Problem: 1976 F-250 with 390 topped off with a recently added Holley 4160. Engine and and truck are mostly stock. Unless I use premium unleaded gasoline the motor pings terribly under moderate (normal driving) acceleration. Timing is set to factory specs. Any thoughts on what I can do to mitigate this problem so I can run a less expensive, lower grade gas. At 10 mpg every little bit helps.

Thanks in advance.

 

Put a timing light on it and rev motor to above 2500 rpm. Watch your timing mark. Does it go to oblivion?? Take the vacuum off the advance and check again. It should stay within reason.

My '67 did the same thing, constant pinging on highway under any moderate acceleration. I found the vacuum was pulling 60+ degrees of timing. NOT good. I disconnected it and left it off(plug line from carb). Now I have it set for about 35 total timing(around 8 intial). It runs like a champ.

Larry

 

A cam swap will help your pinging problem. I'm running a Crane 272 degree Energizer in a 10.5 to 1 390 on 87 octane, where it would ping like your's with the stock cam. Timing's set at 12 degrees BTDC now.

 

Is it possible that the "recently added Holley 4160" is set waaaaay to lean? Initial advance should be around 12 at idle. Whatever you decide, stop that pinging, it will destroy your pistons.

 

William:

Yes, it is entirely possible. However, are you thinking idle mixture or main jet size? I can easily reset the idle mixture using a vacuum guage, but, checking whether or not the carb is jetted correctly for the type of driving I do is beyond my experience. Any further input would be greatly appreciated.

Timing is set to factory spec (12 degrees) using an inductive timing light with the distributor vacuum advance unhooked and plugged.

I did follow the advice of one poster to unhook and plug the vaccum advance line. Doing this did seem to help. Further driving this weekend will give me a better idea.

Thanks

 

It would be main jets (primary). Your carb is most likely setup for a small CI engine and uses small primary jets for max fuel economy. I would re-jet it ASAP! In the mean time back the initial timing to 8 degrees before or buy premium fuel. Just for grins; engine idling and warm; use a hand held propane torch, not lit, and spray propane around the carburetor base and around the intake to cylinder head area, looking for a leak. Do you have an egr valve? sometimes It would be main jets (primary) and that is as far as I can help with Holly carburetors. Your carb is most likely setup for a small CI engine and uses small primary jets for max fuel economy. I would re-jet it ASAP! In the mean time back the initial timing to 8 degrees before or buy premium fuel. Just for grins; engine idling and warm; use a hand held propane torch, not lit, and spray propane around the carb base and around the intake to cylinder head area, looking for a leak. Do you have an egr valve? Sometimes the egr will stick open and lean out the mixture. Usually the engine runs like crap when that happens, huge vacuum leak! Sometimes carbon will build up on pistons and actually increase compression. The carbon gets hot and causes pre-ignition and dieseling. Before your next oil change; engine warm running at fast idle 1800+ slowly dribble a gallon of water down the carburetor. Don't stall the engine.....just run the water through it. This will clean the carbon deposits out of the combustion chambers. Then let it run a while and change the oil. Worth a shot and costs nothing. Ever seen a piston that has been working with a leaking head gasket, clean clean clean!

 

Make sure if you're using propane to check for vacuum leaks that you have a fire extinguisher handy (you should have on in the shop anyway, but...)

Don't forget you've got upwards of 25,000volts running through those plug wires. A stray spark could really ruin your day!

 

I personally prefer to just listen to the motor to tell whether or not it's got a vacuum leak Never did like to spray propane around a spark source Unless I'm trying to kill a wasp nest ( that's not near any other flammable material)

 

Use some carb. cleaner to find those vacuum leaks, you'll also have a clean engine when you're done!

I had the same pinging problem when I replaced the 2700 variable venturi pos carb. on a 302 with a replacement Holley. They pull more vacuum out the vac. adv. port so I adjusted the vac. diaphragm on the dizzy with an allen wrench to slow down the advance.

Barry

 

Barry:

Could you please provide more specifics on how you adjusted the vacuum diaphram on the dist or how I can adjust mine properly.

Thanks for the reply.

Mark

 

Oh, sorry about that. Try sticking an allen wrench down the nipple of the vacuum advance, I forget what size it is. Some are adjustable and some are not, you'll find out if you don't get any allen wrenches to engage inside.

Turn the allen wrench CCW one turn at a time and take it out for a test drive. If it still pings, turn it CCW another turn and test again. Write down the number of turns when you are done so you have a reference. Be gentile when turning the wrench, the diaphragm is very tender.

Barry

 

Thanks Barry:

I will give it a try tonight.

Mark

 

The vacuum advance for the dist helps in part throttle driveability and fuel mileage. I would adjust the vac or get an adjustable unit, don't leave it unhooked.

 

Usta was that the old dists had a removable fitting in the vacuum so you could add (or remove) washers to preload the spring and thus adjust vacuum advance. Yeah, I am old. That said, you can also use WD40 to check for leaks. If I were doing propane, I'd have a rubber hose over the end so I could direct the propane EXACTly where I wanted to check.
If you have the old FoMoCo carb, try putting that back on to see if the ping goes away. Pinging will kill your gas mileage as the pistons are trying to go the wrong way and the ones that have just fired are working against the effect of the ping. If the Holley is new, or not, you should be able to get the jet size recommendations from them for your size engine.
Last thought is that you may have hooked your vacuum advance line to the wrong port... By '76, some dists had advance AND retard connections. Is your dist factory? No vacuum advance at idle is the commonest setting of that time, and if you swapped carbs too, it may just have too weak centrifugal springs.. or too much centrifugal advance.. Dang, too many things changing all at the same time...
tom

 

The older style vacuum advances were more adjustable. You could shim the spring to change the rate of advance and you could also change the spacer to adjust the advance limit or stop. With the newer ones and a allen wrench you can only adjust the rate.

Barry