miles a year on mine, I work a little harder on the gas mileage department. But, if yours is a weekend warrior / play toy, then richening up your main jets is probably what you want to do.

The different styles of squirters is basically a quality thing. The one with the two little tubes sticking out is going to give you a finer stream that gets the fuel further down into the venturi and gives it a finer mist of fuel. But, they cost twice as much. They're something you want to look into if you just really want that crisp, perfect precision. But, for an every day driver, you'll probably never notice the difference without them there. Definitely run them if you want them, but don't sweat it if you don't.

On the power valve, that sounds about right. On mine, if I really lean out the main jets, I'll throw in a larger power valve so that it covers the lean hole quicker, but on a normal engine, 6.5 is a good, safe bet.

On a rebuild kit, if your carb is in good shape, I'd get the least expensive rebuild kit. It'll just come with all the standard gaskets and you'll be able to freshen it up.
One thing you'll run into with the Holley 390 is the secondary diaphragm. ALL rebuild kits are tailored for the 600cfm carbs, so the secondary diaphragm that comes with it is too long. I don't remember the size (I think it says it on the arm), but you'll have to order it separately.

Before you order any parts, check out allcarbs.com. You'll notice that almost everything is about half as expensive as anywhere else.

For example:

PUMP NOZZLE TUNING KIT

HOLLEY MAIN JETS
 

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Discussion Starter #19
Well I got the carb back together and finally put on the Bronco (been sick for a couple weeks). Went with 51 jets and a 31 discharge nozzle. 6.5 power valve. The idle screw is adjusted all the way to where its no touching the linkages and the idle mixture screws are turned all the way in and it still idles at 1500rpm.

I'm gathering this is a vacuum leak, and as I've already replaced two lines, I'm planning on replacing the other couple. If this doesn't help, where should I check next?
 

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Yo,
Did you test vacuum level with the gauge?
Vacuum Gauge Readings


Could not find high altitude vacuum diagram, call local dealer parts & ask for one.

1985-86 4.9L engine


985-86 4.9L engine

Vacuum Acronyms: not all apply to your year:
TAB is the Thermactor Air Bypass Solenoid
TAD is the Thermactor Air Diverter Solenoid
ACV (Air Control Valve) is the TAD (Thermactor Air Diverter) Valve (AIR Bypass (AIRB) and AIR Diverter (AIRD) valve combinations);
MAN VAC is Manifold Vacuum;
FPR is Fuel Pressure Regulator;
EGR is Exhaust Gas Re-circulation Valve and/ or EVR is EGR Vacuum Regulator
SOL V: Thermactor Air Diverter (TAD) Solenoid (SOL V) (AIRD) & Thermactor Air Bypass (TAB) Solenoid;
VRESER is the Vacuum Reservoir Can (Tank);
V REST on later years is Vapor Valve (Roll-Over);
V REST on earlier years such as SEABRONC's 83 is a Vacuum Restictor/delay valve, it delays vacuum for a certain amount of time;
Carbon Can is Carbon Canister (Charcoal Canister, Vapor Canister, Evaporative Emission (EVAP) Canister);
MAP is Manifold Absolute Pressure Sensor;
AIR BPV is the AIR Bypass (AIRB) Valve, also called the TAB (Thermactor Air Bypass) Valve;
CPRV is Canister Purge Solenoid Valve/ Canister Purge Solenoid (CANP);
VCKV is Vacuum Check Valve; CAT is catalytic converter;
ENG is engine;
PCV is Positive Crankcase Ventilation Valve
MAN VAC is Manifold Vacuum
CAT is the catalytic converter
ENG is the engine

Acronyms, Carbureted
(not nec. for your year/engine)
A/CL: Located in the air cleaner
A/CL DV: Air Cleaner Diverter Valve
A/CL BI MET: Air Cleaner Bi-Metallic Valve
A/CL CWM: Air Cleaner Cold Weather Modulator
ACT: Air Charge Temperature Sensor, prior to 1992
ACTS: Air Cleaner Temperature Sensor
ACV: Air Control Valve
AIR: Thermactor, short for Thermal Reactor. CA vehicles are installed with it as standard. Air Injection Reaction is what it stands for.
AIR BPV: AIR BPV is the AIR Bypass (AIRB) Valve; also called the TAB (Thermactor Air Bypass) Valve
BV: Bowl Vent (on top of the float tanks)
CARB: Carburetor
CBD: Closed Bowl Distributor
CPRV: Canister Purge Valve
DIST: Distributor
DPFE: Differential Pressure Feedback
DV-TW:Two Way Delay Valve
EFCA: Electronic Fuel Control Assembly/ (Fuel Pressure Regulator (FPR)
EGR: Exhaust Gas Recirculation
EGRC/EGRV: EGR Vent/EGR Control
EXH HCV: Exhaust Heat Control Valve
EVP: EGR Valve Position
EGR: Valve Position
EVR: EGR Vacuum Regulator; also called EGR Vacuum Solenoid
FLTR: Filter
FPR: Fuel Pressure Regulator (check vacuum line)
HEAT VLV INT: Heat Vacuum Control Valve Intake, (next to carb)
HICV: Hot Idle Control Valve
IAT: Intake Air Temperature Sensor (Air Charge Temperature (ACT), prior to 1992
IVV: Thermactor Idle Vacuum Valve
LCV: EGR Load Control
MAN VAC: Manifold vacuum-Vacuum source; usually ref. to as Vacuum Tree atop intake manifold]
MAP: Manifold Absolute Pressure
PCV: Positive Crankcase Ventilation
PURGE: Vapor Canister Purge Valve
PV: Ported Vacuum
PVS: Ported Vacuum Switch (PVS) Valve
S: Spark port
SA-FV: Separator Assembly Fuel/Vacuum
SOLV: Thermactor Solenoid Valve
SV CBV: Carborator Fuel Bowl Solenoid Vent Valve
TCAC: Thermostatically Controlled Air Cleaner
TCP: Temperature Compensated Accelerator Pump
TK: Throttle Kicker
TVS: Throttle Valve Solenoid (aka Solepot, Dashpot)
or
TVS: Temperature Vacuum Switch
TVV: Thermal Vacuum Valve
or, TVV: Thermostatic Vacuum Valve (aka Ported Vacuum Switches; Thermal Vacuum Valve)
VAC: Vacuum
VCKV: Vacuum Check Valve
VCV: Vacuum Control Valve
VDV: Vacuum Delay Valve
V CK V or VCV: Vacuum Check Valve
VOTM: Vacuum Operated Throttle Modulator
VRDV: Distributor Vacuum Delay Valve (VDV, VRDV, DV-TW)
VRDV: Vacuum Delay Valve (VDV, VRDV, DV-TW)
VRESER: Vacuum Reservoir (coffee can style in earlier years; replaced by plastic style)
VRESV REST: on earlier years such as SEABRONC's 83 is a Vacuum Restictor/delay valve, it delays vacuum for a certain amount of time
VRV: Vacuum Regulator Valve
WOT: Wide Open Throttle Valve
....

"A detailed examination of all connectors, wiring and vacuum hoses can often lead to a repair without further diagnosis. Performance of this step relies on the skill of the technician performing it; a careful inspector will check the undersides of hoses as well as the integrity of hard-to-reach hoses blocked by the air cleaner or other components. Wiring should be checked carefully for any sign of strain, burning, crimping or terminal pull-out from a connector.

Checking connectors at components or in harnesses is required; usually, pushing them together will reveal a loose fit. Pay particular attention to ground circuits, making sure they are not loose or corroded. Remember to inspect connectors and hose fittings at components not mounted on the engine." by Auto Repair Reference Center
 
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