Again, I apparently find something I simply dont understand. After dissecting a distributor to a million pieces on my chevy 216, I understand just about how everything works except the octane selector. It simply doesnt appear to do anything? I made a sketch to reference by. From articles I read, in the days of yore gasoline octane was not especially standardized. Some days you could end up with lower octane than others. Apparently one could adjust this with the octane selector and make due, until some good stuff is found. It read, Now that octane has an minimum value of 89 it is no longer necessary to use this feature? Anyway? How does it work? I see only slight mechanical force? Perhaps the grease being forced behind, or withdrawn from the Bakelite bushing causes some resistance to the shaft, and allows the vacuum advance to either move with more of less resistance? Anyone know? And can one just ignore the adjustment now-a-days? Just set to zero? Do tell! Thanks again!
Your sketch shows an old fashioned, manual greaser. Screw it in a bit, to lube it. The octane selector, as i recall, is a timing adjustment scale, where it is numbered, to give the owner a sense of where the timing was, and is.
I guess I just don't see now it can change the timing? The is no "positive" mechanical action? It only forces the bakelite bushing against the shaft? How can that change the timing? It has me baffled??...lol
It's a manual grease fitting. It is not a timing deal. From memory 3-4 years back on a friends 48....Set timing to the flywheel "ball" with the timing "scale" set at zero?. Now, if the owner gets low octane, he himself, can set his own timing "back" or retarded, to get rid of the pinging. Then he gets better gas, looks at the scale, then adjusts it back to where it was.
Well that makes sense! No wonder it doesn't appear to do anything! Thank you so much...Told yeh it was for dummies! Peace!
I could understand the function if it terminated at the dizzy plate but terminating at the shaft doesnt do squat other than to wear out the bakelite .... maybe it is meant to reduce wobble in the shaft.
The drawing shows the grease cup for the distributor shaft. Has nothing to do with adjusting timing whatsoever. Per manual it should be tightened down 1 turn every 1000 miles, to keep the shaft properly lubed. When you can't turn it anymore, unscrew the cup and refill with grease. The 'octane selector' is part of the vacuum advance and simply allows quick adjusting to different octane fuel by just loosening the bolt that holds down the vacuum advance. One step, engraved on the vacuum advance, equals 2 octane points up or down, from the zero setting. Earlier chevy engines had a thumbwheel adjustable octane selector with set screw, that didn't even require loosening a bolt. It's true, that with today's supposedly stable and accurate octane fuel from modern pumps, you don't have to worry about adjusting your distributor anymore after initial setting. Adjust the timing by listening for slight ping under heavy load, like pulling up a moderate hill in high gear at full open throttle. Then turn the distributer back so it just stops slight pinging and bolt the distributor clamp/vacuum advance down. Btw: The shaft is supposed to wobble a bit to aid in preventing wear against the cam shaft gear. That's why there is no means to tighten it down to a hard fixed position.
Chev48-Ditto what everyone said. Depending on whether or not your vacuum advance unit was replaced or not, some of them don't have has marks & numbers stamped into them. You can still set the timing roughly and time it by ear & feel once you get it running. Setting mine straight up and using 87 octane, I was still getting a little ping. A little fine tuning and it was fine. As long as she's out, its a good idea to check your vac. advance unit with a vacuum pump to see if it works. While you have the distributor all apart, make sure all those bakelite bushings are intact and not cracked or crumbling. Some of those, especially on the side of the housing, can really be a pain to replace once its stuck back in the car. Over time those can break down and cause some fun electrical issues down the road. Just wanted to throw that out there to help a guy out.
The octane selector is just a fancy name for a way to advance or retard your initial timing. If you loosen the bolt holding the octane selector to the block you can turn the vacuum advance unit which will change your timing at idle by turning the entire distributor. There is no breaker plate in the distributor so the whole distributor will turn. We just went through all this on my buddys 1952 bel air. If you pull the distributor to replace the advance unit you'll obviously wanna check the timing. Here's what we did and it seems to be working fine so far: Put the #1 cylinder on top dead center of the compression stroke (so both valves are closed) Install the distributor so the rotor is pointing to the #1 spark plug wire terminal of the cap once the distributor is seated all the way down. Put the cap n rotor on and fire it up to check the ignition timing. (There is a window with a pointer on it on the front of the bell housing you can see from the passenger side of the engine compartment). If you have a timing light with an advance option set it to 5 degrees advance (chevy specs out of shop manual) and it should line up with a steel timing mark on the flywheel. Once that is set tighten the distributor shaft clamp which is the flathead screw clamp on the bottom of the distributor shaft. Leave the 1/2" bolt holding the octane selector loose. It will probably stumble off idle if the octane selector is not positioned properly. The shop manual says to set it at zero but it was still stumbling during acceleration off of idle so we retarded it a couple degrees on the scale on the octane selector and it runs fine now. Hope this babbling helps you out
When your 48 Chev was built the best gas was under 80 octane, regular under 70. So the engine was tuned for 65 octane gas. If you got a load of petunia juice that was under 60 octane you could back off the advance to prevent pinging. Today there is no chance of getting gas that low in octane. You can set the timing for the best performance or best vacuum reading and leave it there.
I know this post is from years ago but maybe these 2 pictures will help someone Octane_selector[ by gmpenn posted Aug 7, 2017 at 10:03 AM Octane_plate by gmpenn posted Aug 7, 2017 at 10:03 AM
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