Almost sure of this, but thought I could run it by you gurus.

New to me: 1983 f150 i6 4.9

Runs rough and slow rpm at idle which I originally thought was just the carb needing a good cleaning. Which it may still need.

BUT

Finding out now that when on the road, it stumbles and loses power/bucks? any time I get into a higher rpm in any gear. e.g. When driving in 2nd, i'll make it to about 20 before it happens, 3rd about 35 before it happens, and in 4th about 55 and im sputtering and 'bucking'.

This I thought made sense to be fuel related, but I find it to go up hill with no problem. I can practically put the pedal to the floor uphill and it seems to be getting the fuel cleanly.

Also, downhill, for example on the highway, in 4th gear, I have to just hold my foot on the accelerator just enough to give the engine a little gas to not be pulling against the engine, but not too much that it goes over 55mph without it sputtering and ****tin around.

Sound like a vacuum leak?

I should of known better getting a 'deal' on this truck.

 

Don't complain about getting a deal just yet. Your symptoms are probably something relatively simple that won't take much to resolve.

It sounds like a fuel starvation issue, definitely. But, it could be faulty emissions equipment or a plugged exhaust or vacuum leaks, etc.

Usually with a vacuum leak, the motor will idle a little high, it'll stumble when you give it gas in park or neutral, and it can cause bogging problems that result in an aggravating vehicle to drive.

The best thing to do at this point is to go visit the auto parts store and get a vacuum gauge. With a vacuum gauge, you can set fuel mixture, timing, check for vacuum leaks, and some of them can even test fuel pressure. But, before you do that, let me run some tips by you.

Take a 1/2'' wrench and tighten down the carburetor. The Carter YFA only has two bolts that hold it down, and these come loose periodically on some vehicles. Once those are nice and tight, grab the carburetor and see if it wiggles. If it does, then the 4 screws that hold the throttle body onto the carburetor are loose and are creating a vacuum leak. At this point you'll want to remove the carburetor and tighten the four screws on the underside of the throttle body. Now would be a good time to clean it and rebuild it if you like.

If neither of these solve the problem, check for any unhooked or cracked vacuum hoses. Check your spark plug wires and make sure all your spark plugs are tightened down correctly.

If no vacuum leaks or any apparent issues aren't found, you'll need to look at your emissions equipment.

A faulty EGR valve will cause problems, and is a cheap and simple replacement. Likewise, you'll want to check your PCV and any other emissions equipment.

 

Going back and re-reading your post, pull off the air cleaner and work the throttle to check and see if you can see a strong jet of fuel squirting out of the accelerator pump in the carburetor. You'll need to look down the carburetor with the motor off and work the throttle.

Get a timing light and check the timing. Your timing could be a little too retarded.

A vacuum gauge would let you check the fuel mixture as well. Doing it by ear is a poor way, but it can get you close.

 

Patience has never been once of my stronger traits. I went ahead and pulled the carb and all the vacuum lines(labeled them all), and the 'mount' for the carb which had a large flare nut fitting on a roughly 1" tube.

I had went ahead and done this to hopefully get a better look at the manifold bolts. Some along the top were not hand loose but definitely not to torque.

If I'm this far, is it much harder/worth my time to replace the gaskets for the manifolds?


The carb was down tight before removing it. And from what I remember the other day, looking down the carb throat with the engine idling and hitting the throttle it had a good spray.

I'de like to to a basic rebuild with gaskets etc. for the carb while it's out.


Where can I get a kit for this single barrel, think it said motorcraft on the side? Stock?

 

I went ahead and rebuilt the carburetor and it runs much better and doesn't stall etc. but still stumbles when trying to go fast at all. I also torqued the manifold bolts down while I had the carb/filter out of the way, and don't know if it is going to seal a leak coming from this area, but figured was worth it. Going to source a vacuum gauge and try to tackle that. Any tips on diagnosing this with the vacuum gauge would be helpful and appreciated.

 

Check all possible sources for vacuum leak(s), not just the carburetor. With the engine idling, try spraying something like WD-40 or carburetor cleaner around where the manifolds bolt to the engine as well as the bottom of the carburetor and EGR adaptor. If the idle speed changes, you have found a leak.

Check the vacuum hose going to the brake booster. Listen when applying the brakes for a hiss. Sometimes the booster diaphram leaks.

Check the PCV valve and its hose.

Check any and all vacuum hoses and make sure that one or more of the rubber caps on the manifold vacuum trees as well as on the firewall have not cracked or fallen off [this is not uncommon!].

 

I timed the engine to 6 degrees BTDC, and replaced the cap and rotor.

The instructions for the timing procedure called for pulling the vacuum line off the distributor and plug/ cap the line.

When doing this, the engine purrs like a kitten at idle and runs clean as well.

Still stumbles at higher rpm's in every gear, which seems to be a problem of it's own.

But runs clean.

So, with all the vacuum lines on and in seemingly good condition, what could be causing the engine to run sputtery with the vacuum line on the distributor but off and capped run smooth?

Oh yeah, and I installed a tachometer and set it for 6 cyl for my 300 4.9 and am only getting a reading of about 400 rpms and it sounds like much more than that.

 

Like my 82 4.9, you have a bad vacuum advance diaphragm.

 

Just checked the vacuum advance diaphragm by fist disconnecting the original vacuum line and hooking up a length of tubing to the inlet?/outlet? of the diaphragm and then proceeding to suck the diaphragm until vacuum was created on my tongue. It then held the vacuum to my tongue until I pulled it off, and I could see the arm moving all the way in and out.

However, the vacuum lines pulled off the diaphragm had no vacuum or pressure that could be built in them, sounded like they wide open on the other end, somewhere in the carburetor area. That's normal right?

So, so far, I am realizing that something in this vacuum system is creating a stutter/stumble like idle, and I am also having a possibly separate problem that does not allow the engine to rise high in rpm in any gear without REALLY stumbling and giving nothing. I say 'possibly separate' because it does this with or without the vacuum lines hooked up to the advance on the distributor.

Where can I find some solid information on how to tune this single barrel carter? It's got vacuum type and electrical switches on the exterior that are likely not to spec/adjustment.

Thanks all so far.