1982 F100 4.9L 6cyl. Recently failed MD emissions test for high hydrocarbons (unspent fuel). It has felt like it has had a slight miss for a while and it diesels a little when I turn it off. I changed the EGR last year, but never got around to a full tune up. I changed the plugs, wires, cap and rotor, and bought a timing light with a Tach. I have disconnected the vacuum hose and plugged it while setting the timing. However, the stutter is still present and the engine still diesels when I turn it off. The sticker in the engine compartment says the speed for timing adjustment is 800 RPM max. Unfortunately this thing is idling at 900-1000 RPM while I am setting the timing. I haven’t got a clue how to lower the idle speed to set the timing, or if that would even do the trick. If anyone has an idea what I am missing or how to properly set the timing (assuming I am doing it wrong) I would really appreciate the feedback.
Patrick

 

It could be that your timing chain is stretched,and or your carb is out of adjustment, herd somewere on here that exsesive carbon build up can cause dieseling aswell.

 

A year and a half ago when I last smogged my truck, I had high hydrocarbons too. Found out that my carburetor was no good. Had to get a new carburetor. Also had to put in a new PCV valve in one of the vacuum hoses. After that and adjusting the timing, it passed with hardly even reading on the scale at all.

I found out the reason why your truck diesels when you shut it off. It's idling too high. Mine idles about 1000-1100 when it's warm, but when I shut it off, if I let it stay at 1000-1100 it will diesel when it shuts off. I have a manual so I let out on the clutch a little until my rpm is down to 800 then turn it off. Shuts off every time without dieseling.

You would probably have to adjust your fast idle speed to correct that problem. My dad or I haven't figured out how to do it. We have all the Ford deluxe service manuals on the truck, but just haven't had the time yet to read through them.

 

Thanks for the feedback. I believe that the 4.9 has solid gearing for the timing, so that there is no chain to wear out or replace. I was afraid that someone would tell me it was the carb. Has anyone had any luck rebuilding the carb on a 4.9 1bbl?

 

I have tried rebuilding my 1bbl carb, but didn't seem to do as good as getting another remanufactured carb. I know they cost a couple hundred even remanufactured, but mine has lasted all this time so far since I did get another one. Good luck to you.

 

According to my truck engine service manual by Ford, my 4.9L does have a timing chain. Timing chains could always have cracks and/or worn or damaged teeth. I should sometime check mine to see if it's worn. Haven't ever replaced the chain yet and the engine has over 165k on it.

 

I also have had little luck rebuilding the single or 2bbl carbs. It costs as much to rejet and calibrate a single than to buy a good quality rebuild. Danman7 is right. The diesel effect is from fast idle. Even my 460 will diesel when shut off at 1K rpm. One more thing to check on your 4.9 is the intake manifold. The intakes on these engines are notorious for fractures causing an imbalanced air/fuel mixture thus not allowing it to idle at 850 rpm. I know this is not good news. Be sure your intake is good before you invest over a hundred in a rebuilt carb.

 

If I need a carb which one do I have? How will I know?
A)w/ Carter Carburetor, w/ Feedback Valve, w/ Automatic Choke
B)w/ Carter Carburetor, w/ Electric Choke
C)w/ Carter Carburetor, w/ Climatic Choke

 

300-6s does not have a timing chain. It has timing gears. It is possible the gears are worn enough to throw it [the timing] off.

 

I so look forward to the day my state will jump on the politically correct band wagon so I can fret and toil trying to get Big Ugly to meet federally mandated standards........Yes, put 2 new cats on the wish list......thanks Uncle Sam.......

 

I had the same problem, ended up with a new carb and egr valve. Also the idle was too high and it was dieseling. Problem there of course is a setting for good idle warm is a tad lumpy when it first comes off the choke, got to baby it till i get on the freeway. And yes, no chain, gears.

 

victorcharlie
Grin

Our state went from driving on maypops with a blue cloud trailing behind ya as "you might be a redneck"

To:

These are the Federal Laws and you WILL comply or ELSE!

Gawd... what a culture shock.

 

82StepEffie

One good thing I can say about the folks at the local part stores, (some are sharp but that is rare) is that they know how to use a cross reference manual. Give them the numbers from the tag on your carb and they can tell you which carb will cross reference. I tend to trust my local Ford Dealer Parts counter best for this type item even if I do not buy directly from them.

 

82 SetpEffie

How to set the timing. Locate the distributer which is the dealy with a wire running to each spark plug. At the base of the distributer is a lock bolt with a U or pie shaped clamp below the bolt head. Mark the distributer shaft at the base with a marker so you will know where it is set now. Slightly loosen the hold down lock bolt enough to reorient the distributer left or right with a medium hand twist motion. Disconnect the vacuum advance line on/ at the distributer (below the cap level) and plug the line with a pencil or small bolt. Start the engine, place the light on the timing mark and note the timing reading and the tach reading. Slightly move the distributer by twisting it either left or right, slightly and note again the timing change < > from the original reading and the reading on the tach. This will tell you if you are advancing or retarding the timing, note the reading on your tach. You now know how to do this on your own. Go practice!

 

82StepEffie

One thing I failed to mention is the carb and how it also has an effect on your RPM.

All carbs have at least one of the following adjustment screws:
air/fuel
curb idle
fast idle

With the engine off, locate the air/fuel mixture screw, you can find a diagram for location here on this site or about anywhere on the web. Note the orientation of the slot of the screw now. Turn the air/fuel screw in whole turns (one 360 degree slot revolution) clockwise counting how many turns it takes to bottom out the screw (stop turning) do not force this screw tight. Write down the full turns plus any partial turn it took to bottom the screw. Now back it out the same number of turns. The idle screw which is on the throttle connection side of your carb adjusts how fast the engine runs (tach) assuming fully warm engine idle not cold. The orientation of this screw is unimportant. Fast idle screw is normally on the opposite side of the a carb that has a choke plate (most do) Normally this screw rarely needs adjustment unless the engine fails to automatically kick down from fast idle when engine warms up.

One very important note here:

Never make more than ONE adjustment to the air/fuel mixture screw(s) at at time. Always briefly open the throttle to wide open letting it snap back to idle before making another adjustment. This also holds true with the curb idle screw. Always note the tach reading when engine settles back.