Small block 350 horsepower , just filled up with 93 octane highest I can get at $3.15 per gallon but it pings when accelerating. Does it mean my timing needs to be adjusted.
Any suggestions?
Thank you,
Kurt

 

yep, retard your timing.
try 2 degrees of timing at a time until the pinging stops.

 

Ping means that the fuel/air mixture is burning prior to the spark. In the old days, lead was there to counteract that (it cooled the combustion chamber), and today's fuels have a higher octane to combat it.

When you're running the highest octane that is practical, then retarding the timing will be your next course of action.

Of course, this does assume that you're not running hot at the time when this occurs -- as a overly warm engine can cause this, and then the timing can help reduce temps, but it might be "fixing the effect, not the cause".

Brian.

 

Coolant level good? This also could case a ping. WB

 

 

Start with your vac adv, unplug it from the carb and plug the port then go for a drive, if the pinging stops then you need a diff vac adv can, if not adj the timing down as suggested, once the pinging stops hook up the vac adv can and drive it again, if no pinging you are good to go.

 

hi, I'm not disagreeing or saying your wrong, but can you explain the reasoning for a different vacuum can.
Assuming the can is working and that it's the correct can for his motor, each vacuum can was specified for different motors based upon the advance curve of that motor.
Changing to a different can will change the overall timing curve.
The vacuum can simply adds up to a specified amount of additional advance based on the specs of the can for a particular motor.
If he is pinging and assuming no other "problems" with the motor such as overheating, backing his timing down should cure the problem without having to change to a different can and therefore changing his entire advance curve.

On a '68 L79 motor I believe (someone correct me if i'm wrong) the correct vacuum can on todays replacement units is a Echlin VC-1810 can. If you are saying to change out to a replacement can of the same specs because his can may not be functioning correctly that is one thing, but I'd see no reason to need to change to a can of different specs.
It's easy enough to check to make sure his can is working - simply disconnect the vacuum line off the can and if the rpms drop the can is working (I believe the '68's were still hooked to a full manifold port rather than a timed port??)

 

Unhooking the can is quick and easy and will let you know if your initial and mech adv is set right. When setting your timing you do it without the can hooked up, then you hook the can up and if it pings you need a can with less timing added, a can the releasing the added timing faster or a combination of both.

Just retarding will work but may yeild less performance. Of course it is up the car owner which route they choose to take to combat the problem.

 

i'm well aware to unhook the can when setting timing.
again, assuming he does have correct can installed currently there is no need to change out to a different spec'd can, all he needs to do is back off the timing slightly. Most likely a simple 2 degree retard will do the job for him.

It's easy enough to find out if he has the correct can installed. either look at the number stamped on the mounting flange or if there is no number, disconnect the can, see what total timing is, rehook up the cam and than see what the new total timing plus the vacuum advance is. That difference will give the spec on the can and we can determine if he is running the correct one.

 

I am assuming this "can" you are refering to is the pie plate shaped thing sticking on the side of the distributor. It is the one that came with the car in 1972 when I bought it, and it was on the distributor when it was sent out to a machine shop to be checked. If there was anything wrong with it wouldn't they have noticed that? The distributor is the correct one for the car and since I have owned it (1972) it has always been in the car and never replaced or altered, including the "can".
Does that help?
Kurt

 

Might help to know, did it always ping, did it get worse after last tank of gas, did you do anything else to the motor? Any chance your vaccum line to the distributor is disconected, engine temp reading hotter than usual. Double check the points to make sure they are not burnt enough to make them arc.
Just a couple of things to check before you start moving the distributor on a motor that did not ping till now.

 

OK if the the adv can is factory correct and it should not be changed why would you change the timing if it is set factory correct?

 

You could run some SeaFoam through your fuel system to help eliminate any excess carbon deposits which would raise the compression ratio causing detonation. If it still pings and the vacuum advance is working as it should you'll have to turn the distibutor clockwise a little bit to retard the timing. In a way vacuum advance is something of a misnomer since it's purpose is to retard the timing under load.
My engine runs great on 91 or 93 but starts to ping a little with 87.

 

yep, that's the thing we are talking about.

