63 _ Need to reduce distributor mechanical advance limit
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Re: 63 _ Need to reduce distributor mechanical advance limit
Steve, Look for a chevy distributor from a Impala and mine the weight base/cam, many times it will have a smaller slot and that will work. Also, search and see if there are offset bushings available that may work.- Top
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Re: 63 _ Need to reduce distributor mechanical advance limit
Have you calibrated your initial timing taking into consideration that the 63 timing indicator tab degree marks are not each at two degrees? How are you arriving at the total mechanical degrees? Using a dial back timing light?
Stu Fox- Top
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Re: 63 _ Need to reduce distributor mechanical advance limit
In the "Similar Threads" box at the bottom of the page, I just read through the thread started by Duke July 18, 2006. There is a reference to the cam plate having been drilled which served to limit the advance. I assume this means that someone created an alternate (shorter) slot and used it instead of the factory slot? Has anyone done that?- Top
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Re: 63 _ Need to reduce distributor mechanical advance limit
Ken _ Thanks
The slot measures about 7/16 in (0.4375). So if the reduction is linear, to reduce from 32 to 25 would require 7/32 times 0.4375 = 0.095, or approximately one tenth of an inch fill in the slot. From a practical standpoint, I assume you braze part of the slot and then use a file to tweak the advance to where you want it?- Top
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Re: 63 _ Need to reduce distributor mechanical advance limit
The pin may not be at the end of the slot in the fully retracted position. Fully retracted is when the weights butt against the cam, or in some cases each other.
Make a twelve degree cardboard template with a protractor to determine how much added material you need to grind to get about 12 degrees to the end of the slot from your datum. As stated brazing is SOP. JB Weld might work, but I've never tried that.
The optimum spark advance map for all OE mechanical lifter small block cams is the same as the 365/375 HP engines- about 24 centrifugal all in by 2500, with initial timing to bring total WOT advance to the 38-40 range if it doesn't detonate and a B28 VAC.
Duke- Top
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Re: 63 _ Need to reduce distributor mechanical advance limit
A very simple solution is a dab of JB weld in the slot. File off any excess.Leif
'67 Coupe L79, M21, C60, N14, N40, J50, A31, U69, A01, QB1
Top Flight 2017 Lone Star Regional- Top
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Re: 63 _ Need to reduce distributor mechanical advance limit
The shaft is relatively easy to remove: first remove the tach drive cross shaft, then simple drive out the roll pin retaining the drive gear on the shaft (be sure to note the position of the dimple on the gear relative to the rotor position aligned to the terminal and also the number of shims that may be present under the drive gear).
Stu Fox- Top
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Re: 63 _ Need to reduce distributor mechanical advance limit
Steve, the 532 stamped into your cam piece indicates 32 degrees mechanical max advance. This total is determined by the length of the slot in the cam piece that the pin from the mainshaft can travel in (discounting the affect of any bushings). So, theoretically on a Sun distributor machine, this piece would allow for 32 degrees; so, if you want to limit it, the slot length needs to be reduced. I have found 1 degree is about .0006". Therefore, to reduce from 32 to 25 would require a reduction in the length of about .0042". Bet if you take an inside caliper, presently the length is about .0375-.380". From experience, I find .0330-.0335" will achieve 24-25 degrees total. The springs & weights determine the mapping or how much and how quickly the advance comes in. Brass is a great way to "fill" the slot, but I find it hard to machine (by hand), so I use epoxy (have tried a couple of brands, find "RapidFix" to be the best-harder/stronger than say JB Weld); there is little stress here and have been using this method for over 20 years. Make a little mold with masking tape on both sides of the outer portion of the slot and fill the slot about 1/8" and machine (round file) until you achieve the length you desire. I check it on one of my Sun machines to insure, but .0330-.0335" should get you where you want to be. If no Sun, a timing light with a total meter/gauge will do, just more hassle to pull the distributor each time to adjust. Good luck! BTW Stuart: position of the lower gear; dimple on one side of the gear next to the pin hole, should align with the square hole of the rotor.
Michael Z- Top
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Re: 63 _ Need to reduce distributor mechanical advance limit
Mike _ thanks for the detail on epoxying the slot
Stu mentioned the proper position of the dimple on the drive gear relative to the position of the rotor. I had not pulled the shaft yet, but I decided to just check the position of the dimple. I found the the dimple to be aligned with the back side of the rotor instead of the front. I have had the car for many years, and the distributor has not been disassembled during that time, so the engine has been running with the distributor in this configuration for all that time. I found this comment on the subject from Duke in the archives:
If the gear is indexed with the dimple pointing 180 degrees away from the rotor tip you cannot rotate the distributor enough to achieve the correct timing. The "fix" is then to advance or retard the distributor one tooth and re-index the wires. This will allow proper timing, but rotor-cap misalignment results in misfires and poor performance. This drive gear indexing issue is a source of no end of problems for the last 40+ years, and they continue to this day.
After reflecting a bit, I realized that each time I had pulled the distributor and then re-installed it, I always had to advance it a tooth to get it to start and run. I never re-indexed the plug wires, but it never misfired and it has always started on about half a crank and I thought it ran well.
So, I removed the drive gear , rotated it 180 degrees and re-installed it (it lines up with the square hole on the rotor as you said) and re-installed the distributor. Tomorrow I will re-check the advance measurements and see if anything has changed; then I will work on the slot.
Steve- Top
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