Howdy folks, first to get the technicals out the way: 86 F150 with a 300 and 4-speed manual trans. I've had this truck for a coupla years, bought it out of a field it had been sitting for 4 or so years. This is the first vehicle I've ever worked on, and I've learned a lot, mostly how much there is to know that I don't! Anyway I've got it to the point that it'll start and drive just about every time, now I'm working on improving performance, esp. MPGs. Maybe 5-6 months ago started working on the exhaust, as the muffler was exploded like a looney-toon, and I figured the cat was clogged and no good. So I chopped it just before the cat and sure enough the thing was empty, save a little pile of platinum dust. So I started slowly accumulating the parts for the job, and in the mean time continued driving it with about 3.5 feet of exhaust. Prolly not the best thing to do but it seemed to handle it alright(no emissions regulations in the county I live in BTW). A month or two ago the performance really took a nose dive before the engine heated up it would stall out returning to idle, and occasionally flood out totally. Did some research learned all about throttle position sensors and how to test them and discovered that my TPS only went up to 2.5 volts out of the 5 its supposed to get at full throttle. Problem found! Put a new one on and it ran great much more responsive, hopped on the highway and could easily get up to 75. No stalling returning to idle, no chugging, could climb inclines with ease! Started it the next day and everything was back, spluttering, sluggish, backfiring and all, 'cept now it doesn't die every time I put the clutch in, just almost and then it saves itself. I've played with the fuel mixture screw and as far as I can tell one way makes it run ****ty and the other way kills it. When driving it's kinda like somebody is flipping a switch: it'll run good, and then bam like hitting a wall all the wind goes out of the sails. I'm kinda thinking it related to the silly feed-back carb, some sensor somewhere, or the result of running a sawn-off exhaust. I got a code reader and did the key on engine off test and it came back clear. Any thoughts?

 

Has it been cold and rainy where you live lately? Does it act up the worst after you first get it running, and after you finally nurse it along and get it warmed up, turn it off and then restart it later, does it run better?

 

There is a certain procedure for setting the mixture on these feedback carbs...

1. Have you rebuilt the carb yet?

2. What is your base timing set to?

 

I would run the full test for codes and post any numbers good or bad it pops out.
If you are not getting any codes then you need to use a test light to pull the codes.
Dave ----

 

Yup Pacific North West winter: cold and rainy! When I first start it it runs with a high idle, til I tap the gas again and it'll drop back down into rough RPMs. After it heats up/drives around for a while it runs better(I'm guessing the hot engine can deal with the rich condition better.) after I park and restart it runs pretty good, but that depends on how long it sits off. Sometimes starting for the second time in a day it'll be fine and a minute or two into driving it'll start running like garbage. I rebuilt the carb last winter. I'll run the code tests this weekend and verify timing.

Thanks y'all!

 

I have had these symptoms many times before, it sounds like carb icing. When it is cold and rainy outside, the venturi of the carb and the atomizing fuel has a cooling affect, even though it might be 40 degrees outside, frost can start building up inside the carb. This fouls it up and is the reason it runs rich. If you finally run it enough or turn it off and come out later and restart it, heat from the engine can finally warm the carb enough to prevent the ice from forming.

Ford used several different components to prevent this from happening, one of the most important was the aircleaner. Most of these engines had a pipe that ran from the aircleaner intake down to the exhaust manifold. There is a vacuum door in the aircleaner and this was controlled by a temp sensor mounted in the aircleaner. When the air is below about 100 degrees, the sensor would apply vacuum to the door and that would let the engine pull air from around the exhaust shroud and up the pipe. The exhaust shroud would warm the air as it came up the pipe to prevent carb icing.

 

 

Hmmm interesting. The pipe goes from the sheet metal bit around the exhaust up to the air filter housing right where the air hose from in front of the radiator, yea? I've noticed where there ought to be a heat riser type deal, except mine came with no pipe. Whats worse is some time ago when I was replacing the alternator I couldn't get it out on account of the J-shaped bit, where'd you'd hook up a hose, blocking the through bolt from coming all the way out.( Why the through bolt goes in backwards on that bracket is beyond me!) Anyway in my frustration I cut that sheet metal bit off to pull the alternator. Another thing I was thinking is that it might be my choke, which is of the electric/heat activated variety. I have another heat riser tube(copper and insulated) that also isn't hooked up to anything, rather is just kinda shoved down amongst the manifold. I've read that the electric part of the choke is just an assist and doesn't work under 60 degrees.

 

You are correct, your choke works mainly from that insulated copper pipe. There is vacuum from a little hole in the carb that sucks on that round black piece. They hook the copper pipe to that housing, and then run that copper pipe so it gets heated by the exhaust manifold. Usually the copper pipe wen through a hole made into the exhaust manifold, most of them corroded off long ago. If you can't get a new piece of copper through the hole, just take a length and wrap it around the exhaust manifold. All you are doing is heating the air that the vacuum is sucking through this copper pipe, and the heated air warms up the round black choke housing and makes the choke open up.

You can easily see if any of this is your problem. When the engine is throwing a fit, take the air cleaner off and see if the choke is open. If it's not open, take your finger and pull it open and see if the engine clears up. If the choke is already open and the engine is still running rich, I suspect carb icing.

You can get new flexible tubing for the bottom of the air cleaner from the store. I like using the aluminum stuff. And it does not have to be fancy. Too bad you took the original shroud off, but what you can do is split the new aluminum tubing on the end, flare it out a little bit, and slip it over a convenient spot on the exhaust manifold. Just so it pulls air from around the exhaust manifold, that is all you need. When the system is working, when you first start the engine you can put your hand on the new tubing and feel it getting warm. The sensor in the air cleaner will actually modulate the vacuum door, keep the air the correct temperature.

 

Just ran the KOEO test again, and this time it pulled a 63 which according to the book is incorrect volt from the Throttle Position sensor, which I ain't thrilled about. I was going to run the full tests but it says not to proceed unless it passes with an 11. Timing was some where in the 7-10 degree range, although I couldn't find anywhere that would hold without jumping around. The sheet metal shroud is still there I just cut off the nub you'd hook a hose to with a hose clamp. Should be easy enough to shove some tubing in the hole like you discribed.

 

Did you pull the spout connector when checking the timing? You need to do this to take timing control away from the computer, then you set the timing, and then plug the spout connector back in place.

 

The copper tube goes to heat the choke. You should also have the hose coming off the exhaust manifold supplying air to the intake. Having fixed these problems on my 4.9, your truck would run way better with it.