Howdy again all. So 1973 f100 fe390 is starting to act up. Morning starts are getting more difficult and with winter coming and lots of wood to still cut I need to get on this now . So morning starts, I will pump the pedal 3 times, turn key and she starts right up, but in a few seconds idles down and dies. To keep it going i have to feather the pedal for a minute or two until it will hold it's own idle (400-500 rpm cold) and once its plenty warmed up, if i blip the throttle it kills the engine. Really taking a long time to warm up also with very low idle, and occasional misfires. When warmed up idle is a very smooth 800-900 rpm. Recently upped the fuel ratio a half turn on 2100 carb because on routine inspection i noticed plugs were grey/white, and exhaust stopped having any smell for some reason. What i have done so far is change fuel filter, replaced float with brass, set float correctly. Am i on the right track here. To me it seems starved for gas, but now that I richened it up a little it smells very strong, but acts the same

 

I would test fuel pump- maybe its going out. ideally you would get a fuel pressure gauge on there when its running. When in doubt I always go back to the basic tune up- pugs, plug wires, cap, rotor, points, air filter, you already got fuel filter, etc. If its been worked on recently double check plug wire order. The engine may be tired and a leakdown test will usually show this. I had a lifter go bad on my 360 causing it to run rough and misfire.

 

white is running lean. not enough fuel. float sticking or needle sticking closed or somehow not filling bowl up. partially plugged jet or jets. filter restriction. or possibly sucking some air before pump back to tank somewhere . low idle when cold? increase fast choke idle screw a turn or more. and make sure choke is shutting when cold so that the fast idle cam screw is setting on fast idle cam, and that the choke is operating correctly. my 73 390 has manual choke and its staying that way. if it was weak ignition issue then plugs wouldnt just be white they would be at least wet. and it would be running weaker and probyl rough at higher rpm especialy. etc. white is burnign all the fuel available but its just not alot of it. or the plugs are too hot of heat range. 45 plugs right?

 

For sure in the manual choke. I removed heat choke setup last year, and installed manual, but never had to use it until around 40 or less degrees out. But I know the fast idle isnt even set up, so will do that before it gets cold. Funny right after you mentioned lifter, I ran to hardware store and it had its first tick! All the way in back on pass side gonna check all that has been mentioned. and crank down header bolts to see if it's the exhaust since I never re torqued header bolts after install over winter

 

Can I do a leak down test with a compression gauge? Or do I need to buy the special one? Motor could be tired, truck has 178000 miles on it , and have no clue what has been done. It was sitting in an old stable with a tarp on it for 7 years after my neighbors husband died. My guess is it hadn't been as the water pump I just replaced last month was the origional ford blue color(didn't need it, just had one lying around in box)

 

Sorry one more thing. Should the fuel filter be full at all times? It it almost always half full. (One of those big round ones with the paper element)
And plugs I dont know right now(at work) they are ngk and gapped at .045

 

It's always a package deal with a tune-up. Gotta do the whole thing, step by step and in proper order. If there are any shortcuts, I haven't found 'em and I doubt anyone has. A Ford Shop manual is best, because it has the exact specs for your particular engine, but some of the period general repair manuals are pretty good and have the same stuff.

The first thing to check is always engine compression. This is important because it tells you what you have to work with. If it is poor or uneven, a tune-up is pretty much a waste of time and money. On the other hand if it is satisfactory you should be able to get it running really well. The vacuum test with a mechanic's vacuum gauge is super useful for determining the state of tune and diagnosing faults. If the manifold vacuum is off spec, it will never run right. There aren't very many adjustments really on the carburetors but every one of them is important. Choke pull off, idle mixture, fuel height level in the bowl, etc, and it has to be clean, and no intake or gasket leaks. Carburetors are totally dependent on everything else being in spec, ignition and ignition timing, no leaks, temperature, manifold vacuum.

Poor grounds and weak charging system means weak ignition, accessories, lights, and slower warmups, as does a "lazy" thermostat, or the wrong temperature thermostat. Ignition timing defects are very common and should be checked out.

 

Gotcha. I have a compression gauge, but never used it, so after work that's happening. Valve cover, intake, riser, carb gaskets were replaced 3 months ago. Carb rebuilt last year, and looked brand new inside, thermostat is 195° and temp gauge has never moved off 196° oil pressure has always been at the bottom first line of factory gauge once warm, but have aftermarket gauge halfway installed, as NONE of the gauges ever worked correctly. Added 6 ground straps, grounding everything to everything, I also have a fuel pump meter that has never been used, so that also is happening today. I will post numbers, and results of tests once complete as far as charging system, alt is new 2 months, battery 4 months, voltage regulator 2 months, battery/starter cables 2 months, solenoid, 2 months, pertronics flamethrower coil? Unknown age, plugs/wires/rotor cap 6 months. This truck has been stripped down it seems. There is no vacuum. Lines other than brake booster, trans valve thingy, and one to distributor., all replaced 4 months ago.. anyway, I will give some numbers in a bit. Thanks!

