Distributor Advance Weights

[Duke W.]

Re: Distributor Advance Weights

I've used Sun machines in the past, but what I can do with a dial back timing light and Mighty Vac yields the exact same data.

To test the centrifugal, you disconnect the VAC find the point where where the centrifugal starts and then take readings every 500-1000 revs. This may require free-revving the engine to 5000+ with some OE curves, and some guys don't like to stick their head next to the fan with the engine free revving that high, but there's nothing really unsafe about it if the OE components are in good working order. Fan/front end accessory disintegration events are very rare.

From the data you subtract the initial, and plot the centrifugal curve.

Pumping down the VAC with a Mighty Vac on the engine is just like pumping it down with the vacuum pump on a Sun machine. You end up with the vacuum vs. advance curve, but you must be careful that as you pump down the VAC revs do not exceed the starting point of the centrifugal or you data will not be valid. The same caution applies when using a Sun machine.

Assuming the VAC complies with the "Two-Inch-Rule" all you have to do is verify that it's within reasonable tolerance of spec.

Detonation problems on OE or OE equivalent rebuilt engines can usually be solved by retarding the initial timing a few degrees. If this is insufficient and the VAC is the least aggressive that complies with the Two-Inch Rule, then slowing the centrifugal is the next option.

Detonation problems on ported vacuum advance engines can often be solved by converting to full time vacuum advance, but this often requires a different VAC to comply with the Two-Inch-Rule.

Ported vacuum advance increases combustion chamber boundary temperature at idle and low speed/load. This will increase the tendency to detonate if you go to high throttle opening after spending some time slogging in traffic.

Duke

[Tom P.]

Re: Distributor Advance Weights

For those of you experienced with teardown and rebuilding of a distributor, what do you fill the grease well with that's under the breaker plate/felt-plastic seal?

[William C.]

Re: Distributor Advance Weights

Unfortunately, my only source is from used distributors, and I haven't been able to cross reference them to GM part number. If you could pick up a set from a 60's chevrolet, you would at least be in the ballpark, but as you say, no easy solution without a machine, other than using the engine to spin the distributor and using a timing tape and tachometer to chart the curve.

[Stuart F.]

Re: Distributor Advance Weights

On mine - nothing! But then I use a Dyna-Flite ball bearing plate. I lube that with a little Lucas Oil whenever I have the distributor out.

Sorry, I just had to throw that out there Tom as I'm just so smug about still having one of those suckers.

Stu Fo

[Duke W.]

Re: Distributor Advance Weights

For those of you experienced with teardown and rebuilding of a distributor, what do you fill the grease well with that's under the breaker plate/felt-plastic seal?

GM 1960954 was a special grease that GM used for this application and some others like starter motor bushings, but I think it's been discontinued. I believe it has some silicone content.

I lieu of the above I'd use a full synthetic NGLI #2 GP grease.

The felt seal should be soaked in oil, then allowed to drain for at least a day before installation. It's primary purpose if to lubricate the breaker plate bushing on vacuum advance distributors.

Duke

[Joe C.]

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.
MichaelZ505

Mike,

The advantages of the "Sun Machine" are mostly ergonomic; however, the contraption is not "needed" in order to properly set up a distributor, as has been pointed out.

The big advantage of doing the adjustment "in-situ" is that it is easier to road test the car between tweaks, since the distributor doesn't have to be R & R'd after each iteration.

As I'm fond of saying: you're entitled to your opinion and I respect it, although I don't agree with it.

Some folks prefer using a dial back light and others prefer laminating a degree tape to the balancer. Both methods work equally well. The reason I like the balancer tape are twofold: first, there can be no mistake in electronics (rheostat employed for dial back) since physically moving the distributor bypasses internal electronics in the timing light and is directly read from the degreed balancer; second, I have owned the same high quality induction timing light for more than thirty years now, and see no reason to invest in a new instrument.

[Joe L.]

