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I have been experiencing engine "pinging" during hard acceleration under load and have begun a process of elimination on the possible causes. I have only just begun my search and amongst other things I am also looking at the distributor as a possible cause. Since opening it up every thing looks OK, although I will be purchasing a new set of Advance Weights and Springs just to prove that they are not the cause. Which brings me to my question. How loose is the fit between hole in the Advance Weights and the studs which protrude from the Cam Weight Base Assembly? Should there be any play from side to side of the Advance weights when fitted with the Springs are attached?
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Re: Distributor Advance Weights
Hey, Lou! Expect to see some 'slop' in the advance weights and their mounting. The more important issue is that the weights move in/out freely without binding/sticking...
That's typically the result of dried/caked grease/oil which is easy to clean up. -
Re: Distributor Advance Weights
Louis, I don't know where you are going to look for "new" weights and springs, but as a caution, I have not seen ANY weights/springs from aftermarket "high performance" sources that even remotely match the oem designs either in appearance or performance. Would recommend finding a shop with an old-fashoned Distributor machine that could check yours out against the factory specs and adjust to suit.Bill Clupper #618Comment
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Re: Distributor Advance Weights
Lou-the weights should have a little "slop" as Jack indicates, and Bill is correct the aftermarket weights are not quit correct. Now, what needs to be addressed is; what limits the advance, is the pin at the underside of the mainshaft top "football" (top side has the pins the weight springs attach to). This bottom pin usually has a rubber sleeve and moves within the oblonged hole or slot in the pole piece (point contact cam piece). The pole piece slot and the thickness of the rubber sleeve on the pin dictate and limits the max mechanical advance. Some pins may have a brass sleeve vs. the rubber sleeve, but in any case there is always a sleeve. Over time the rubber sleeve deteriorates and can shrink in diameter or completely fall off, thus increasing the travel of the pin within the slot, causing a larger max advance than intended for your application.
At the very least, you might start with tearing down the distributor and replace the sleeve (available from most of the large supply houses i.e., Long Island, Corvette Central, Zip, etc.). The other issue with these distributors and the factory specs on advance curves is the fuel we can buy today-it has lower octane and thus will "ping" much easier than the factory specs intended-this is amplified at sea level & high compression ratios. You might try backing off on the initial lead a few degrees at a time until the "pinging" is resolved.
Or, you could get your distributor to a qualified tech that has availability of a distributor testing machine, who can customize the advance curve and can fully rebuild/restore your unit. Customized curves can restore/maintain lower RPM performance, but limit the maximum advance under stress loads at full throttle higher RPMs. There are a couple of guys that indicate that service is available on E-Bay-check "TI distributors" or "Corvette TI", and the like; I think you will find one (may have to read the whole details-it's toward the end of the descriptions) or contact me directly/privately and I can direct you.
MichaelZ505Comment
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Re: Distributor Advance Weights
We might be able to offer more specific advice if you tell us the year and engine option, whether the engine is OE or modified, the spark advance map data from the CSM, and the actual measured spark advance map if you can do so.
DukeComment
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Re: Distributor Advance Weights
Thank you all for the advice. The weights have a little slop but do move freely and I have taken on board the information on the aftermarket weights. My machanic has also suggested that we have the distributor recalibrated for the new lower octane fuels that are now available. I just wanted to explore all avenues before I started ripping out distributors.
The car has an original 66 427 BB 390 HP motor with no modifications.
Thanks again to all.Comment
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Re: Distributor Advance Weights
The OE spark advance map for this engine is:
Initial timing: 4 deg. nominal (4-10 range)
Centrifugal: 0 @ 900, 14 @ 1200, 30 @ 5000
Vacuum: 0 @ 6, 15 @ 12
I believe, but am not absolutely positive, that the vacuum advance is full time, not "ported". If it is ported, it should be converted to full time.
This OE spark advance map is fairly conservative, and I don't think many owners of this engine report detonation problems as long as the initial timing is set in the low end of the range and they are using the highest available fuel octane.
You or your mechanic should have a 1966 CSM, and the actual timing map on the car should be checked that it meets the above specs as a first step. Taking about recalibrating the distributor is foolish until you know what it actually is relative to OE spec.
