VCCA Home Forums TECHNICAL - 1945 AND NEWER 1949-1954 Octane selector adjustment

I am trying to adjust the timing properly on my '51 216 CI engine. I understand that the octane selector has to be set to "0" to do this. I am very inexperienced on this type engine so my question is this: does anyone have a picture of the octane selector for this engine? I see a slotted screw beneath the distributor and on the vacuum advance can bracket and a slot just inboard of this. Is this the selector? If this is it, are there markings for the settings? I can't see any numbers at all on this so maybe this isn't it. Sorry for my ignorance but I am trying to learn.
Jeff

Do you have a 1951 shop manual?

The selector is under or a part of the vacuum advance. On my '41, it came with the new vacuum advance. It should have a series of tick marks with at least a number 0 in the center (may have to clean the crude off if you have a dirty engine like I had).

From what I understand, the octane selector is really just a way to advance or retard the engine timing. The shop manual explains it.

My '41 also has a 216 inline six. I timed it by hand by lining up the little ball in the flywheel to the timing mark on the bell housing to match TDC on the #1 cylinder. My vacuum advance was not holding a vacuum so I put in a new one. I set the octane selector to 0. I believe the shop manual says to try other settings if the engine is running rough, you want better performance, or whatever. On mine, once I changed the vacuum advance, the engine started and revved much smoother with no more pops or backfires, so I saw no need to adjust the selector from the 0 mark.

I do have a '51 shop manual and I tried to find the answer there before posting here but was not successful. If you have a page number let me know.Thanks for the response.
Jeff

Thanks for answering. I will do a better job of cleaning although the engine compartment is immaculate. What's not so good is my vision. Maybe I just don't know where to look. I was hoping that someone would post or send me a picture.
Jeff

I'm not sure how to insert an image into a message on this forum. I can see an option for a link, but I don't have a website (anymore) to post the image to.

Essentially, if you're standing on the right side of the engine compartment looking down at the distributor, the octane selector will be a small tab on the left side of your vacuum advance assembly - it will be laying right on the engine and readable from your position if you lean in close and use a flashlight. If you loosen the distributor, you can turn it slightly to change the selector from 0 to however much you want to advance or retard the timing. I assume you do this with the engine running.

If you remove the distributor, the vacuum advance will also be removable. These marks are, as above, on a tab on the left side of the vacuum advance.

Like I said before, I don't know how important it is to adjust this from 0 to any of the other tick marks. I static-timed my engine to 0 degrees at top dead center and put in a new vacuum advance set to 0. Engine runs great. I think the selector is for if you want to time the engine minutely from 1 to 4 degrees before or after TDC, since you can time it at 5 degrees or more through the markings on the flywheel.

In the 1949 Chevrolet Shop Manual with the 1950-51 supplement, in section 6-20 is a photo of the octane selector and that section tells how to adjust the timing.

This is exactly what I needed to see. Thanks you for pointing me to this.
Jeff

http://chevy.oldcarmanualproject.com/shop/1949_51/4951csm0620.html

That is exactly where I found the picture. I was looking at the wrong location. I did see this bolt at the rear of the VAC but didn't see the markings. I will get a better look tomorrow. Thanks for all your help!
jeff

The octane markings on the vacuum advance are to the left of the bolt in the photo. They show the octane selector set on zero.

Thanks for all your help. I did finally find the same photo in my 1951 manual and was successful in adjusting the selector. I added about 8* of advance and was able to slow down the idle and got a very smooth idle. I couldn't take the car out since it is a miserable day here in Detroit but I moved it around the shop with no issues. We'll see if there is any pinging under load on the road using 87 octane gas. From what I've read, the octane rating the car was designed for and available in 1951 was about 75 and today's regular is 87. It will be interesting to see.
Jeff

Your 13 degrees total initial advance should work fine with today's 87 octane gasoline. You could actually run more a few degrees more without creating a knock.

The problem is that more initial advance can cause starting problems. The engine is turning fairly slowly when driven by the starter. It will "kick-back" because the plug will fire so early in the compression stroke.

I tried to run my '37 with an additional 10 degrees on the octane selector. The starter would often disengage due to the kick-back especially on a hot engine. It starts fairly well at 8 degrees on the selector. When things are very hot I might get an occasional kick-back when starting.

I know that this could be a problem so I let the car warm up well and tried to start the engine again and it spun perfectly. Of course it isn't 90 degrees and humid now so we'll have to see.
Jeff

Welcome Jeff!

Where in SE Michigan? I'm in Bloomfield Twp.
Jeff

I'm in Warren.