Re: Distributor Advance Weights

For those of you experienced with teardown and rebuilding of a distributor, what do you fill the grease well with that's under the breaker plate/felt-plastic seal?

Tom-----


This. But, it's long-since GM-discontinued.

[Joe C.]

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.

Yes................that's very true, as dwell should be set between 28-32 degrees on a single point distributor with 8 firing positions per revolution. The tighter the points gap, the longer the dwell (number degrees points are closed per firing), and so, the larger the points gap, the farther advanced the timing becomes because the points "break" sooner, which collapses the magnetic field in the coil and sends secondary voltage to the rotor. When I had points, I always liked to set the dwell angle in the middle of the range, or 30 degrees, although setting them at 28 degrees provides somewhat more longevity between adjustments due to wear of the nylon rubbing block. As the nylon rubbing block wears, the spark timing advances. The change in dwell from 28 to 32 distributor degrees translates to an ignition timing difference of 4 x 2/2 or 4 crankshaft degrees.

The reason I bring up the carboned chambers is only based on the assumption that Lou Rokas doesn't know the history of his Corvette, AND that it has fairly high mileage. If both of these are true, then eliminating the likely possibility of carboned chambers is where I would begin....... The chambers can not be seen, of course, so the assumption must be made that they are carboned. In the final analysis, the cleanout of the chambers will enable more spark advance if optimum cannot be achieved, all else being equal.

[John H.]

Re: Distributor Advance Weights

For those of you experienced with teardown and rebuilding of a distributor, what do you fill the grease well with that's under the breaker plate/felt-plastic seal?

Tom -

I pack the well with strips of gauze (bandage-type), then saturate the gauze with 30W oil before installing the plastic seal and oil-saturated felt washer.

[Tom P.]

Re: Distributor Advance Weights

OK, thanks.
Over the years I've used a good chassis grease only, and so far, ZERO problems. BUT, if there is a better lubricant avaiable to pack into that well, I'd prefer to use it.

[William C.]

Re: Distributor Advance Weights

You could always use the original Delco lube, still readily available, looks like it has a significant graphite content.

[Nick L.]

Re: Distributor Advance Weights

Unfortunately, my only source is from used distributors, and I haven't been able to cross reference them to GM part number. If you could pick up a set from a 60's chevrolet, you would at least be in the ballpark, but as you say, no easy solution without a machine, other than using the engine to spin the distributor and using a timing tape and tachometer to chart the curve.

There are not many available out there. If I ever get my hands on a distributor machine of my own, I am going to undertake this investigation and produce springs that match the OE specs. The speed shop kits are useless on a street car that runs vacuum advance and pump gas unless you shorten them to increase the static load they apply.

[Timothy B.]

Re: Distributor Advance Weights

Nick and all,

I have had some good luck with the speed shop springs to achieve a curve approx 24-26* that's all in at 2800-3000. Starts about 900 with the intermediate about where the GM spec is.

[Nick L.]

Re: Distributor Advance Weights

Timothy,

I have been able to achieve a good curve on my 67 L-68 with modified speed shop springs, but have not found any that will deliver a suitably "gradual" curve in standard form. It seems the spring rates available only allow a curve window of less than 1000 RPM from where they start to allow movement in the weights to full deployment. The big problem I have found this to cause is audible detonation under part throttle acceleration between 2000-3000 RPM where centrifugal advance is coming on strong but the engine is still experiencing some vacuum advance due to low throttle angle. This is compounded by the fact that the currently available B26 vacuum motor actually specs at 16 degrees of crank advance where the OE "360" can only had 12 degrees.

I have a curve that starts near 2300R, and is all in by 3500R. Total centrifugal advance is 22 degrees. Base timing is set to 10 degrees with a B26 vacuum canister connected to full manifold vacuum. This combination pulls very strong and smooth with no stumble or hesitation and runs perfectly on 93 octane pump gas. I arrived at this combination through a lot of experimentation. To be any more aggressive than this with the centrifugal advance results in the onset of knock between 2500-3000 RPM.