Hey guys Adam here. Just bought a 86 f150 with the big 300 in it. Love the truck but the truck has some minuses. The truck has only roughly 39k on the Leo. When i got it it looked as if it sat and it ran rough. It has the dreaded feedback carb. That's a minus all on its own. But anyway. I did a full tune up which included oil and filter change , spark plugs, cap and rotor. Done other stuff like new sending unit, water pump, new belts, power steering pump, and etc. I have adjusted the carb hundreds of times . It does not hold a tune as it were. Some days runs like a kitten. Then other days it coughs puts pudders, limps, and drags home. Some days it idles perfectly and somedays its like I got a huge cam in the engine makin it wanna just die. I work 10 miles one way to home. An I am averaging about 8-10 mpgs. I look at the forums an see 100-200k 4bbl 300s running 12-16mpg in town ! I had to clean up a lot of the vaccuum lines bc someone butchered them . Routed wrong. Lines not meant to hold vacuum was having vacuum. All kind of stuff plugged off, wires butchered. But I got that all straight. The truck still has some pep but not v8 pep that deserves to get that kinda mpg. I have been thinking hard on the Duraspark 2 swap with a 1 bbl yf carb for now. Then up to the offenhauser intake an cam and etc. But if 10 mpg is my fate I mind as well have the power to go with it. I mean I BABY this truck through town . I have tried it all. So someone please help me . Oh an the rear end is a 3.08 gear ratio . 8.8 rear end . Thanks guys an I love the site packed with easy goin Ford guys.
I'm responding to this post not to help you, but to let you know I'm going through the same EXACT thing (dreaded feedback carb, bipolar idle, Carb that likes to cough at me, etc). I just recently picked up the truck 3 weeks ago, and I've realized I'm getting barely 10.1 Mpg, and this is after Spark plug change, etc. (Admittedly I haven't done a motor oil change yet, that is being done this weekend be as though I just got the money for it). I'm running what I believe is 3.55 gears in the rear, although I'm not sure how to identify it without opening up the differential. I'm definitely subscribed to this, I want to see what the great collective hive of a mind that IS FTE has to say in response
O2 sensor would be the first culprit I'd suggest swapping. It's a tune up item that should be swapped every 35,000 to 50,000 miles in this year range.
The feedback solenoid on the carb or a carb rebuild is the second if the O2 sensor didn't fix it.
The computer will fight with you, so to adjust the fuel mixture you have to adjust it, reset the computer and adjust it again. turn the screw in untill it begins to stumble then out untill the engine idle smooths out. This should get you in the ballpark. Shut engine off, disconnect the battery for 5 min, then re-adjust again.
Sounds like the computer is going into limp home mode, and a bad O2 will definitely cause that.
If you decide to go with the DS-II swap those things would be considered moot.
Good job in all the work you have done. Do you have a timing light? If so, warm the engine up (short drive), then check the timing. Timing should be about 20 degrees BTDC at idle. If you raise the RPMs to 2000 or so, timing should advance to 30 degrees or so (right off the scale).
If timing does not advance with engine speed, then the ignition control computer is unhappy and has locked the timing advance to 10 degrees BTDC, the so called "limp-home mode". This mode is there to keep the ignition control computer from hurting the engine, but performance and gas mileage will suffer in this state.
If timing is not advancing with engine speed, then you can figure out why your computer is not happy. It could be something simple, like the O2 sensor as 81-F-150-Explorer suggests, or the temperature sensor. You can retrieve codes from the computer (OBDI), to find out why it is unhappy. That can direct you to potential fixes.
Properly running, you dreaded feedback carb system can deliver close to 20 mpg. That is darned good for a truck of this size.
1986 F150 XLT Lariat
2WD SWB 4.9L Carb
I thought the same thing about swapping to a DSII when I got mine, which isn't a bad thing if you don't have smog checks in your area. But got to looking, and started reading allot an realized mine still had all the smog equipment. So decided to start checking thing. Found you could pull the codes so did that then replaced or fixed what was indicated. Then I bought a vacuum gauge (best investment I ever made) and used it to set timing and adjust the carb. Also opened the plug gap as far as I could without pinging, mines set about 52 now. All in all I got the TFI with the dreaded feedback carb to do +/- 14 in town and 18+ on the highway. And still looking for more. From what I've found pulling the codes and fixing what's broke goes a long way.
