Timing

[greg g]

If the cylinder is firing after TDC, the piston is already heading down, so when you light off the mixture late there is more room for the air fuel mix to expand into.  This would create a less violent explosion with less force on piston pins, worn pistons, bearings etc.  Might that cancel some noise from an old motor? Seems like a reasonable assumption.  

There are some other things in the equation like reduced compression, worn timing chain and gears, slower flame front with modern gas, think whoomph instead of bang.  So if you have it adjusted to start, run, and sound happier let it lay.

[greg g]

My backup distributor is an IAT4102.  I believe the vacuum advance is non op. So I popped of the cap to check it out.  Everything moves freely, bit I don't have a vacuum pump to test the diaphragm.  So I was hopping to take the canister off and investigate further.  Got the screws out but can't get the arm off the breaker plate pin. Is there an OMG NOSEEUM spring clip holding it down or is it a wiggle just right, with you tongue at the corner of your mouth interference  deal.  I can't tell even with a magnifying glass.  

 

Your inSight will be appreciated.

[chrysler1941]

Mine 4202 has a small horse shoe clip.

Testing chamber, not scientifically but for function, I connect a hose, place in mouth and imagine it's cold milkshake. It will move. ?

[Merle Coggins]

@WPVT Earlier you stated that you believe your mechanical advance and vacuum advance are both working properly, but you get detonation at higher speeds. It may be worth while to double check your vacuum advance unit. Years ago I started having a detonation problem with my truck at higher speeds, especially under load such as climbing hills. Eventually I found that my vacuum advance worked, but also leaked. This vacuum leak seemed to cause a lean condition which led to the detonation. After I replaced the vac advance unit (purchased from Kanter) and gave it a good tune-up, it ran great again. 

[WPVT]

In looking over the specifications that Merle provided, I see that on the engines with vacuum advance, the ignition is advanced about 11 degrees at full vacuum (steady speed). The mechanical advance at higher rpm's is about 10 degrees, giving a total advance at steady, highway speed of 21 degrees.

On models without vacuum advance, the numbers are very similar, except the total is 11 degrees, not 21, because there's no vacuum advance. 

Initial timing in the spec's call for retarding the spark 2 degrees on the non-vacuum advance models, making their total advance 9 degrees. 

So it would seem that with the same engine, at steady highway speed , on some trucks they run at 9 degrees total advance, and some 21 degrees total advance. Seems like quite a spread. Could someone explain the intent here ? 

[greg g]

Guess the diaphragm is toast as I got no resistance through the tube.  Looking at another site some one said he adapted one from a slant six to work on his distributor  but it was an IGS.  But those chambers are out there for《 20.00.

[JBNeal]

As far as I know, vacuum advance for the B-series was only on 1/2 and 3/4 tons.  The larger trucks were treated as workhorses that didn't need speed as much as torque to move a load.  This means the engines were operated under load near peak of the power band a majority of the time, when vacuum is mostly near its lowest.  So as a cost savings, vacuum advance was not included on truck flatheads if its performance improvements were negligible.  As a reminder, these trucks pre-date the interstate highway system, and the best improved highways had posted speed limits under 60 mph, so the larger trucks were not the gear-jammin' diesel-burning speed demons that menace are at work on interstate highways today.  The flathead trucks were geared so they could make a run at a hill, not flatten it

[kencombs]

I don't think there is as much difference as it appears.  In real world use with a heavy load,either in a truck, pump or some other industrial use there would seldom be enough intake vacuum to engage the advance.  Most hit full advance around 16-17 inches while a loaded truck would probably see closed to 6-8 on average.  And much less uphill!

[WPVT]

Here's my answer to my own question, as I see it. Without the vacuum advance, the total advance is only 9 degrees, as opposed to the 21 degree total advance on the 1/2 and 3/4 ton models. It seems an engineering choice designed to make the engine less spritely, but last forever. In my case, that's relevant because I found that retarding the spark significantly put less strain on the engine and made it quieter at higher rpm's. So today I did just that. I disconnected the vacuum advance, and set the initial timing at 2 degrees after TDC, as specified for the non-vacuum advance models. At 2600 rpm I now have only about 10 degrees of advance, which quiets the engine down considerably. 

 

I intend to rebuild the engine soon, but meanwhile I can run it with less strain by duplicating the set-up and spec's of the non-vacuum advance heavy duty trucks that were designed with longevity rather than speed in mind. 

 

This has been an education.