I have attempted to adjust my distributor (purchased from Advance Auto Parts) to fit the specification given in the factory manual using a tachometer, vacuum pump, and timing light.  I have obtained results that are at least close to specification, but something is wrong.

If the car is idling in park and I slowly and gradually increase the throttle, the engine is smooth and fires evenly until around 900 rpm all the way up to something like 1400 rpm.  In that range, it misses, shakes, and is generally unhappy.  It seems to improve at higher rpms.  If I accelerate through that same trouble range with the vacuum advance disconnected, it does not miss or shake.  The vacuum advance is giving far too much advance at those speeds for an unloaded engine.

I have never known an engine in good tune to miss and shake during any reasonable range or load of operation, or, specifically, when slowly acclerated at idle in park.  So I conclude that something is wrong.  But it does not demonstrate the same problems when driving.  It is generally smooth and responsive.  At high road and engine speeds, say 70mph (3k rpm) it feels like something is off though.

The vacuum pickup on the carb (it's original, autolite 2100) is above the idle screw hole but below the transition holes, so it sees manifold vacuum almost the moment you ease off of the idle stop.  What is so puzzling to me is that all of the pieces of the system seem to be set according to their intended design, yet the actual performance of the system is flawed. 

Any thoughts about what, if anything, is wrong here?

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

The roughness is probably due to too much advance at that speed.  Are you sure the damper hasn't slipped and giving you a false tiiming reading?

John - "The Hoosier Hurricane"

What John said about the damper, but could also be carb related. You may have a problem as the carb transitions from idle.

Glen Henderson



Freedom is not Free

Letohatchee, AL

How much is the vacuum advance advancing the timing?Should be in the range of 6-10 maximum.The vacuum can is adjustable through the nipple using an allen key.I believe counter clockwise reduces the amount of advance but you will see quick enough as you adjust it.Total advance,initial+vacuum+centrifugal should be no more than about 44 total.The distributor centrifugal advance could be 26 or 36 depending on which slot in the cam assembly is engaged in the stop post inside the distributor.You will have to take the breaker plate off and see which it is.It sounds to me like the vacuum advance is giving too much advance.Something I didnt realize until I started playing with my distributor is this.Vacuum advance comes in at low RPM when vacuum is high.Comes back in at high RPM cruise conditions when engine vacuum is high again.As an example-if initial is 10 BTDC,vacuum advance is 12 maximum and distributor is 36 you will have 58 total advance at high RPM criuse condition when vacuum is high.Way too much whch will cause the engine to missfire.Have a look at the vacuum advance and see what it is advancing the timing by at just off idle as you are using ported vacuum source.You can also use direct(non ported) vacuum source if there is an outlet somewhere on the intake for fulltime vacuum.This will advance timing as soon as you start the motor.Hope I dont sound like a Know-it-all in this post but this is what I learned while working on my own distributor problem awhile ago.One other modification I did was install a pair of light centrifugal advance springs from Mr.Gasket(about $5.00 for kit)which gives quicker advance.Doesnt change total advance just allows quicker advance.Makes a noticeable difference in power.

  

The advance "can" on the new distributor can supply 10° distributor/20° crank at 15 inches hg. Depending on where you set your initial advance with the vacuum line disconnected / plugged - you could be considerably over-advanced. Chances are pretty good the replacement distributor came with its "can" set wide open. Most of the newer versions can be (gently) adjusted with a 3/32 Allen wrench inserted through the vacuum nipple. Older models with the hex nut on the line entrance have to be disassembled and shimmed.

Where have you set the initial advance? Your new distributor will not have come with the same internal settings and springs.

 
 Steve Metzger       Tucson, Arizona

Greenbird56, you are quite right, the distributor came adjusted for who knows what engine and I did not trust for a moment that it was calibrated properly for my engine, thus the efforts to adjust it. 

Everyone who suggested that the spark is over-advanced is definitely correct.  I'm just not sure why it's over-advancing.  Let me give some additional detail.

