I picked up this '85 f150 with the 300 recently. I figured I was going to have to do the duraspark swap because I despise the feedback carb and the EEC system. Well to my suprise, the truck had an older industrial 300 in it with duraspark already installed. I looked closer and it still has the feedback carb on it with the TPS and all the rest of the plugs cut off. The computer and all related wiring had been taken out too.

So my question is, With duraspark as the ignition system, how is that feedback carb doing its job right? The truck seems like it's very sluggish. I did the duraspark swap on an identical truck a couple of years ago, but I switched carbs at the same time and everything was fine.

Is the feedback carb the issue?

Thanks!

 

not sure what the problem is but I guess you should get the correct carb with connections

 

Of course the feedback carb is not doing it's job right, but a lot of people use the feedback carb for awhile and the engine will run with the wires disconnected. I believe the fuel trim feature of the carb is what will not work.

One of the biggest problems I have heard people talk about when swapping to the DSII and using the feedback carb is there are no correct ported vacuum connections for the DSII dist vacuum line on the feedback carb.

 

Yes, the feedback carburetor is the issue. You can still use it, but performance and economy will suffer without the computer there to tell it what to do. The same thing happens when you change the feedback carburetor to a conventional carburetor but leave the computer and TFI ignition in place. You have to either run it as it was designed, or do a complete Duraspark swap, which means distributor, ignition module, and carburetor.

If your truck has the Duraspark II ignition swapped in correctly, you will need to swap to a simpler, conventional carburetor. The feedback carburetor has a feedback solenoid that looks for signals from the computer to adjust the air/fuel mixture. Since the computer is now gone, the carburetor can't do its job, and the feedback carburetor is not really designed to be used without input from the computer. What usually happens is the carburetor will run rich most of the time, no matter how you adjust it.

For a simple bolt-on replacement, you need to find a Ford Motorcraft or an older Autolite carburetor from a 1983 or older 300 engine.

If you would like a performance gain for your 300 engine, I would recommend the Offenhauser intake manifold with a Holley 390cfm 4V carburetor. There are a lot of guys over in the Ford Inline Six Forum that run this combo with fantastic results.

 

The short answer is the Carb is not working like it was original intended to. Do you have to pass emissions testing? The carb will run rich as is… about .7-.8 volts reading on the O2 sensor at 35-45 mph cruising speeds.

But it also might not be the source of your sluggishness, might just need a tune up.

I have been running a feedback carb for 2 plus years after I did the DS2 dist swap and still haven’t been able to come up with a good reason to change it out yet.

For 3 months I ran it with a push button switch on the feedback solenoid while I monitored O2 sensor with a DVM. Once I hit 35 MPH I would energize the feedback solenoid with the push button switch until coming to an Idle at the next stop light. Any chance I could get I would force the mixture full lean, leaner than the computer would have. At freeway speeds above ~55 mph it was always LEAN all the time as is. My daily commute was 17 miles one way, with about 14 freeways and the rest 35 mph. During the 3 month testing I could not see any change in gas mileage, which was 15 average for my normal driving.

Here is a good read.
https://www.ford-trucks.com/forums/9...-ignition.html

I would not put any money of effort into fixing of repairing your current feedback carb, if you have any carb issues just replace it with a carb for an 1983 or older as has already been suggested.

Jim

 

When I got my 1985 F150, the previous owner replaced the stock feedback carburetor with an older Motorcraft non-feedback version. Everything else was left as-is, meaning EEC-IV computer with all associated sensors and the TFI ignition was still there. I drove it this way for about 8 years, and it seemed fine, except for running somewhat rich. What I didn't know at the time was that since the stock system was tampered with, the computer locked the advance, and the truck was basically in a fail-safe "limp home" mode all the time. This resulted in poor performance and fuel economy. After realizing what I had, I decided to finish what was started and do a complete Duraspark swap. After I did the swap, I felt I had a completely different truck! Faster starts, I picked up about 5mpg, and gained a lot more power with a functioning distributor advance again.

I am sure I would have experienced the same gains in both fuel economy and performance had I went the other way and simply replaced the carburetor that I had with the correct feedback model. The problem was the cost. Since I didn't have a core, a correct rebuilt feedback carburetor was over $400! Plus some of my emissions components was missing, courtesy of the previous owner. All engine sensors and emissions components must be hooked up and working for the feedback system to work properly. Keep in mind that most mechanics have no idea what to do with the feedback system, since it was only used for a few years. Even the Haynes and Chilton's manuals I had avoided the feedback carburetors. Luckily for me, there is no emissions testing where I am, and with the help of some really good guys here, I successfully swapped to the older Duraspark ignition system.

BEWARE: If you have emission testing in your area, this swap will not pass.

 

Change the carb to a non feedback type.

Most of those problems, including sluggishness should go away.

My feedback solenoid died on my original carb, and it was severly sluggish.

 

It all makes sense. The first swap I did was on a truck with almost 300k miles on it and it felt way peppier than this one. The distributor is probably not getting the correct advance...making the sluggish condition. It seems to struggle even getting up to 55mph. The duraspark system is great...I've even used it to convert a points equipped truck.

Thanks a bunch for all the input!