 

OK, sounds like you're tracking pretty good. Remove all the spark plugs, make sure the battery is charged up good, block throttle and choke all the way open. Supposed to be checked with a warmed up engine but most people don't like to burn their knuckles. Anyway checking cold, if it is acceptable pressures cold it will only get better hot, I would think. Ideally see nice and even all cylinders around 150 say. 135 wouldn't be unusual with some miles on it.

What we don't want to see is something like 120, 140, 75, 60, 100, 100. 120, 140, numbers all over the place and a couple duds to boot. Record the cylinders, see how many "puffs" it takes for the highest reading on the first cylinder, and use that for the rest. It's better to see the needle swing up to a high reading quickly on the first couple puffs, than slooow and lazy.

 

Wait, so this truck has 170,000 miles? (I checked earlier posts), most likely or hopefully engine has been rebuilt or replaced along the way. Compression should tell the tale. If it checks out OK there's some other tricks in the FTE toolbox.

 

Start with a compression test for now. NGK are good plugs. Low compression can be caused by high mileage, worn piston rings and bores. But it can also be caused by gum and carbon sticking the piston rings. Engines that have sat for a long time are prone to this. A solvent can help to free stuck piston rings so they function again, but if they are worn badly you're looking at an overhaul.

 

Ok so did everything but a compression test as took too long chasing vac leaks, and after starting to do comp test, then realizing how long it was gonna take, had to put off. Vac reading was a steady 15 at warm idle. Started off at 10, but sprayed some carb cleaner and found stupid vac tree leaking from everywhere possible. I just need to ditch the 6 port tree, and replace it with on that only has 2( one for test plugin, and 1 for trans) the caps I put on 6 month ago all were dry crumbly, and as soon as I touched they imploded. Also note that 15 vac reading is at 8000 feet elevation if that matters.
I think I can get that higher, as spraying the choke area slows idle, but cant figure out where it would be leaking in. Old warm choke tube inlet is filled with epoxy, and dont see anything else, but might be high idle link going Into carb.
Got high idle set to 1300 rpm probably have to raise it once snow comes. Plugs looked dark and carbon, so turned mixture back half turn to where it was last week did a comp test on front drivers cylinder but did not take all plugs out, and got 60 instantly, but did not go up(or down after leaving it 7 minutes) the manual says to hold the throttle wide open (which I did not do)what will that matter?
Anyway I will do comp test after work tomorrow, but now fear some bad news

 

One of the important things with a vacuum test is that it is a steady needle, versus an absolute number, but 15" at 8,000 feet elevation is excellent. That's good news, as it generally indicates good compression, at least indirectly. It obviously had a ring job at some point. Pretty sure on that, could be wrong. There's some more specific tests one can do the gauge that show this a little better. Goose throttle very quickly to wide open and return to closed. The needle will quickly drop to maybe 4 or 5, and then rebound to 25 if the rings are in good shape. Maybe 20 at your location I guess, but you get the idea. It doesn't substitute for a compression test, but it's a good way if a compression test is impractical. I'd still recommend performing a compression test, because then you'll know, and have a baseline. Always measure, don't guess! Wish I would have taken that advice when I was younger.

What carburetor is this exactly? One thing overlooked generally and specifically at elevation is the power valve. I'm assuming an Autolite or Holley here. Probably want a 4.5" or maybe 5.5" or it will waste LOTS of gas. Manifold vacuum is what holds it closed, when the engine is under heavy load, the power valve opens up to provide extra fuel necessary. Engine vacuum is used for quite a few things, especially as a signal, because the level is load dependent. But at higher elevations, the engine vacuum is proportionally lower, and a flatlander power valve installed will never close, and it will dribble fuel constantly. A vacuum gauge is useful for tuning when driving around, can see how it reacts in different driving conditions, slight grade, climbing hills, deceleration. Knowing what the engine's manifold vacuum level is when cruising on flat ground allows one to really nail the correct # power valve. The key takeaway is that the power valve is not open, even a little, when loafing along at 60 on flat ground.

As always your spark plugs will tell you what's going on inside the engine, they don't lie. It's very tricky with modern gasoline. There is little color when it is tuned right. Some, not very much. If they look like they've been hit with black spray paint, it's way too rich.

 

Gonna have to look into the power valve then. From what you describe, it sounds like mine could use changing. It floods quite often, and while I expect low gas mileage, and don't care if its 10mpg it seems to be going through alot more than it should. If I drive somewhere and shut it off, if i dont start it within say 15 minutes it is flooded. Like it creates some kind of suction dripping gas into the intake. An hour later though it is fine. All fuel in intake has evaporated (or whatever it does)
So if I'm correct in what you told me, the higher elevation, the higher number valve? I live at 8000 feet, but take the truck above 10000 on a regular basis hunting for wood, exploring fire roads, etc

 

The # of the power valve indicates at what vacuum it opens, so the higher the number it opens earlier. Most carburetors and rebuild kits ship with a 6.5" but that is probably too high for 8,000' and above. Can download carb manuals that go over basic setup and adjustment. It's important to have them adjusted and baselined for a good fuel mixture and jetting, optimum tune does mean better fuel economy and range but it has other benefits, no fouled out spark plugs or washed out piston rings, diluted engine oil.