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I am working on timing the 41 eight and have carefully read the Shop Manual and other references on the procedure to adjust the distributor. There seems to be 2 different ways of adjusting the distributor,and need to know how this works. In the timing section, the shop manual talks about a "distributor holding plate" and about a "distributor clamp", clearly stating they are different components (see photo # 1).
The 2 parts related to adjustment are shown
I understand how the clamp at the bottom of the distributor works regarding adjustment. I did in my car and the only position in which the engine improves, is a rotation in clockwise direction, all the way to the max, which corresponds to advancing. However, the timing light shows me the engine is still retarded 5 degrees and there is no further rotation possible to advance more.
Therefore, my question is: do I have to adjust this "holding plate" to further advance the timing? How does this "holding plate" work in adjusting the timing of spark ignition?
There is only a very brief reference regarding this holding plate in the shop manual. Photo #2 shows a close up of this plate behind the distributor. Notice the marks at the top showing 10 degrees advance or 10 retard.
I found a curious paragraph in the Owner's Manual stating this plate is used to adjust the engine depending on the octanes contained in the gasoline product that the driver wants to use. Again, no explanation on how that works. (See photo # 3)
So,I decided to experiment and after losing the single bolt that holds this plate to the engine block, I moved it counterclockwise toward the side that says ADV (advance) a total of 2 lines (I think that is 4 degrees). To my surprise now the timing light shows the indicator coincides with steel ball on the flywheel. I don't understand how this works but after that the engine has improved substantially.
When I receive a Dwell meter ordered online, I will try to adjust the points exactly where it should be and go over the timing process again. For now, I just want to learn what this "holding plate" mystery is. I would appreciate your comments and provide some insight. Thank you all. Manuel
I have the same adjustment plate on my ‘41 eight. I think you’ll find the “clamp” they’re talking about is actually the bolt clamping the “distributor adjusting plate” to the distributor body. If you don’t have enough movement in the slot on the adjusting plate to get the timing correct you’ll need to loosen the “clamp” at the bottom so you can move the distributor body independently of the “adjusting plate” to get some more movement. I hope that makes sense.
Leigh
Didn't MaineOlds run into something similar with that "octane adjuster" plate on his 49 303?
Gasoline in the 40s was about 75 octane from what I've read and compression ratios were low enough most engines had no problems using it. I can see where that "octane adjuster" would be beneficial if your area's fuel was good enough to allow more spark advance.
I have the same adjustment plate on my ‘41 eight. I think you’ll find the “clamp” they’re talking about is actually the bolt clamping the “distributor adjusting plate” to the distributor body. If you don’t have enough movement in the slot on the adjusting plate to get the timing correct you’ll need to loosen the “clamp” at the bottom so you can move the distributor body independently of the “adjusting plate” to get some more movement. I hope that makes sense.
Leigh
Yes, the "clamp" is the bolt under the distributor that when loosened up permits the distributor to turn for the adjustment. The instructions for timing state that the first thing to do is to center this "adjusting plate" at 0 (zero) and tighten it in that position with the bolt clearly seen in my 2 photos and then loosen up the other bolt, under the distributor, to release the clamp and rotate the distributor in what ever direction is needed. The "adjusting plate" octane selector seems to work independently of the clamp. What I want to understand is how it works, what is it doing to the distributor, and how should I adjust this for the current high octane fuel (I use ethanol-free, 92 octanes) in an effort to make the engine work smoother. I hope the question makes sense.
As I said in your other thread, I used to work on 1940s & 1950s farm tractors and occasionally tinkered with a couple old straight eights and some of this comes from memory a very long time ago. You are following the procedure in your shop manual correctly. You now know what the steel ball looks like, where the timing "window" is located, and the best timing for your engine (I can't recall what it is when the timing is adjusted correctly). But when the steel ball (timing index) aligns with the timing mark you're at TDC and moving distributor CCW or CW will advance or retard your timing. I can't recall on an old Oldsmobile straight-8 if CCW is advancing or retarding - you can figure that out.
To address the "octane selector". The octane selector is a primitive distributor vacuum advance. Obviously you know by now your car ran gasoline of the 74 octane to 76 octane range back in the 1940s. In fact, it may have run far out of that range based upon the technology of the day - you may have received 68 octane one day and 78 octane another day. The point I'm making is this. The octane rating is related to the ability to fire off compression stroke in the engine and at the desired correct timing. If you had a very low octane rating one week/month or whatever based upon type of fuel they delivered, you could adjust the distributor vacuum advance to accommodate for disparities in fuel octane ratings. You're reading it correctly and adjusting correctly (to zero "0") when you initially adjust the timing. If you need more vacuum advance (to accommodate for a less desirable octane rating) you can adjust the octane selector accordingly. I believe, I don't recall this exactly, but I think the octane selector really provided the most benefit at "slow idle"; I don't believe it really had much of any effect at cruising/road speeds. I think the actual adjustment effected an orifice inside the distributor which opened or closed vacuum - I really can't recall, it may have been a small mechanical selection inside the distributor where the vacuum advance is mounted to the distributor.
Here is a decent enough discussion involving the "octane selector" for a straight-8. Read the entire discussion and you'll be ahead of the game on its functionality:
The plate at the back is centered on a vertical line marked on the engine block. The distributor is rotated separately to see the no1 cylinder timing mark line up at the flywheel viewing port. The distributor clamp is then tightened. The plate at the back bolted to the distributor by the clamp is then rotated to the advance postion 8 degrees for our modern fuel. The engine will have a lot more power. This plate with the distributor can be difficult to move as the advance mechanism is bolted to the distributor and has a rather large vacuum pipe attached to it. I use a thinner copper pipe to make this adjustment easier. Excuse the late reply.As you say each notch is 2 degrees.
June 11th, 2021, 11:31 PM
