82 F100xl 300 inline 6 2wd C6 3sp tranny / non-feedback carter carb / duraspark ignition / 92k miles

Has anyone successfully stopped pre-detonation pinging in a desmogged 300 inline 6 and is this related to my ridiculous 10mpg? I'm about to give up. Get pinging during normal acceleration after it shifts into 3rd. It shifts at about 20 mph which seems very early to me. Pinging stops when I get to cruising speed. It even pings when running 93 octane. Truck runs very good. Here is what I've done so far:

No vacuum leaks - moved timing between 6-10 degrees - removed cat converter - moved to manifold vacuum - distributor vacuum canister Allen screwed all the way in - installed a "stop" on the distributor vacuum actuator rod to prevent throwing to much timing - seafoam thru carb to release excess carbon buildup in cylinder

The carb was recently rebuilt before i got truck. Could problem be related to Carb in some way? Should I just drive it and forget pinging? I sure would appreciate any advice. Thank you

 

Pinging is a bad thing and should be stopped ASAP --- little secret mine pings too but only when under a heavy load at low RPM.
The only thing I see different from mine is you are running manifold vacuum, try ported vacuum like the factory had it and see what happens.
Also what plugs are you running and the temp of the stat? Higher heat range plugs and a hot stat can add to the pinging.
I just had someone put a lower stat in his car to stop pinging and it helped. Factory should be 195* I believe so maybe a 180* would be better?
I have not tried a lower stat, just lazy to swap it out I guess. I run normal heat range copper plugs like the factory wanted.

Now on the low MPG it can be a bunch of things from the numbers you are using for the math you use.
If the speedo / miles are off the math will be off. The speedo can be off because the tires have been changed to non-stock size.
You are going by the fuel gauge for the gallons used, had a used that just did this and had poor MPG.
You need to take down the milage at fill ups, drive till you need fuel again and take down milage and gallons. Do this 3 times to get an AVG. of MPG.

Your driving style can make the MPG bad. You heard drive like there is an egg under your gas pedal foot? I find city driving make my fuel gauge drop pretty fast but most of my driving now is city or pulling a 20' enclosed trailer in the city so I am not too worried about the MPG like when I was working and putting 400 miles on the truck in a week.

Just because the carb was rebuilt does not mean it was not done right. Mine was a mess when I got my truck, as was the truck that had missing parts and wrong screws.
You can adjust the high speed needle / jet to lean out the carb but you dont really have a way to know where you are at or where you are going without a AFR gauge (air fuel ratio) to tell you if you are rich or lean, yes I do run one in my truck.
I do run mine on the lean side being 15 to 16 AFR and it seams good. When I was following my MPG I AVG 15 MPG. I seen a high of 17 and a low of 12 and have no clue how I got either MPG?
I also know my speedo / milage is dead on as I have checked it with my truckers GPS and Google Maps App.

It was easy to install the AFR gauge as I am running EFI exhaust manifolds and used the threaded port the O2 sensor used.
If you are running the factory exhaust then it may be a little harder as you would need to add the bung for the O2 sensor for the AFR gauge.
If you are good and have the right heat range plugs you might be able to read them to see if running rich or lean but it will take some doing.
Now the factory and the Ebat knock offs you can adjust the needle and seat and not need to remove the carb but not knowing when type of screw the needle has you could just remove the top, again no need to remove the carb, and see / adjust the needle. the needle needs go down in the jet to lean it out and up / out of the jet to make it rich. IIRC the screw work backwards from what you would think it does.
Once you know what screw head the needle has if you look at the top of the carb where the filter gasket sits there should be a plug over where the screw would be.
Pop that plug and use the screwdriver to adjust the needle as needed, yea it is fun trying to get the driver on the screw head

Make the vacuum timing change, check the plugs for heat range and if rich and maybe a lower stat, I would not go lower than a 180* and see if that helps.
BTW running high test gas almost cut the pinging out on my truck but try to only run it when pulling the trailer if I can.
Dave ----

 

