I am in the middle of trying to eliminate a troublesome ping under load in my 79 F-150 that showed up after changes to the cam and ignition system. The long and short of it was my truck came to me through a friend of mine who had swapped in a 5.0 ho motor with an unknown cam that did not run well from idle to 3500 rpms, but ran like a buzz saw above. Because I am using a toploader and use the truck on the highway for work I beat the lifters out of the thing and when I went to rebuild the top end I swapped the cam. Also the old stock points dizzy with pertronix was dead and I went to a MSD ready to run with a new coil. The ping did not exist with the old setup, but now it has appeared.

I swapped to an e cam and I am really happy with the change, but in top gear when trying to accelerate rapidly from cruise, or under very hard acceleration from stop the ping will appear. Accelerating from cruise it shows up in top gear. From a standstill it usually shows up in 3rd before shifting into fourth.

I am trying to adjust the carb because to me it would make sense I am hitting a dead lean spot under acceleration under load. I have tried to play around with timing and I am at 14 initial, which the engine likes, and 18 mechanical advance and no vac advance because it pings more with the vac advance than without it.

I have been running the silver spring for the secondaries. My next move will be to try a lighter spring to see if bringing in the secondaries sooner will fatten up the mixture to eliminate the ping.

My question is about power valve selection. I am at 14" of vacuum at idle. I am running a 65 power valve (6.5" of vacuum to activate) . Holley generally recommends dividing idle vacuum by two and using that power valve. However, I am also reading that it is ideal to run a power valve that is 3" below idle vacuum, which for me would be a 95. If I am reading this right, going 3" below idle vacuum would open up the power valve sooner and also fatten up the mixture, which may help in eliminating the ping. Does this sound right?

 

You could try it. I am working through PV selection in a stock engine and everything online as far as info is geared for WOT and drag racing, Holley sort of gives conflicting info, (or maybe the racing folks do) Sort of get the impression that the folks suggesting a PV 2 points below idle vac have lumpy cams and "idle" at about 10 inches. I think the standard 1/2 rule works for vacuum idle above 13. One thing the carb guys stress is get the timing dialed in first and only then work on jetting and PV. Another thing that impressed me today was how reading plugs can determine if there's too much timing, I'll try to dig up the link.

http://www.4secondsflat.com/Spark_plug_reading.html

 

Thanks for the reply. I think I have been on that sight as well. I have two distinct marks on my ground strap right near the bend.

I have always used either the 18 or 21 degree bushing and kept total timing between 32 and 36. I have started with initial as low as 10 and still it pings... the e cam definitely likes 14 degrees or more initial timing in my engine.

I have iron heads on this thing, which are far less forgiving than the al ones when it comes to heat and lean spots.

Because this ping either comes in under very hard acceleration, or accelerating from cruise, it am pretty sure it is caused by a lean spot in the fuel delivery and therefore can be tuned out.

I am going have the vac gauge in the truck and I am going to watch it as I drive and see what it says.

 

Hi,

Seems you are on a good path to fix your problem.

Only thing I can add is: What is the Timing curve with your new Ignition setup?

When I re-curved the Dissy in my truck I read everything I could find online then started experimenting. I first checked my timing curve form Idle to 3500rpm every (500rpm) to establish a baseline curve. My stock Dissy didn't have my total timing (36*) in until close to 4000rpm.

I started swapping springs as I wanted total in at 3000rpm. Anyway, I put to light of a spring in and experienced the same ping condition that you are describing. I checked the curve with the light springs and the total was in at 2250rpm. This would work fine for a light car but for a heavy truck... nope.

I swapped the spring combinations a few more times until my 3000rpm goal was reached. Engine runs great with no ping.

I recommend that you check the advance curve and see if it's coming in to early thus causing the labor knock, ping, detonation you have.

Take care and let us know what you find.

 

Jet the carb up 2 sizes.