It's my understanding that this is caused by the air being a lot denser in the winter than the summer. My default thinking is to retard the timing, but I've been wondering if changing the metering rods a size smaller would compensate enough to make it run less lean as an alternative?

 

I have only had noticeable pinging on one around 8 to 1 compression Olds V8, a 307 I ran super lean jetting on. Otherwise, I have ran 60 plus degrees on 87 octane. You could get Cliff's 44 primary rods or you could just pull the plug out above the APT and turn it counter clockwise till the pinging goes away.

 

If your 403 is stock and well tuned it shouldn't need that, unless the timing was advanced in the first place.

403s were designed for 87 octane, but if timing has been advanced better gas quality is necessary to offset the modification.

 

I did advance the timing a little but that was only to smooth it out. Even though this is totally stock it wasn't happy with the timing where it was, though i can try resetting it as i marked the distributor position. It was idling a little rough and timing advancement smoothed it out, even after readjusting the idle speed to keep it at 550 on drive. I'm also not 100% sure what I'm hearing actually is spark knock just because I have heard intermittent noise I could trace to the speedometer. Sometimes I wasn't sure exactly. There's only one throttle position I seem to hear anything out of the ordinary at all regardless of the source.

One thing I noticed that i don't understand, it's my (limited) understanding that when finding the timing that works best you can turn the distributor to where the the vacuum pressure gets to the highest until it starts dropping again. But I found when advancing it that no matter how far it was it never dropped, but rather just peaked and plateaued. Is my understanding wrong?

 

You could try putting the timing back where it and switch from ported to manifold vacuum. It should smooth out the idle like you want. And it won't give you too much total and probably cause your pinging. You could probably get away with a couple more degrees timing. I have ran near 50 at idle, around 60 part throttle and around 40 at WOT with no noticeable pinging on a 8 to 1 403.

 

A couple things I'd try before changing metering rods would be a higher octane gas or gas from a different station. Ethanol content may vary and who knows how accurate the rating are on the pump placards. Another thing would be to shine a light in the air cleaner snorkel and make sure the flapper for the heat stove is opening at operating temperature.

 

It is not. The density of dry air at 32F is about 1.28 kilograms per cubic meter (kg/m3). At room temperature, the density of dry air is about 1.20 kg/m3. Difference is about 6%. Back in the day, no one had to change their engine's timing every time the season changed. IF you have spark knock, and you say yourself that you're not sure that what you're hearing actually is knock, it's due to something else.

 

Being able to maintain 550 rpm drive idle is impressive. She must be running pretty good. My 350 is set at 575 rpm drive idle, and fluctuates from 550 to 600 depending on her mood and how aggressive I drive. But when she is around 550 she seems close to stalling at times....

Here is what pinging sounds like. (It's the undertone chatter occurring on acceleration.)

Yes and yes. Great points shiftbyear.

illumined, gas can noticeably vary regardless of octane match, from station to station. Those of us with original 1960s engines, can hear, and feel a considerable difference. From bad gas to good gas, from low octane to high octane. My car pings and has run on with 91, with 93 no run on but occasional ping. Adding octane booster reduces the occasional pinging to almost non existent. 100 octane blended with 93 the engine runs flawlessly.

And this video below may provide a refresh for you with regards to flapper. 5:40 minutes to 7:40 minutes.

You Tube

P.S. speedometer cables can be noisy, especially when they are about to go. I've been through it.

 

Great suggestions above, you can also consider going to a colder spark plug for testing.

 

Sorry I hadn't been able to work on this for a while. I put premium in just to see what would happen, made no difference. However, I think you're right about the speedometer cable. I heard that noise again under low throttle but I was able to pay more attention to where it was coming from, it was the speedometer, no doubts this time. I don't think the engine was pinging at all so this was a false alarm. Better safe than sorry.

 

The speedometer cable noise will get louder till it snaps ... At least that's my experience.

If the majority of gas in tank is still low test, you may not see an improvement if there is one to have ...

And if pinging has been ruled out great. And thanks for responding back. A bit dejecting when advice is given and there is no follow up. At this point some of us have an almost equally vested interest as the questioner to see if the issue has been resolved. Plus its really nice when others search threads like this and actually see the confirmed resolution.

 

I ran it down to less than 1/4 tank and refilled the rest with premium, I would think that would have been enough, overwhelmingly it had 92 in it.

 

Definitely, even if the 1/4 left was crap you would notice a difference with that much offset.