Good info Dave. Thank you for taking the time. I have 31" tires so add about 10% to milage when figuring mpg. I installed 1 level cooler spark plugs. I'm a bit intimidated going into the carb but that might be the next project. I've tried ported vacuum too.... pings like crazy. Not sure what temp thermostat is have but I'll check and go cooler if it's a hot one. I drive very easy.... no hot rodding here. I know nothing about an afr gauge but maybe that's in the cards too. Running factory exhaust so don't know what would be involved in tapping threads for the AFR sensor. First thing I'll do is take a look at plugs. Appreciate you. Thanks

 

I'm not much of a plug reader but if someone can offer their opinion? plugs have only been in for maybe 4-500 miles. Some loose carbon around body of plug but electrode looks clean. Thank you

 

To me that plug looks really lean but they say you need to look down into the plug.
What happens if you dont run vacuum, just plug the supply so you dont have a vacuum leak.
Do you know what your rear axle ratio is? How about the RPM at say 55 MPH or 65 MPH? I wounder with the 31" tires and the gear ratio you might be running the RPM too low?

On the AFR gauge do a Google search on how and where it needs to be mounted in the head pipe. Some bungs use a strap to hole them in place but most have to be welded.
The bung did not come with my gauge so you would need to come up / buy one.
Dave ----

 

See pic but I think I'm running a 2.75 rear ratio which is very high supporting the theory of too low rpm's. Don't have a tach but will a soon as i can get to parts store. Not sure what you mean by:

"What happens if you dont run vacuum, just plug the supply so you dont have a vacuum leak"

I have run with dizzy vacuum plugged and no pinging all all.... if that is relevant.
I'll do some research on an afr gauge and see how it would be mounted in my exhaust. Big BIG thanks for your time...

 

I assume 2.75:1 ratio. 9" differential

 

On the "no vacuum" you did just what I wanted to know. With no vacuum to distributor you get no pinging. How did it run power wise?
Maybe just run with out vacuum as there are many motors running with out vacuum to the distrbutor.

No vacuum and maybe bump the timing up a little bit more and see how it goes.
As long as you dont get pinging when driving or kick back on hot start.
On mine running high test gas helps cut pinging a lot when pulling the trailer.

I also have the same rear gear ratio, 2.75, but I have a granny 4sp manual transmission and added over drive.
At 70 MPH the motor is only turning 1800 RPM and that is the same RPM of running direct at 55 PMH.
I do run over drive at 45 MPH, 1200 RPM, but the pinging starts to happen when you hit a little hill so I kick it out of over drive till over the top.
Dave ----

 

Most who have been into these big sixes will tell you they cannot tolerate too much timing. Keep on driving it like that, and eventually....well, timing will no longer be an issue. You'll have bigger problems
On my rig, I have 32° total once all the advance is in. It can tolerate 34 or 36° on premium fuel, but it's not always happy. After dinking with a GM HEI distributor for a while, ( which worked OK only after changing the advance curve inside the distributor) I have since converted to Holley EFI, and I now control timing through the EFI system. I have it dropping timing out under low vacuum, high load conditions. Initial timing (at idle) is around 15°. With this, my fuel mileage is about 13-14 in town, and a bit better than 20MPG on the highway. 10mpg would just be unacceptable. At least to me.

With the fuel mileage you are getting, my guess is the motor is working hard against itself, making the base problem worse.
So as Dave "FuzzFace" suggests, run it without vacuum advance, and see how it works out.
Another thing to try is running the vacuum advance on manifold vacuum rather than ported vacuum. This way, timing can be a bit more at high vacuum low load conditions, while dropping some advance out under low vacuum high load conditions.
If you don't know the difference between manifold and ported vacuum, it's ok. None of us were born knowing everything. There are a bunch of youtube videos on that subject. You will see pros and cons of both approaches.
Good luck with it!

