I decided to write this after my experiences with converting from the Feedback Carburetor. The information is out there, but not all in one place and I had to do some digging to get all the information to complete my project. I'm sure I missed something in this guide, but it does get everything in one place. If anyone sees any obvious errors or has suggestions, PM me and I'll edit this post. The EVAP section will be updated shortly, once I complete the process on my vehicle.
Disclaimers: This is the process I used on my 1985 F150 4.9. Your application may be different. I used a DUI system as I could not locate a DS2 harness.
Carburetor / Ignition Conversion from the Feedback System
Carburetor: National Carburetor #1506 (Carter YFA)
Distributor: Davis Unified Ignition
Plug Wires: Davis Unified Ignition Live Wires
Gaskets: EGR Plate base gasket and carburetor gasket
Choke Tube: Choke Tube Kit (to relocate choke tube)
12v-40 Relay and pigtail connector: For Ignition Relay (if needed) (see that section)
Replacement carburetor has choke heat tube fitting on driver side. Original carburetor has inlet on the passenger side. (Make new choke tube.)
DUI Distributor requires full 12v, not supplied by factory hot wire. (Install ignition relay)
Throttle Cable may be too long for proper operation with the new carburetor. (fabricating a new bracket seems to be the best option here)
Normal Hand Tools, Tubing Bender for choke tube
i. Remove old carburetor and gasket. On mine this just required a ½” wrench
ii. Remove EGR plate gasket (gasket under EGR spacer)
iii. Now is the time to remove the smog pump and / or EGR valve if desired. (See those sections for specific information)
iv. Replace EGR plate gasket, carburetor gasket and bolt down new carburetor.
v. Make & Install ‘new’ choke heater tube (don’t forget to connect power to choke also)
vi. Install Choke Fresh Air line and Bowl Vent line (See EVAP). Also plug any unused vacuum ports (you will need to a ported vacuum outlet)
vii. Pull distributor cap and note location of rotor. Remove plug wires (note locations) and remove distributor. Be sure to note where the rotor is as it clears the engine. Also be careful here, the oil pump drive shaft may come out as well and drop in the oil pan. Just be slow and steady; the shaft should be held in by a clip but not always..
viii. Install new distributor, making sure rotor lines up in the same location as the original. Line up rotor with location it was in as it cleared the engine. Due to the helical gear, it will be different than the location when its installed.
ix. Mount and connect coil relay (see subsection – Ignition Relay)
x. Remove Spark Plugs and Re-Gap to .050 to .055. Re-install plugs.
xi. Connect spark plug wires (you did note the positions before pulling distributor and wires, right? Important for firing order)
xii. Connect ported vacuum source from the carburetor to the distributor vacuum advance.
Smog (Air) Pump Removal
On my vehicle, this was a fairly straight forward process, but other vehicles may differ somewhat. In my case, the smog pump was driven by an independent belt. In other trucks, it may require a pulley to replace the pump, in order to keep the original belts, or you may need to get a shorter belt. With that in mind, read on.
If you plan on removing the computer, this system won’t function without modification as it is controlled by the computer; hence the removal. If you still have catalytic converters, this system is needed for them. Also, if you have emissions testing, this system may be needed. If either of those are the case, you can leave the computer installed to control this system. If leaving the computer, it can also control the EGR and EVAP systems if desired.
¾’ Plug, Pipe Thread
Normal Hand tools, Vise Grips, C-Clamp, 2 small pieces of wood, tubing cutter
The Process: i. Remove the hose from the air pump to the EGR plate
ii. Remove the flexible hose from the air pump to the exhaust tube
iii. Remove the air pump and belt (install pulley or shorter belt if needed).
iv. Remove the valve assembly with the hoses attached, disconnect the vacuum lines from the valve (these control lines feed into the harness and go to the solenoid bank on the valve cover)
v. Seal the exhaust tube (like folding over toothpaste tube.)
a. Cut the tube using the tubing cutter
b. Start to flatten a section of tube using the vise grips / c-clamp
c. Put a small piece of wood on either side of the partially flattened tube and clamp to fully flatten it
d. Bend tube over to seal
vi. Plug the hole in the EGR plate that was left after removal of the air hose and valve. This is a ¾” Pipe thread, so a plug can be had easily.
In my case, the EGR valve was bad and I decided to delete the EGR system. This system is computer controlled but can be converted to use vacuum control. If you decide to delete the EGR system, read on. To keep the EGR and make it run on vacuum instead of computer control, see the EVAP section note.
To delete the EGR, you will need an EGR gasket and will need to buy or make a block-off plate (use the gasket as a template). In any case, you’ll need to bolt a plate where the EGR valve was to block the ports.
There will be a line coming from the exhaust and going to the EGR plate, just under where the valve mounted. It will pretty much be frozen in place. You can attempt to remove it, but don’t need to as the blocking plate will seal the exhaust. By leaving it in place, it’s a simple process to re-enable the EGR should you decide to.