Wow guys thanks for all the swift replies. No i haven't checked timing yet. I have bad luck with timing lights an snap on tools already got me by the *****. Lol . Also pulling the codes is something I should try to do today. How do I go about doin that on this truck? I have a tercel with obd1 and all I had to do was jumper tc and cg and the cel would flash so many times. An just count it out. But how do I do that ? All of the smog stuff is removed by the previous owner. Am hearing wag better projected mpg numbers an I am geting anxious to short this out. Rebuilding carbs is something that I am a little intimidated by tackling. Am goin to look into that later. The truck was a companies' work truck I believe. So the body isn't perfect but its not horrible. Thanks guys I was going WTF!!! saying this can't be right. Thanks for all the swift an knowledgeable answers.
One more very common problem with both the feedback and non-feedback YFA carbs is a loose throttle body, if you can move the top portion of the carb, then the screws have backed out and the gasket is probably destroyed. This will make the truck run rich all the time. When checking timing, there is a connector near the distributor to disable the computer controlled timing, to correctly set it, you have to warm the engine up, cut it off, unplug the connector or shorting plug, restart it and set the timing, should be 10 degrees BTDC, then shut it off and reconnect the wires or reinstall the plug so the computer controls things again.
A 300 should do better than 10, that's down in the 460 territory.
Bill Vose (Old School Hot Rodder) 1986 F350 crew cab DRW 460ci EEC-V MAF/SEFI, E4OD, PMGR starter, 160 amp 3G alternator, 12X3 1/2 rear brakes, Traction lock 3.55
1995 Lincoln Continental (project car), 1994 Taurus LX (daily driver) http://www.oldschoolhotrodder.com
"growing old is inevitable, growing up is optional"
Wow guys thanks for all the swift replies. No i haven't checked timing yet. I have bad luck with timing lights an snap on tools already got me by the *****. Lol .
Regardless of the grasp "snap on tools" has on you, you'll kind of need a timing light to diagnose your 25 year old truck. You will want the inductive kind (wire that clips around spark plug wire), and they can be found used for cheap on ebay.
Rebuilding carbs is something that I am a little intimidated by tackling. Am goin to look into that later.
I wouldn't worry about rebuilding the carb until after you have the ignition control system figured out anyways. But you should check to see if the carb is loose on its mount, as 85Lebarent suggested. Two 10mm nuts hold it down.
1986 F150 XLT Lariat
2WD SWB 4.9L Carb
All of the smog stuff is removed by the previous owner.
That's a big problem. This will throw the computer in limp home mode as well. (rotten fuel Mileage). I wouldn't bother with pulling the codes yet as there will be numerous codes due to the missing parts.
You need to either replace all the smog equipment that was taken off, then run the codes, or you will need to go with a DS-II swap to properly fix this.
Ctubutis and galendar thanks a lot an everyone else for the input. Cutubutis thanks for those helpful links sorry for asking a question you already covered clearly. I will check eBay for a timing light snap on came by my job today an he told me $350 for the cheap one and $500 big ones for the good one. Hopefully this Saturday I can pull a Duraspark 2 with all the goods. This afternoon on my way home. I hopped in the truck an she fired right up. No stubbling, or surging, or backfiring. Just fired up. As I drove it home she ran beautifully . So I kinda got carried away with the throttle. I drove it good. An I could literally WATCH my gas hand fall. Literally. That was discouraging.
Last edited by ctubutis; 03-08-2012 at 08:01 PM.
Reason: $; u = you; please don't write in Text-ese, OK?
That's a big problem. This will throw the computer in limp home mode as well.
That's a good point, I forgot about that!
86bullnose - Don't get a Snap-On timing light unless you work in the industry and will use it all day every day... $350-$500 is obscene for a timing light, a good one should easily cost you under $100.
If your talking about a timing lite for the 300 why not use a vacuum gauge? That's all I use anymore and it works great. Apparently my harmonic balancer has slipped, which I have read is a common problem with the 300, and that was my only option. It's a lot easier than the light is I think. Just hook it up, set timing to maximum vacuum then back it off 2-3 points.
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