So, I have been trying to get things set just right on my truck and can't seem to get the pinging to stop. I verified TDC and set the timing to about 8* BTDC and it seemed to run great, however, while at speed if I blipped the throttle it would ping slightly and then stop. If I mashed down on the throttle I didn't experience this. I have been moving the timing around between TDC and about 10* BTDC trying to get rid of this and still haven't been able to eliminate the ping. It doesn't seem to happen under normal acceleration unless I go beyond 10*. Maybe what I am hearing isn't a ping, it is hard to tell at freeway speeds, things get kinda loud. My other thought is that it could be the valves floating a little bit but without a Tach I am unsure as to whether I am in the valve floating teritory or not. I have been running Plus gas through it.

Truck is a 72' 240ci with a 3spd that is stock from top to bottom as far as I can tell. If anything internally has changed I would be suprised. It is running the stock ignition, carb, and exhaust manifold. Got some EFI manifolds I will be putting on there as soon as I get the bugs worked out and plan to run dual 2.25" pipe with some glasspacks of some sort.

Any ideas on my delema would be appreciated. Come to think of it I think it may just be the valves floating when I blip the throttle.

 

Have a look to see if your timing advance is functioning.
Pop off your distributor cap and twist the rotor lightly.
Make sure the mechanical advance is still hooked up, and is springy.

Then while the truck is running, pull the vacuum hose off the
vacuum advance diaphragm. When you do this, watch for the mechanism
to release, retarding the ignition.
This might not be visible if your truck is pre electronic ignition.

 

forgot to mention, if I take the vac line off and suck on it I can get the timing to advance. Haven't checked the mechanical advance, but the distributor is supposed to be "new" according to the prior owner, I'll check to see if the springs are working/stiff enough. Ignition is points, condenser. I wonder if when I blip the throttle it is cause the vac to spike and making the vac advance too aggressive?

 

It's actually the other way around.
When you hit the gas, your vacuum pressure drops, and then retards timing.
You loose vacuum pressure with the throttle open because,
well, the throttle is wide open.

The reason I mentioned looking at both mech. and vac. advance,
is because they work together like this.
You're cruising down the street.
Your rpms are high, so your distributor is mechanically advancing your timing.
You have to give it a lot of gas for some reason.
Now your throttle is open, your rpms are still high, now your vac. adv.
retards the timing six or so degrees so you don't get pre-detonation, or ping.

If your system is doing this correctly, and you still hear pinging,
timing advance shouldn't be an issue.
You might have to look at something else.

Let us know,
Murph.

 

so when I blip the throttle I should be losing vac momentarily decreasing the amount of advance to avoid pinging? If the vac diaphragm is sticking it could cause this? That would make sense. Sometimes after cruising around and coming to a stop the idle seems to be on "high idle", or perhaps, it is just the diaphragm sticking open, advancing the timing increasing the idle. I usually give the throttle a tap to get the idle to come back down. The carb was recently rebuilt so it shouldn't be the cause of the high idle issues I seem to experience occasionally.

 

The diaphragm sticking could cause ping.
Seems odd that it would stick in that position though.
The spring that it would be keeping compressed is a pretty strong one.
Stranger things have happened.

This shouldn't have anything to do with your high idle though.
That probably has something to do with you throttle linkage.
Something may just be hanging up a bit.

If you think the vacuum adv. is sticking, you need to check it with vacuum gauge.
Call around parts stores and see if they rent or loan them.
When you get one, here's what you do.
Pull the hose off of the diaphragm.
The truck doesn't need to be running for this, so if you need to
pull the distributor cap off to see the vac. adv. in action, that's ok.
Put the hose from the vacuum gauge on the nipple you just
removed the other hose from (diaphragm).
Slowly pump the gauge and notice how much pressure it takes
to move the vac. diaphragm through it's entire travel.
Should be around 6or 7 inches of pressure. That is fully advanced timing.
In your case here's what you're looking for-
When you release the gauge, watch to make sure your diaphragm snaps back.
Do this a bunch of times. If it doesn't snap back, that's the sticking we're talking about.

Hope this helps,
Murph.