Evaporative Emissions System (EVAP)
For those that don’t know, the EVAP system is in place to deal with fumes and venting the gas tank pressure. When the engine bay, or fuel tanks get warm, or fuel is sloshed around, pressure builds as do fumes. To deal with this, enter the EVAP system. It consists of a charcoal canister that is connected to the fuel tank and carburetor bowl vent. Fumes are filtered by the charcoal canister and later purged and burned. We’ll get into more detail on that later, but understand this is one system you want to keep. To do so, it will need to be converted from computer to vacuum control (if you are getting rid of the computer). It helps to understand how the system works in order to understand the conversion.
When fuel in the tank is heated (driving, sitting in the sun, etc) pressure is built. There are also fumes in the tank to deal with. Rather than vent this to the atmosphere, we use the EVAP system. A similar situation occurs with the carburetor bowl vent. While the engine is running, it’s not an issue. It comes into play when the engine is shut down and warm from either having been running or from the sun. Without the EVAP system, those fumes vent to the air, and you smell fuel.
The bowl vent is routed to the EVAP charcoal canister where fumes are ‘trapped’ until purged. We don’t want this to happen unless the engine is off and warm, so there is a thermal valve in the line that controls this. There is also an electronic solenoid in the line that is open when the engine is off, to allow venting if the valve is also open. When the engine is on, the solenoid closes to prevent the purge vacuum from pulling from the bowl vent instead of the charcoal canister. The purge vacuum is supplied as a line from the PCV valve, also controlled by an electronic solenoid. That device doesn’t allow purge vacuum to be applied unless the engine is warm. That also is computer controlled.
So what to do if you are getting rid of the computer? You get rid of the solenoids and go old school.
EVAP Conversion / EGR (See Part 2)
The connections coming from the oil pressure sender and engine temperature sender are part of the computer harness. If you are removing that, you need a way to get the gauges to work.
There will be a round connector on the driver side fender well that has 4 wires. One is larger than the others and this is the coil HOT wire that will be used for the ignition. Two other wires will go to the temp and oil gauges.
The connector needed to plug into this port is ‘Standard S-698’. I bought mine from Ebay. I then found I could use the existing plug from the old harness. I just unraveled the harness and separated the oil / temp wires with the plug and cut the other two wires on the plug (left them long enough to pigtail).
When using the DUI distributor, its coil needs full alternator voltage. I used the KEY ON power at the existing 4-wire plug that I used for the gauges. If that doesn't work for you;
Mount a 12v-40a relay to the firewall (I put it on the driver side fender well). Connect the old coil HOT wire (the larger wire on the round, 4 wire plug) to the ‘trigger’ input of the relay. Connect the input side of the relay to the battery, and the switched output to the coil positive on the distributor and ground the relay. I made a simple harness running from the NEG battery and POS side of the starter solenoid, across to the relay. Then I just connected the old coil HOT to the relay for the trigger and ran the output of the relay to the distributor coil. Use 12g wire here. The DUI distributor came with the connectors it needed.
The theory of operation is simple. When the key is on, the coil HOT wire will have between 10 and 12v. Mine didn't have a ballast resistor, but the voltage does drop during cranking. While this won’t work for the DUI coil, it WILL work to trigger a relay, which when active will switch and connect the distributor to the battery source.
Ran out of space in the first post and will need to break it into sections.
To keep the EGR and convert it to vacuum operated, just run a vacuum line from the ported vacuum switch (may need to T off the output) to the EGR valve.
The EGR valve controls when exhaust gases are recirculated to be burned. We only want this to happen when the engine is warmed up and above idle at cruise but not when at WOT. Ported vacuum only exists above idle and below WOT so there you go.
The vacuum switch only 'turns on' the vacuum to the EGR valve when the engine is warmed up, and since the source vacuum is ported , the EGR won't open up unless the engine is both warm and above idle and shuts down at WOT.
Pretty simple really.
When an EGR valve goes bad, it can be stuck closed, open or leak, If stuck closed, it's like it's not there so no big deal.
Stuck open and it will allow exhaust gas into the intake at the wrong times; causing rough idle, issues with fuel / air mixture, etc.
If it's leaking, it is a huge vacuum leak.
For me, it was just simpler to remove it. If I start to get pinging or other issues, I'll just replace the block off plate with e new valve and operate it as stated here with vacuum.
my 83 f150 xlt 4x4 has no egr system with the carter yfa carb runs better then it has new new starter and head and intake gaskets valve cover gasket removed all the air and vaccum lines and modded the kickdown on the trans run it wide open and have fun lol
Was curious if you have increased the spark plug gap after installing the d.u.i. . I increased mine today to .50 but haven't had time to put any miles on it yet to test driveability but did notice that I may have to increase my idle? Any info regarding this would be appreciated
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