I know there is a lot of posts on here about feedback carbs, I know I've read a bunch of them, but I have a specific question. 1985 F 150 Explorer 4.9 is my truck. My question, can I disconnect the feedback solenoid and somehow tune it to lean up? Also go with a HEI distributor? It looks like there is an unused vacuum port on the bottom of the carb that can work.

 

I ran a feedback 2150 that was disconnected from the ECM for a few months; it made no real difference to the engine........no power change, plugs didn't soot up etc.
There isn't a very wide mixture change caused by the computer/solenoid, at least in the Motorcraft carbs; the mix is still mostly governed by the main jets in the float bowl.

 

Welcome to the forum, JOlee!


When working as designed, the feedback system works quite well and is plenty lean enough. It has to be in order to deliver the excellent gas mileage it is capable of [4.9L six], when driven conservatively.


Before messing with the carburetor, be sure to check all other components. When is the last time the engine was tuned and the Oxygen sensor replaced?


Why do you believe the carburetor needs to be more lean?

 

Most of the emissions control has been removed from the truck including smog pump. I live in an area that checks emissions but only at the pipe. The CO level was at 5.8. I called a mechanic friend he told me that was probably due to too much fuel. I get about 10mpg as well.

 

You have a computer-controlled engine and you disabled/removed many of the computer's methods of controlling it... so it's defaulting to a rich mixture condition.

Look into a DS2 swap; you're hosed in your present situation....

 

Ok, I know the DS2 swap is the default answer. I can get a one wire HEI distributor cheaper with less time involved. So can the feedback carb be modified to work with that setup?

 

Yes, the feedback carburetor can be used [contact forum member, JimsRebel]. I believe he has used his for several years with good results.


I am going to attach a link to a project I am continuously working on. My truck is a 1986 with 4.9L engine. When purchased, the first two tanks returned 11 and 9 mpg [yuck!]. Currently [6+ years later], it averages between 17 and 23+ mpg.


https://www.ford-trucks.com/forums/1...-9l-300-a.html

 

I had to use a push button switch to manually control the feedback carb solenoid to pass emission testing as I was just slightly rich.
About a 50% duty cycle on the switch worked, this was with the back wheels on the roller load drums at 35 mph. You just monitor the O2 sensor output with a DVM while pushing the switch.
The lowest cost fix is a new DS2 dist with a GM 4 pin module.
Does the emission testing ever stop? In WA they only go back 25 years.
Passing emission testing will be your biggest challenge. What RPM do they test at?

You should read this thread and scan my other posts for more info as we have the same truck.

https://www.ford-trucks.com/forums/1...-feedback.html

Jim

 

Thanks guys. I love driving the old truck. I would love to make it economical to drive. If I could get around 17mpg I'd drive it every day. Shoot the air conditioning still mostly works. It does have that silly C5 transmission but I still like it.

Jim,
We test two idles here. The first when you pull up and the second after holding at 2500 rpm for 30 seconds. They test anything that came factory with a cat.

Thanks again.

 

The moderator in the 6 cylinder forum [AbandonedBronco] has experience passing emission testing with the 300 engine without feedback. He has a different intake manifold, 4 barrel carburetor and EFI exhaust manifolds.


I think he attributes his emission testing success to having installed a new 3-way catalytic convertor.

 

Question: Can I disconnect the feedback solenoid and tune the carb to any level of accuracy? I currently still have spark advance with the stock distributor. Would I lose spark advance without the solenoid connected?

 

I believe you would. If the computer senses any missing input [sensors/feedback], it reverts to the "safe" mode.

 

What does the feedback solenoid control? Vacuum? Is that how it controls fuel mixture by regulating the vacuum on the diaphragm to the metering rod?

 

It has 2 internal vacuum circuits, Idle and cruise, both get actuated at the same time when the feedback solenoid is energized by the computer. I don't know how the vacuum internally causes it to lean out , but I don't think it has anything to do with the metering rod.

On some carbs the metering rod is adjustable externally through a hole in the top of the carb, it has a plug in it that is removable .

I one point in time I was thinking about tweaking this metering rod adjustment, but then I realized it was going lean at around 55-60 mph as is... so there would not be any gas mileage improvements thus I never did mess with it.

Jim

 

I'm fighting to get it to pass emissions. HC 184, CO 5.60, CO2 10.98, curb idle 700 rpm, timing 10 btdc. That tells me I have a lot of unburnt gas. I get about 11 mpg. CO has to be blow 2.2. Everything else is in. I was just trying to figure if I was working against the solenoid.