First, this is how I have calibrated my distributor.  I looked up the centrifugal specs in the manual.  It calls for x-y* advance at z rpm.  I have taken these values to be DIST degrees at DIST rpms, so I doubled x,y, and z for each data point.  With the vacuum advance line plugged, I then would rev the engine to the spec'd speed and use my timing light to check for the spec'd advance increase over initial.  I did this mainly for the low-speed spring, because my tach only goes to 2k rpm so I just sort of assumed the high-speed spring was where it needed to be

Then, to set the vacuum advance, I idled the engine as slowly as I could to prevent the increased engine speeds due to forcibly advancing the spark from bringing the centrifugla mechanism into play.  The manual specifies x* advance at y "hg vacuum, and again I doubled x.  Using my hand vacuum pump, I supplied the spec'd vacuum, then looked at the advance.  My advance can is NOT the allen key kind, and as it turns out the spring rate of the supplied spring was too soft and would never produce the spec'd curve, so I obtained several other springs from the hardware store and found one that had almost exactly the desired rate.  The washers move the curve up and down, and ultmately I got all the data points comfortably within their ranges.

I have the initial advance set at 8* by the damper.  Who can say whether it has moved?  I chose the 8* mark partially on assuming the damper hadn't moved and partially on the way the engine ran.  It would run faster at idle if I advanced it more, but I did not want to max out initial advance.

I should also note that at hot idle, there is not enough vacuum to the advance can to move it, but just a touch off of the hot idle stop i get 15"hg.  So, around 1000 rpm, the timing light show over 30* advance--waaaaay too much. 

The way I see it, either my calibration method is wrong or this thing was designed this way--and I'm more inclined to believe the former than the latter.

Oldcarmark, the centrifugal advance is set to an 18/36* maximum, and the vacuum advance maxes out at about 12/24* (dist/crank degrees).

Thanks so much for your help so far everyone!

1954 Crestline Victoria 312 4-bbl, 3-speed overdrive

That cam can be switched to the 13L slot which gives 13/26 crank dgrees.The other end of the cam should be 13L.If you stay with the 18/36 slot you will end up with over 50 BTDC at highway cruise speed when you ad all 3 factors together initial+centrifugal+vacuum.Steve(Greenbird56) likes to use the 13L slot not the 18L one.I have mine on the 13L slot.

  

The use of the 18°/36° is going to be way too much total mechanical advance - add the 8° initial you mentioned and we are at 44° mechanical - and the engine won't like that a bit. 

With a little disassembly - and you've been down to that level already - you can swap the advance mechanism over to the lower setting of the advance, is it a "13L"? That's 13° distributor/26° crank. That plus an initial of 10°-12° will net a maximum mechanical of 36°/38° which is close to correct for one of these stock engines. Mark is right - use the proper springs to get all of the 36°/38° at between 2500/3000 rpm (with vacuum advance disconnected and plugged). The engine should be both run-able and tune-able with these spec's and no vacuum advance.

To get economy you will want some vacuum advance added to the engine at periods of low load - cruise. You will find that with the quicker mechanical curve it works best to restrict the vacuum advance at the crank to something like 7°/8° so the total available at cruise / low load over 3000 rpm won't exceed 43°/46°.

I have found that putting an orfice in the vacuum line to slow the speed at which the vacuum pot makes its additions and subtractions of advance - is extremely beneficial to driveability.  

The 18L/36° slot is provided for use with an engine set up at 0° initial advance. You won't like the way the engine runs with that sort of initial advance - it will have a very hot idle and drink fuel.

 
 Steve Metzger       Tucson, Arizona

I just wanted to make a suggestion.Are the other style "allen key" type advances not much easier to setup?I am surprised the rebuilt distributor came with the spacer-spring type advance.Dont see them much anymore.Why not switch advance units and make it much easier to tune?By the way,you can do better than "factory" specs to really dial in the distributor for your particular setup.Don't get hung up on trying to duplicate what original specs call for.The Y-block likes a little more initial advance and setting up the advance curve can do wonders for the way it runs.Something else to check.When I put my rebuilt dist. in I was having trouble with the timing marks being unstable.The timing marks would move back and forth when checked with the timing light.After much aggravation and not finding the reason,I noticed there were specs listed in the shop manual for dist. shaft endplay.How far the shaft can move up and down in the dist. body.Not side play-endplay.Specs are I think .026-.050 measured between the bottom of the body and the retainer part on the shaft.Mine was over.100-more than double maxpecs.I purchased a washer kit from Mr.Gasket which gives 3-4 thin washers yo shim between body and collar.I got mine down to minimum-.026.You have to take gear and collar off.What was happening was the  shaft was riding up and down due to the action of the cam gear on the distributor gear.timing would change  as the cam went up and down opening the points.Problem fixed.You might check yours while you have it out.Specs are listed in shop manual.

  

Does anybody have a part # / brand for a replacement vacuum advace can. Sorry for the hijack but this is exactly what im dealing with. My advance allen stripped out.

Kenny  Onalaska, Texas