My 75 F-250 has a 390 2v and electronic ignition. Everything in the motor is completely stock. I get a pretty good ping about 1/2 throttle, but it you open the throttle all the way is opens the secondaries and all is well. Same off idle, light throttle is great but half and it starts to ping pretty bad. I just put on new plugs and wires, set idle mixture and idle rpm and set timing at 9 deg. I think stock timing is lower than that, but 9 is about where the motor likes it. For what it's worth I retarded the timing to where it would hardly run and still got pinging.

One thing that concerns me is the original owner (who i just bought the truck from) replaced the motor with a recon engine a couple of years ago. He said after he put the new motor in he had some problems and found he had two dead cylinders. I was told the machine shop ground the valve seats to far or something along those lines. He then bought two recon cylinder heads from the local napa store and supposedly all was well.

Only reason I mention the rebuilt engine is I'm a little concerned that maybe the wrong heads are installed and there's a compression/pinging issue. Any ideas?

Thanks! (sorry for the long post)

 

Hi welcome to the site!

Two things I can think of right now, one is your EGR is not functioning and the other is too much vacuum advance.

I don't understand when you say secondaries, isn't it a 2bbl?

 

my 390 does the same thing....gas gas gas...use GOOD GAS. Mine doesn't ping on GOOD 93 octain. I need to play with the Mallory Unilite distributor too..but I don't know how to adjust it to reduce some of the advance. I also retarded the timing and it would still ping at part throttle. I can run down the road at 60 just fine but as soon as i come to a slight hill where i have to give it just a little more to hold speed is when it will ping...drop the throttle and it stops. If you find a good solution let me know because it is bugging me too.

 

Try some seafoam. You can find out about it here. Just use it in the carburetor as directed, should clean out carbon deposits and reduce the tendancy to ping.

 

a co-worker said to put water in a spray bottle and spray the water directly into the carb ( slowly ) at idle...rev motor slighty if needed...and the steam created during combustion would also clean out the combustion chamber. I did this and it did help. I have also used Amsol Power Foam...not sure the results were worth the $$

 

You ever have the valves out of an engine and notice big pits in 'em on the bottom? That's caused by too much water. Dont use water, it's lubrication properties are nonexistent and it will cause damage to your cylinder walls. Not a good thing to do. It may help now, but down the road I think it'll end up causing more problems than it solves.

The seafoam has some light oils in it to prevent damage.

Edit: OBTW, all Amsoil products are overpriced.

 

What about motors with water injection?

Besides that.......... When I had the intake off of my truck 2 weeks ago, the tops of the intake valves indeed were cruddy...Maybe I will try the seafoam.

 

I would also remember that water does not compress - put too much of it in your combustion chamber and something has to give - If you are lucky just the head gasket but could bend a rod or do other expensive damage.

 

Guys, it if was deposits wouldn't it ping on full throttle too and not just part-throttle?

 

BB, don't forget the vacume advance on the dist - on part throttle vac is high and timing is advanced, full throttle vacume is low and timing is retarted.

 

WillyB, so you think it's a combination of deposits and too much vacuum advance that's pushing it over the edge?

 

Ok..this I can understand. Now...how in heck do I adjust the advance on my Mallory Unilite distributor? Someone mentioned springs and some sort of key....which I don't have...

 

I am far from an expert but this is covered in most tune up guides - the mechanical advance built in to the distributor has weights that centrifugal force moves outward, advancing the timing as the rpm increases. The rate of advance is factory set by the strength of the springs that hold the weights back.

Changing to a weaker spring will cause more movement, therefore more advance for a given RPM, while a stronger spring will cause less movement and therefore less advance. You can watch this when you set your timing by leaving the light on and speeding up your engine.

 

Sounds like you need to adjust your vacuum advance unit only. Try inserting an allen wrench into the nipple of the vacuum advance (I forget what size it is, try a few). If one engages, turn it CCW one turn at a time and take it for a test drive with the vacuum reconnected. If it still pings only at part throttle, try another turn until it goes away.

 

I dont know what carb you are running but if its a HOLLEY you may want play with power valves . Some times an earlier opening power valve will take care of a part throttle ping. Also holley has some 2 stage power valves that are usefull .
Possibly your distributor has too much advance too fast. Or possibly you are lean in part throttle @ may have to richen your main jets.
I hope this helps HOTWRENCH