 

pull the distributor cap off and check the operation of the distributor...make sure the shaft the weights are located on is moving freely and able to rotate....these often slowly sieze up and stop functioning perhaps yours siezed up with full advance....if this is siezed you will not be able to fix this timing issue until the shaft moves freely

 

The original vacuum advance was selected based on the intake manifold vacuum characteristics of the particular engine/transmission combination and how it was expected to perform in daily use. Based on his L79 engine, and going by what he has already said, he has the original and correct vacuum advance for his motor. Changing that to a different unnit will change either the amount of advance or how soon the advance kicks in changing his overall distributor advance curve.
Why re-enginner a system that was perfectly fine to begin with? Doing so you are only asking for trouble and poor performance and drivability. These vacuum advance units selected for each engine/tranny combinations were choosen on a whim or because Chevy got a good deal on a certain spec can so they decided all the cars would now get that same one.

Changing the timing is another matter. By retarding the timing back approx 2 degrees or so is not changing the curve of the whole system, it's only shifting it slightly down to avoid the timing being too high and having detonation which can easily destroy pistons if it isn't eliminated. Besides, how do you know his timing is currently "factory correct". although from the sounds of it he has the original vacuum can SOMEBODY, at SOMETIME has tuned up the car and readjusted the timing since it's left the factory! who knows what it's set at now. Also, look at the specs listed for the timing in any shop manual it it usually gives a range of values to set at - for example, on my '65 L76 motor the manual lists initial timing should be set at BETWEEN 12*-14* and total timing BETWEEN 34*-36*.
What if it was set currently at 36* and I backed it down 2* to 34* total. I'm still within 'factory correct" specs.
Another factor to consider from the early cars to todays cars is the difference in the fuels. Without the lead and the relatively lower octanes many, if not most of the cars, need to run slightly retarded on the timing to avoid detonation. This can be done without messing around with the correct advance curve for a particular motor.

 

Guess I just like things simple, pull a vac line and take it for a drive.

 

make it even simpler - leave the vacuum line and just drive. Oh wait, still need to get rid of the pinging first before we can do that.

sorry, I've just learned the hard way, in a very short amount of time doing my own work on my cars that there aren't any real shortcuts to doing it the correct way and end up with the correct and proper results.

One reason I gave up taking my Vette (starting with the '65 and now the '78) to mechanics is that I went thru a good number of them and NONE of them seemed to have a clue about working on older cars, especially with points ignition systems and getting them set right.
their idea is setting the timing and setting the proper curve when they saw the timing was too high was to plug the vacuum hose to the can with a ball bearing so I had NO vacuum advance only. I'm not kidding, I gave up on the first mechanic that did that and tyried a second one. He did the same thing so I tried a third one. When he did the same thing I gave up and started to learn how to do my own work. Doing something like that just proves that they knew nothing about the workings of the vacuum advance system, why it's there and how it operates.
I'm NOT grouping you in with thses Bubba mechanics by any means, I'm just making a point that while there may be multiple ways to fix the symtoms of a problem, there is usually only one correct way to properly fix the problem itself.
Changing out to different can may eliminate the ping if he used a can with less total advance in it but that's changing the correct amount of advance that that distributor and that motor should have in it.

 

Well, from reading what he's saying, I think the whole issue of a 'factory correct" anything on timing curve is a moot point. He says it's a small block making 350 HP and it has a '72 distributor in it. Those two things don't go together...so I'm guessing that this motor is NOT stock and has some aftermarket parts. Plus he said dist. was sent out somewhere to be checked...no telling what mechanical curve is in it now.

Give us some more info on the engine including compression ratio, heads (iron/aluminum?) intake, cam, carb, trans, gearing, A/C and other accy's etc etc and we can help you come up with what will work.


JIM

 

where does it say that?
He says it's a small block making 350hp and his profile lists the car as a L79 motor so that would be correct. I don't see where he said the distributor was from a '72 - he said the distributor is the correct one and the same one that was in the car when he bought the car in '72.
He also mentioned the can itself is the same one that was on the car when he bought it in '72.
Now, is it possible that in the four years the car existed before he bought it that the distributor was changed or the can was changed? Sure, anything is possible, but from the sounds of it it seems that he has the correct distributor and the what is most likey the correct can also.
In four years prior to his ownership who knws what else was changed in the distributor but chances are it was probably only springs if anything to maybe gain a bit of performance at the time. I agree though that the springs should probably be checked to assure they are the correct ones for a correct curve on that distributor.
In fact, after all these years it wouldn't be a bad idea to pull the entire unit and have it rebuilt, check the end and side plays, shim if needed, relubricate the unit, and get it on a Sun distributor machine to set it up for the proper curve.
on the other hand, if he checks the dwell and it's not fluctuating all over the place, than it should be fine for now and he could hold off on a complete distributor rebuild until next winter when the car is stored for the cold weather and just retard the timing a bit to eliminate the knocking and the detonation which is not good for the motor