 

Thanks so much for the posts. Got a tach and turning 2000rpm @ 60mph. 28.75" tires and 1:1 3rd gear so think i do have a 2.75 rear gear, agree? Tried switching to Ported vacuum and pings just as bad. Again... pinging pretty loud when accelerating normally after C6 shifts to 3rd at around 20-25 mph . Have not checked stat temp yet. Today i plugged and disconnected dizzy vacuum and the thing runs great without any pinging. I honestly can't tell any difference at any rpm. Timing is set at 10. Maybe I should add more timing until pinging restarts to establish baseline and just drive it there? Really don't want to burn high octane due to cost. Thoughts appreciated

 

Another idea... during a normal acceleration, my c6 tranny shifts into 3rd at around 20mph. Does it make any sense to adjust the transmission vacuum Modulator to shift at higher rpms reducing the low rpm lugging?
thanks again

 

Why hasn’t anyone discussed checking the mechanical advance in the distributor? Back in the 80s I worked in one of the big repair shops in Bend, OR and we had a distributor machine. I checked and recurved every performance build we did. All everybody talks about is the initial timing setting but that is fairly irrelevant. It is all about how fast it advances and how much mechanical, the vacuum only helps the fuel mileage on cruise. Yes I am that old.

 

Are you sure that modulator is getting the vacuum it needs? Pull the hose off t the valve to make sure you dont have ATF there.
I do think to shift into 3rd at 20 MPH is a little low, I would like to see it at maybe 30 MPH and adjust up / down as needed.

You know this is not the 80's and good luck finding anyone that has a distributor machine even pushed into a corner of the shop.
The other problem is what do you use for springs and maybe weights for the 300 distributor?
You said performance so I bet 90% were bow ties and that maybe low. Next would be Ford then MOPAR and I would bet my life savings you never saw an AMC
Remember they were the BIG 4 back in the day and yes I am that old too.
Out of all the distributors you did how many were for a six, straight or v6? and I will not even ask if any were for a 4 cylinder.

When I did the springs in my distributor I think I used Ford v8 springs and just did a lot of pulling dist. and trying till it felt good.
Yes I did also use a dial back timing light and noted what the timing was at each PRM in 500 sets. It is a lot of work when you dont have a machine and then who has a chart of what works for a street driven Ford 300 six in a truck that is also used as truck at times.

If the person is not going to pull the distributor, take it apart as the springs & weights are under the pickup coil plate to make changes that may or may not work and put it all back in then we can out guide to kick back the timing a little, run higher octan fuel and get the RPM up higher so it will not ping. When pulling the trailer and it starts to ping I have to drop half or a full gear to make it stop. I have over drive where I can split gears and why the half gear drop or going up too.
Dave ----

 

Don't forget carbon in the combustion chamber can lead to hot spots causing detonation.
valve guides or valppve guide seals can allow oil to contaminate the mixture and cause combustion issues

 

@gwp36530
I'm not running any Vacuum Advance on my '78 Ford 429.
I had it built to 9:1 compression with mid-range Crane Cam and dual springs and roller rockers. _ _ it pinged real bad.
That was 1993. My recollection is I read an article in the Bob2000 website where he explained to options for advance arm settings.
I may have gone to a lighter spring for advance to come in earlier. ( if I did, I think that could have been a Problem ).
Anyways that dang thing pinged so bad, I jjust kept retarding timing until it quit, while also disconnecting the Vacuum Advance.
Well, now, it runs cool with the large radiator, and today 20026, it still runs very nice as the 385 - Series engines do sound throaty at idle.

My guess is here, that with the Vacuum advance disconnected, you're not running total over 25-degrees, and in my opinion, is ok.
I towed several loads of oak in the Stock Trailer with NO-PINGING.

Now, for the record, on my '77 F350 engine which is a 300 inline Six Gasser, with Alum intake and Holley 600 4-bbl, I am running Vacuum Advance.
FuzzFace suggested that when I set my IDLE MIXTURE screw to peak Vacuum at idle, to then jack the mixture screw OUT one-more-turn to enrichen the IDLE CIRCUIT to prevent possible Hesitation, or Surge which may also reduce pinging.

Anywayz, the 429 with no Vacuum Advance has been happy for 32-years. _ _ ps - I probably pay more $$$ in mikeage, but that's what Vintage engines do. _ LoL