first post here, i am a diese lguy and this carburator/distributor stuff has me confused.lol the truck is a 1981 f100, 300 with a 3 on the tree and 3.08 gears. was having carb and emission equipment issues, so i figured to replace everything with new intake and carb. so got an offy intake(dual plane) and a autolite 4100 carb (1.08, 475cfm). the rest of the engine is stock for now. only has 81000 miles. truck runsgreat but i still have a little ping i cant seem to make go away.

the carb is a rebuilt unit to stock 289 specs. figured it would be close enough for the 300. unsure of the size of the jets. i have set timing everywhere from 2* to 12* , still cant get it to go away. seems to do it the most at cruising speed with an ever so slight throttle aplication. if i psh he pedal down more it goes away. right now i have the timing at 3 * and it seems to be the best it has been. the dizzy is still all original, along with coil.


any ideas? thanks in advance.
corey

 

Pinging can occur when warmed up under high-gear (lower-RPM) acceleration if you don't have the EGR installed and everything configured properly. You can try retarding the ignition timing but you should make sure the harmonic balancer hasn't slipped and is feeding you false readings.

Aisde from that, you can re-curve your distributor to give a timing advance that doesn't result in that problem.

I'm assuming this truck doesn't have California emissions? If it does, it's computer-controlled (or was from the factory).

 

truck did not have cali emmissions, nor the feedback carb. this afternoon after work i am going to set everything with a vacume gauge instead of timing light, try things that way to make sure the balance hasnt slipped.

 

Chris is right in suggesting having the distributor recurved. That way, you don't have to give up the initial timing advance which gives more get up and go at the lower speeds.

After recurving of the distributor in my 1986 F150 with 4.9L, the initial is @ 12 BTDC [actually set by vacuum gauge] and there is no ping. By the way, that is @ 19.75" vacuum on my 1 barrel carburetor.

By the way, welcome to the forum!

 

The 49-state trucks didn't begin the feedback carb stuff until, um, 1985 (when EFI was an option) or maybe 1984, I forget for certain.

Regardless, it would have still had an EGR valve, it would have just been controlled by temperature-actuated vacuum-control valves.

 

yes it had an egr valve, and air pump. the pump was bad, egr valve stuck and carb had issues. looked like water sat in the bowl for a while. pitted bad. thus the reason for the swap.

 

The 300 is finiky about fuel and prefers premium octane due to its fairly long stroke and side draft manifold. Also check for water content in the fuel. The factory single barrel carb was a turd so you did good to replace it but being tuned for a V8 I'm thinking the power valve is the wrong size. Try installing a slightly larger power valve in the carb to eliminate pinging under load. If it pings all the time you may go up 2 sizes on the jets and adjust your timing to factory +2*.

 

go back and reread what i said in the posts. it isnt under load. just cruising it pings. if i am under load it goes away. factory +2 timing is way worse than where it is.

 

Where do you have the vacuum advance hooked up to?

 

Sounds like your vac advance is adding too much timing to the centrifugal advance. Shorten it.

You can test by connecting the vac advance and checking with a timing light to see how much it adds to basic timing, then add that difference to your total max centrifugal advance. It should be no more than 45 to 50°.

 

There should be a wee hex head screw inside the vacuum advance can nipple that lets you adjust where the advance kicks in. Use a hand vacuum pump to check the vacuum advance curve, compare to the vacuum reading at cruise, and adjust the vacuum can until it's pulling less advance at cruise.

Make sure the secondaries are completely shut too. Worst case you may need to go up on the primary jet size.

 

After you remove the EGR the distributor is going to need to be reworked.

If you are crusing in high gear and you slightly open the throttle and the engine begins to lightly ping. This is way too much advance too soon. Can damage an engine over time.

If you cut the engine off and it wants to run on, it will probably need to be corrected with larger jets. Think much larger. Ethanol takes larger jets.

 

To correct the distributor advance you have to add weld to one side of the slots in the pivot plate then file it to the correct width gap. There is some good articles on the internet on how to do this but a good distributor mechanic can change the plate and springs out and test it .