Spark Knock:

[40ford]

I am getting a spark knock(pinging)when my truck(1952 F-1,completely stock 8RT flathead) gets a slight load or pull on it.Never had this problem before.I do have about 12,000 miles on points and condensor.Maybe this gas today has got worse?Could timing maybe be adjusted to help,if so which way to move the distributor?I am guessing my plugs probably have about 25,000 miles on them.Can it be time for a complete tune up?

[38 coupe]

Winter fuel is definitely a different mix than summer fuel. Not sure if that would result in pinging.


I would pull the plugs and check them, and also verify ignition timing and point gap.

[RalphG]

Only time I can remember a spark knock (ping) was when I had the timing too far advanced on an International V8. Never heard it on a flathead. I'd try adjusting timing on yours.

[cadillac512]

40- You'd turn the distributor clockwise as viewed from the top to retard the timing to lessen knock.



Terry

[Gene1949]

Given the mileage you have on the points and plugs I am surprised it even starts.
Before you change the timing or blame that evil gas (think of the garbage we used to burn back in the day) I would replace the points and plugs.
Timing is DIRECTLY effected by point dwell

[3twinridges]

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If you are running the stock distributor, your advance springs could be allowing too much advance on higher load and vacuum. After you perform a basic tune-up, and if those conditions continued, I would look at that.

The advance is controlled by two adjustable studs on the breaker plate, and you need a distributor machine to tune them.

JB

[petehoovie]

Originally Posted by Gene1949

Given the mileage you have on the points and plugs I am surprised it even starts.
Before you change the timing or blame that evil gas (think of the garbage we used to burn back in the day) I would replace the points and plugs.
Timing is DIRECTLY effected by point dwell

Gimme some of that "garbage" gas we use to burn back in the day, any day!
Corn gas sucks!

[35fordtn]

Be sure your vacuum advance is working. If so use higher octane fuel or retard your timing a tick

[40ford]

Originally Posted by 35fordtn

Be sure your vacuum advance is working. If so use higher octane fuel or retard your timing a tick

I thought about the vacuum advance maybe gone bad.What would be the easiest way to check it since I have a metal line going to it?I did replace the complete distributor probably 30 years ago.Truck is my daily driver and usually used every day.

[38 coupe]

Originally Posted by 40ford

What would be the easiest way to check it since I have a metal line going to it?

Take the vacuum line loose from the advance can, pop your distributor cap off, rotate the points plate against spring tension and hold it there, put a finger over the advance can port, and let go of the points plate. If the points plate rotates back the whole way and you don't feel any suction on your finger then the diaphragm in the can is bad. If the points vacuum advance holds the plate from fully returning and you feel suction on your finger then the advance can is good.

[rotorwrench]

A person can check the advance vacuum can with a mityvac or some other vacuum source using a rubber cone tip to fit in the fitting bore. With the distributor cap off, apply vacuum and see if the points plate moves or not. Corrosion can damage the advance springs but the rubber diaphragm has a much shorter service life than the springs. New cans are available. If it's moving well then a person would have to check other possibilities.

[JayChicago]

Could also do a quick check by just sucking a vacuum with your mouth. Maybe can't move the plate with the slight by-mouth vacuum. But if you are just sucking air, not able to pull any vacuum, you have diagnosed a failed diaphragm.

[supereal]

If you are using a non stock carb, it is likely the vacuum port is not correctly placed below or above the throttle plate. The "snubber" in the early models places a brake on the point
plate to retard timing, while the later carb uses the vacuum to operate the diaphragm that moves the plate,

[Flathead Fever]

Under a load the 1949 to 1953 flatheads vacuum drops off which causes the vacuum advance to back off the timing. Because the throttle is opened further under a load than normal driving the cylinders get more air into them. This increases the cylinders pressure which causes the pinging. The vacuum advance is designed to counteract the high pressures under load by retarding the timing. They could have created all of the advance using a mechanical advance, like the hipo Fords in the 1960s did, but this requires high octane fuel because of the extra advance during a load going up a grade or accelerating. You want that extra advance for optimum power if you are racing using high octane fuel. But the average customer wants an engine that will run on cheaper octane fuel. The vacuum advances name is a little misleading because it also retards the timing. It allows full advance under light throttle driving for better fuel mileage and performance. It was engineered so that if it did fail the timing would retard at idle rather than advance. A bad vacuum advance diaphragm would not make an engine ping. The only way would be if the mechanism that rotates the points plate were to get stuck in the advance position. Make sure all of the mechanical and Vaccuum advance mechanisms are free to return back to their fully retarded position. You can also check the vacuum advance diaphragm by pushing the arm in and then putting your finger over the vacuum port. If the diaphragm is holding vacuum the arm will stay in. If not, the spring pressure will push it out. That was fastest way to check them when I was doing tune ups. You also need to check that you are getting the proper vacuum to it.

Early flatheads with a "vacuum brake" use manifold vacuum. That means the source for the vacuum is below the carb's throttle plate. Vacuum will be there until the throttle plate opens up far enough that the air is all flowing through the carbs venturi. There is no more manifold vacuum at that point The late flatheads 1949 tp 1953 use a vacuum advance instead of a vacuum break. That uses venturi vacuum. That means the vacuum port is located above the throttle plate. There is no vacuum there at idle when the throttle plate is closed. As the plate opens and air passes by the upper port creates a vacuum. You measure the mechanical advance using an adjustable timing light by raising the rpm with the vacuum disconnected and plugged to the vacuum advance. You point it at the timing mark with the rpm raised to specification and you rotate a knob on the timing light which will make the timing mark move back towards TDC. Once it reaches top-dead-center you look at the number lined up with the pointer on the knob and that tells you how much advance it took to get the timing light mark back to TDC, then you measure the advance again with the rpm raised and the vacuum line hooked up and compare it to the specification.

Sometimes you can have an engine with poor performance and fuel mileage because of a failed diaphragm. So, then you replace it and then the engine pings and the customer is not happy. Then you have to find out why it's pinging. Maybe two diaphragms have failed. On modern engines the number one reason for pinging is a failed EGR valve. The timing is usually computer controlled so it's usually not the problem. First thing I check is that that EGR valve is working properly.

The pinging could very well be from the winter blend fuel. Here in CA when they changed it each year my '94 pickup used ping on a warm day. The knock senser should sense that pinging vibration and retard the timing but it wasn't, or it wasn't able to retard it far enough. I would check your base timing, mechanical advance and vacuum advance using an adjustable timing light. Never buy one that is not adjustable. It lets you measure how much advance you are getting. You can back off the timing 2 degrees at a time until the pinging stops or try a higher-octane fuel.