The initial timing needs to be set with vacuum advance signal line disconnected, plugged, and engine speed below 900. Many "mechanics" do not understand these old systems.
DukeLast edited by Duke Williams (22045); February 3, 2010, 05:48 PM.Comment
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Re: Distributor Advance Weights
There is a multitude of factors in play with distributor curves on an L-36 or L-68.
First off, the aftermarket weights suck and I would not use them.
Second, understand the three centrifugal reference points given in the manual do not comprise a linear curve (if you plotted the first two reference points graphically it would be evident). By that I mean the full 30 degrees of centrifugal advance comes in well below the 5000RPM reference point given. In my experience, the full advance in a stock distributor is reached well below 3000 RPM. Along with full manifold vacuum connected to a currently avaialble B20 or B26 vacuum cannister (closest to OE that is currently available, provides 16 degrees vacuum advance or more), you will most likely experience audible clatter under part-throttle acceleration conditions under 3000 RPM where there can be a combination of centrifugal and vacuum advance at play. I say this based on extensive testing and personal experience.
I would begin by verifying the maximum centrifugal advance your distributor delivers with a dial-back timing light. Be sure the limit bushing is in place. Mine has been calibrated to deliver 22 crank degrees of advance. Alond with 10 degrees of initial advance, this yields a (very conservative) 32 degrees total.
Next is to use a B20/B26 vacuum advance unit (or a B10, if you can locate one), verifying it delivers no more than 16 crank degrees of advance when fully retracted (most I have checked are around 18 degrees out of the box and need to be limited with a bushing on the rod).
Finally (and this is the most tedious part), you will need to experiment with springs and the centrifugal advance rate. The tuning kits on the market have some very soft springs that allow for excessive advance at low speed. I have found best results using the heavy gold springs in the Accel kit, cut from 8 active coils down to 4 coils. This provides a seemingly VERY conservative advance curve which begins around 2300 RPM and comes in to full centrifugal advance by 3400-3500 RPM.
I've found this combination to allow operation on 93 octane pump gas with very smooth and strong acceleration. I recently found some B10 vacuum canisters which are replacemetn to the OE "360" can and should only allow for 12 degrees of advance. I will experiment with this when I take the car out of storage in spring. It should enable a slightly more aggressive centrifugal curve.
I also just recently found that the HEI distributor tuning kit from NAPA has totally differnet springs than those available from all the other suppliers. They are quite a bit stiffer and may work better than my "custom" gold Accel springs.
HTH,
NickComment
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Re: Distributor Advance Weights
Frankly, the only way to set a curve is on a distributor machine. GM had many variables in the original distributors that they controlled pretty well to achieve the published specs for a given distributor. I've had many distributors on my machine ofer the years, and nearly all of them were in line with the published specs for that particular distributor part number after the distributor was disassembled, cleaned, and reassembled using the correct parts as opposed to whatever the distributor had been treated to by "speed shop kits" over the years. That said, distributor specs were optimized in original production to achieve different goals in different eras. Entering the late 60's and early 70's there was a pronunced move toward later timing (thus lower performance) to achieve certain results in emmisions testing. Ultimately you need to have a conversation with someone who can put the distributor on a distributor machine and work it to achieve the curve that will support the intended use you want to to make of the car. GM used only a few different weight configurations but many different springs, distributor advance cam configurations, and advance pin locations to achieve the results they desired. These tools are not generally available in "blister pack" configuration.Bill Clupper #618Comment
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Re: Distributor Advance Weights
Much good info in this thread, but a 'FYI' to all; Lou's an in-country fellow founding member of the NCRS Australia Chapter. So, some comments like 'football' need to be sharpened a tad (American Football).
Plus, typical sources for parts we take for granted (say, local scrap yards and NAPA/Echelin parts) generally don't get it for our friends Down Under. Now, if a given part or tune/test proceedure was also common to a GM Holden vehicle, we're cookin' with gas for old Lou!
BTW, I don't think it's the Aussie petrol. I've driven equivalently high compression Corvettes from the era (thanks to the loan from fellow NCRS Aussie members) using pump gas in Australia without experiencing detonation...Last edited by Jack Humphrey (17100); February 3, 2010, 09:41 PM.Comment
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Re: Distributor Advance Weights
William, totally agree. Sadly, funcitoning Sun distiributor machines are few and far between these days so I tune mine on the car. Incidentally, I worked for Snap-on Diagnostics about 8 years ago (which owned the Sun Electric brand and product lines), and I probably could have had several of these machines for free because no one really wanted them. Still kicking myself over that...
