Hi all,

My 1986 F250 is approaching 300K miles on it's original 460 motor and C6 transmission, I try to perform a tune-up every 5000 miles and stay on top any little issues that creep up. Recently, I've been experiencing the following problems:

-Terrible gas mileage, not more than 5mpg highway vs. 10-11 that I'm used to.
-Exhaust backfires loudly, once on deceleration, mostly when slowing down after an extended highway trip.
-These problems have cropped up seemingly overnight, making me wonder if the timing set slipped a tooth or something... is this common?
-It's the middle of the Wyoming winter, but I still see my temp gauge needle creeping slightly past vertical, which strikes me as odd.
-I also noticed that the harmonic damper has rubber material sticking out of it, does that indicate that I need a new damper?

Here's some background info:
Last year (about 6,000 miles ago) I replaced the worn out Motorcraft 4180 carb with a Holley avenger 675 and I think it's tuned pretty well besides the accelerator pump which I still need to tinker with. In your experience, should I go to the 50CC accelerator pump?

I live at 7200', regularly go to 9,000 or 10,000'.

Last month I worked it really hard (lots of time @ WOT in 4 low, 1st gear) pulling a tow truck and two 4-runners out of a small valley in the mountains, maybe something slipped then???

To my knowledge, I still have the retarded timing set, I have found in the past that 8* btdc worked very well for my ignition timing.

So... my question to all of you is: what should I fix?
Do I just need a tune-up or should I dig into the motor and replace my timing set?
If I do that, should I use a top dead center timing set or stick with the retarded one?
My goal is to keep this truck as stock as possible, it's done marvelously up to this point and I don't see much of a need to change things up when they clearly work.

Thanks all!

 

I think a 30cc shot is fine but don't really know about jetting for altitude.

You should replace the timing chain.
A timing set for later fuel injected 460 is 'straight up' double roller from the factory.

I would replace the damper and check the spacer for scoring before replacing the front main seal.

 

Most of the time a 50cc pump is not needed.
First make sure the pump is adjusted right, when the linkage moves it squirts.
Have you adjusted the pump cams? I have not had much luck with them but you may.


Then try going with a bigger squirt nozzle?
I want to say most Holley's use a 31 nozzle go up 2 sizes and see what that does.
Most of the time that is all that is needed.


If that all dose not work then go with a 50cc pump kit and go back thru all of the above again.


Cant help you on high alt jetting or power valve for the same.
Dave ----

 

If the gas mileage has slipped that badly I'd guess that your power valve is blown. That will kill MPG, and that is consistent with the backfiring you are hearing. I would do that before the timing set. And, I'd check ignition timing - if it is where you left it then the chain hasn't jumped.

 

If anything at 7200'+ he'd want less of a pump shot, not more generally speaking. Power valves are also spec'd for different altitudes, if you want to look at it that way, just like jets.

They won't change the fuel flow, but alter when the fuel will flow. For example if a 7.5" is stock for sea level, a 6.5" is stock for 5000'-10000'. And that's for a stock motor. A hotter cam will generally result in even less manifold vacuum. The idea of course is to keep the power valve closed, except when it is needed. You may want a 5.5" or even a 4.5".

 

Older Holley's would also blow up the power valve if you had the carb back fire.
The pressure goes back thru the port and blows it out.
Dave ----

 

Great suggestion, but with one caveat. The crankshaft damper has some rubber sticking out, so the outer rim may have slipped relative to the core. That means the timing marks could be way off now.

My brain hurts trying to figure out which direction the outer rim would usually slip. If the cam jumped the chain one direction and the damper slipped the other, the timing may appear normal at the pointer, yet the engine runs poorly. If both slipped in the same direction, the timing marks could be way off.

The upshot is I wouldn't trust the timing marks until verified accurate. You can easily check for a loose timing chain as follows:

Remove the distributor cap. Use a socket and breaker bar on the nut at the front of the crankshaft. Turn slightly in the normal direction, stop, and note where the rotor is pointing. Turn the breaker bar the other way and see how much the crankshaft turns before the rotor moves. More play equals more slack in the chain, and the more likelihood the timing chain has jumped.

 

Thanks guys, you've given me LOTS of good stuff to chew on and I'm really glad to get others' opinions on the matter. Hopefully I'll be able to start investigating on the truck in the next day or so... I'll keep you guys posted!

In the mean time, I'd love to hear it if anyone else has thoughts on the matter.

...Thanks again! -Chas