Hey all i have a 1971 F-250 with a 5.9L 360. It is all stock accept a flowmaster 40 exhaust and brake booster valve. I am running the truck on regular gas (83 down here in colorado)

For the last 2 or so months what it has been doing is loosing power (engine not electrical) whenever i get on the throttle hard and fast. If i ease on the throttle it revs up with ease.

My throttle has been sticky lately but the truck has this problem wether doing it from the cab (pedal) or directly from the carb. It also backfires and pops trough the carb if i rev it up a few times.

I have heard it could be valve cover gaskets burning out, dirty fuel filter, dirty carb, running low octane gas etc.

 

Check your timing on the dist. And also check for vacuum leaks. I had a leak that caused the motor to just be a gutless pig.

When's the last time u did plugs wires and dist cap/rotor?

 

I don't like to say but you have been force feeding you motor low octane gas which is low death to the valve seats since it's unleaded. These truck ran on regular gas but it was 94 octane leaded gas. This was what the unleaded gas Additive was to help in the aid of not burning the v seats out.. The exh seat leaks on the compression stroke an causes a flash back into the intake side and blows out the carb under load. 700-$800 for most shops the rebuild your head for unleaded gas. 1972-76 ford got smarter an started putting in Harden valve seat. Find a junkyard truck with a 360-390 D2TE-AA or D3,D4 numbered heads is what have the harden seats. But they may be in need of servicing like rebuilding and or cracks if over heated... Try using higher octane, check timing, bad vacuum advance sucking air. Drive like your 91 yrs old to may it last longer. Yea right!.. my 2cents orich
orich

 

I am going to do a oil change, carb clean, and new plugs this weekend. I had a vacume leak at the power brake booster but it would barely run at all so i dont think thats the problem.

And sorry my mistake. Regular down here is 85 octane not 83. Should i run a tank of 91 to see how it runs compared to 85?

 

A Vacuum leak makes it run lean making it ping, run hotter, sparks plugs will be more on the whitish side or cracked center porcelain electoral.. Spray some quick start in areas that could be leaking. If vac. leak engine will race to a higher RPM for a few sec. so you'll know where the vac. leak is. fix and re test with engine idling.. orich

 

Valve cover gaskets? Really? I'd probably stop taking any advice from the person who said that.

One of the best pieces of advice I ever got and freely pass on, cover the basics. Get the engine tune up maintenance done and a lot of problems just take care of themselves. Vacuum leaks are very common problems, easy to find and usually not expensive to fix.

 

Either late timing or lean mixture will cause it to bog and spit back thru the carb. I would check the timing, as suggested above, and also check the accelerator pump. You should get strong squirts of fuel IMMEDIATELY when the throttle is opened.

While you're checking the timing, with the vacuum advance hose disconnected and plugged, also reattach the vacuum advance hose and see if the timing advances. If it doesn't, the diaphram in the vacuum advance might have a leak giving you the double wammy of not advancing timing and leaking vacuum.

 

Ok i will do a full tune up and check timing and look for vacuum leaks this weekend.

Now that i think about it the truck shouldnt have bad valve seald. The truck was rebuilt 20,000 miles ago. Rebuilt at 45,xxx because the truck has alot of stale gas run trough it. My grandpa bought this truck new in 71 drove it till 75 then got a new f350 i believe. So he just shut this truck away in a garage until 1996. (the year i was born) that is when my dad dragged it out of the grave and started to work on it. My whole child hood is filled with memories of taking this truck everywhere in th mountains. We rebuilt the truck in 2001 because it was running a little rough. We kept a full book of recipts. I bought the truck about 5 months ago for $500.

Ok back on topic which hose is the vacuum advance hose? And how much should the timing advance with the hose attached?

 

Good questions. I don't know your engine's spec for the amount of advance from the vacuum advance but, if the engine isn't running right you probably won't have normal vacuum and you won't see full advance. Look for at least a few degrees of additional advance when the hose is attached. Maybe someone can chime in with the spec.

The vacuum advance is the round thing on the side of the distributor and the vacuum advance hose is attached to it.

BTW, if you don't have a vacuum gauge, or access to one, this would be a good time to pick one up. They're handy for diagnosing problems and adjusting the carb. They're not real expensive and they usually will measure fuel pump pressure too.

Also, not all vacuum leaks are the same. A leak at the brake booster could be huge but a leaking gasket or cracked small vacuum line wouldn't have so big of an effect. I wouldn't assume its not a vacuum leak because you're not seeing the same effect you saw with the booster leak.

 

At idle removing vac. advance hose mostly no change if in rpm is 550-600.
But it's advised to remove & plug carb side while doing timing. But you first set dizzy points 0.17 but if it's has a dual vac. advance points 0.21 if you don't have a manual for this truck you better get one so you can figure out stuff for yourself as you may break down on the road and you'll be lost for trouble shooting the problem to get it up and running with out a us you ask.
When the engine was rebuilt if harden exh. seats were not installed then as it adds to the cost of a rebuild it may have not been done. Check your paper work for this being done to the heads.. orich

 

Cough-up through the carb is cam/crank timing....timing chain is likely starting to stretch causing the cam/crank timing to be out of phase- check & replace.

 

Timing was checked and set to 6 degrees behind tdc. Points were checked and set to .017. Im going to get new points, wires and plugs tomorrow. How should i set the air/fuel mixture. I believe manual says 1 1/4 turns out but i could be wrong

 

You want the timing set at BTDC= Before top dead center. Set the points first, then do the ign timing with carb vacuum line off and plugged off at carb as not to suck air into carb. Eng. idle about 650 or less when doing timing CARB Air/fuel Setting turns out 1. 1/2+- is just a good starting point. ..orich

 

I had the vacuum advance off and blocked at the distributor instead of the carb. Its now set at 6 BTDC with idle estimated at 600rpm. I will mess with mixture noe and drive it around and see if the timing slips any. Ill check it tomorrow and in a week or so

 

I didn't read all of the posts but My experience tells me that you need to check your timing gears and chain. The easiest way is to remove the distributor cap and use a 1/2" ratchet and 13/16"(?) deep socket to turn the crank a little bit, each direction and if the distributor rotor does not follow the movement very closely, the timing set must be replaced.
Late timing causes low vacuum and poor performance which is always caused by a bad timing set.