My question is, where (if anywhere) can one source OE distirbutor springs? The springs in all the aftermarket tuning kits I've seen are the same, and all of them too light for use on a street car that runs a vacuum advance and pump gas. The only kit I've recently found to have a different spring assortment is the HEI kit that NAPA sells (and I have not experiment with it yet). I have considered buying continuous extension springs from McMaster Carr and making assortments of springs myself, but this is a very laborious exercise without a distributor machine to experiment with...Comment
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Re: Distributor Advance Weights
How long have you owned this car, and have you done anything which may have resulted in this "problem"? Complete configuration of engine, if known? Questions too numerous to list here, but sometimes "too much" background information can be a good thing.
Timing itself may not be the issue; other factors like fuel mixture, static compression ratio (SCR), coolant temperature, exhaust pressure, valve timing, carbon deposits, fuel quality and octane rating (both at 100 degrees and 300 degrees Fahrenheit, inlet air temperature), etc, etc, etc, have more of an effect on engine operation than spark timing alone.
Some absolutes:
1. You don't need a "Sun" machine if you have a degree tape on your crank balancer, and know the principles of spark timing.
2. Most aftermarket weights are smaller (ie: have less inertial mass) than original flyweights, and therefore require significantly lighter springs in order to deploy properly.
3. If spark advance is at the heart of the issue, a good place to start is to retard the initial timing to the point that detonation disappears and note the difference in crankshaft degrees. This will establish a baseline about which you should work to establish an optimal spark advance "curve".
4. Generally, a healthy BBC in a reasonable state of tune likes about 34-36 degrees of total spark advance (initial plus centrifugal) @ WOT for best torque production. This, of course does not include vacuum advance,which should be disconnected and plugged during the foregoing, and configured AFTER your base and centrifugal rates are determined.
5. As stated above, optimal vacuum advance should be reading a manifold vacuum signal...NOT a ported vacuum signal.
6. If you do not know the history of the car, but have valid reason to believe that the camshaft, pistons, chamber size and head gasket thickness remain per OEM, then you should start by assuming carbon buildup in the chambers and remedying that first.Last edited by Joe Ciaravino (32899); February 4, 2010, 11:10 AM.Comment
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Re: Distributor Advance Weights
Louis,
I would start with all the simple stuff first like point adjustment because this can affect timing. Clean the distributor advance weights and weight base but be careful not to lube much because it will sling and dirty the points. Make sure the advance control is working properly (not leaking) and post the part # for the vacuum advance and engine idle vacuum if you can.
After the car is warmed, adjust the idle A/F emulsion screws to make sure they are set rich enough. A lean carburetor could make ping but do the simple things first.Comment
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Re: Distributor Advance Weights
I can determine the spark advance map on the car with a dial back timing light and Mighty Vac in about two minutes.
There's no reason anyone else can't do the same with the same common equipment.
There are very few Sun distributor test machines around now, but there were zero dial back lights back in the day.
DukeComment
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Re: Distributor Advance Weights
Joe-
I tend to disagree with your thought that a Sun Machine is not needed. It depends on the real problem. My Sun Machine Tester (yes, I have a working one, and can repair yours if needed) can tell me exactly what the advance of both the mechanical and vac are at any point-needed for different engine load events; remember "pinging" results under load, not static. The reason this is important is if the distributor is factory and still running a vac canister, in the real world both advances affect the end result the engine is seeing at any particular RPM and/or vac load. Alot of things affect what the engine requires, which was brought out i.e., C/R, fuel and a biggy is altitude above sea level.
Yes, your method can tell you the total advance-a trick racers have been using for years with the degree tape, which works great when you have it full throttle all the time. I feel if one really wants to know how the curve mapping relates to the factory specs, as Duke has indicated must be done-a distributor machine is a must. You might be able to eliminate the pinging without it, but you would not really know what is going on curve mapping wise and could the performance be enhanced if you really did know?-that is the question. Just my two cents worth.
MichaelZ505Comment
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