'86 300 i6 mileage HELP!
 

Hi Folks,

I acquired an 86 F150 4x4 with a 300 and a c6 tranny and 3.50 rear end on 29s. Long story short, it was very cheap but ran like crap when I bought it. I fixed a bunch of little things and got it running reliably but it get TERRIBLE mileage, like 9.5-10.5 regardless of if I'm cruising down the highway or hauling 1500lbs of wood up the mountain. From what I have seen people are getting nearly double that out of this engine. It just generally runs like an old, overweight basset hound. I know I'm dealing with the "dreaded feedback system" here but bear with me.

Now, for context the truck had a lot of vacuum leaks that I sealed up (intake/exhaust manifold, pushrod cover, valve covers, manifold tee, etc. etc.), basically all of the emissions features were cut out by the PO (no EGR, air pump, no O2 sensor, the crazy wire harness has more cut and capped wires than connected ones) but the cat is still on, the carb is a stock carter YF but I'm not sure if it is a feedback carb or not, the ignition is stock feedback. The electronic choke was also completely disconnected, I installed a functional but less than satisfactory manual choke. The original air cleaner assembly was replaced with one of those tiny chrome ones but when I'm driving down the highway I take it off and it still does not help the mileage or power.

After I sealed all of the vacuum leaks I advanced the timing a LOT to get it to run smooth. I do not have a timing light so I cannot be any more specific than that, sorry. The whole thing is running rich all the time (i think) as I cannot kill the warm engine by turning the mixture screw in the whole way. I have played with the carb and timing for about a month now and cannot get better performance OR mileage, just worse. Also, I can hear a distinct popping in the cat at idle. It was a problem when driving at speed but not throttle, like when going down hill, but the timing adjustment took care of that.

As for general driving performance, it idles OK but has a slight stutter (steady 18in of vac so it's not a leak), she has very little oompf off the line, no high end power when trying to climb in 1st and 2nd but has no issue cruising at 60+mph with a load of wood on open highway once she gets going.

Also I have a DSII ignition that I have not installed. Thoughts on that and the carb are welcome. This is a beater and I'm not looking to dump a lot of money into it.

Obviously I am not a mechanic. Don't be afraid to give me basic advice I wont be offended. Seriously any and all thought are welcome. I can upload pics if that would help anyone.

Thanks

Have a search around the forums/sticky's to confirm what I'm about to say. I've never ran a feedback system or carb but I've heard about this problem enough here to have an idea what's going on.

You have one of two systems.
1. A feedback carb and ignition
2. A non feedback carb and duraspark 2 ignition.

Unfortunately, by the sounds of it you have the feedback system. The carb and ignition are tied together. If the previous owner cut into it, or anything isn't working properly the carb/ignition go into a "limp home" or "safe" mode. This will retard and lock the ignition so it will not advance properly, and the carburetor will run rich.

That would explain why advancing the timing helped, and you say it's running rich.

It looks like you'll either be finding the problem with the current system and fixing it, or buying a carburetor and full duraspark 2 ignition or going aftermarket. (The duraspark is far more superior I'm sure most people here agree.)

If it were me and the budget permitted, I would go for a small 4 barrel, intake and EFI exhaust manifolds. It'll be way more reliable and gain some serious power. Or at least buy some sort of newer 2 barrel and an adapter to go on your current intake, just for reliability.

Maybe it's just me, but I had a lot of trouble with my 1 barrel carb. In about a year and a half, I had one carb rebuild, replaced it, never ending vacuum leaks, sunken float, leaking gaskets, and hard starting/no starting here in our Canadian winters eh?



In short, if you have feedback system, it's probably giving you hard time and I recommend anything but it.

Have a good one, Jason.

I would have to mostly agree with Jason.
If any of the stock feed back system is not working as it should it will cause what you have from what I have seen posted.

I would install the DSII dist and get a non feed back carb.
Thing is do you gat say a pre. 81 stock carb or change intake for a 4v carb and if you do that then I would do the efi exhaust manifold.
Dave - - - -

Welcome to the forum, ptharris4990! :)

I am going to attach a link to my 1986 truck's journey: https://www.ford-trucks.com/forums/1...-9l-300-a.html

Perhaps it will give you some ideas.