Magneti Marelli S159B Distributor Help Needed

Hey Rob,

The labels on the bodies look different from the ones on mine (still the same model - S 159 B. See pic below).

Theoretically, the splines should be in the same spot relative to the rest of the advance components of each distributor, but in the real world, this may not be the case and why we all have to rotate the dizzy a bit on the engine to get them on the timing mark. It takes a long time to dial in the IR triggers of my CraneCams so that when the dizzies are on the engine, they both fire on the marks and the bases are even and flush with the mounting flanges. Did the same thing in the beginning when I ran points. Lots of fiddling around though.

Are you sure you have everything marked properly and lined up when you put the dizzies back on the engine? One tooth of the spline makes a huge difference in timing. I've made that mistake before.

I bought two new dizzys because the ones on mine were so worn out, there was no more I could do to them to bring them back into spec. Thankfully, I got them just when our Canadian dollar was at it's peak value late last year, so the four-figure price tag wasn't such a blow to my wallet!
Image Unavailable, Please Login

 

If this is the case, move the rotor by 1 spline as Peter suggested (although you'll have to figure out which way). I can't recall how many splines there are (would someone please post the number of splines), but my recollection is that they made the adjustment slots large enough such that (if the distributor is properly set-up -- i.e., the point set opens when the rotor arm is pointing at the red notch) you can always get it to work properly (even if the distributor housing outline does not wind up well-aligned with the cam cover shape) -- i.e. the phase of the splines relative to the cam lobes can be anything, and you should be able to get it to work.

IMO, you should be able to set/confirm things manually to just a few degrees of error before even running the engine (and I'm assuming that you are still running an R1 points only set-up in your "new" distributors?):

1. (On the unmounted 1-4 distributor) Confirm that the R1 point set opens when the rotor arm is roughly pointing at the red notch (just using an ohmmeter measuring between the R1 electrical post and the dist housing should be good enough).

2. With the engine at ~7 deg BTDC from PM1-4 and the #1 cyl ready to fire (i.e., at the end of the compression stroke) -- Mount the distributor with the rotor arm still pointing at the red notch.

3. Rotate the dist housing to the position where again the R1 point set is just opening up -- tighten down the housing and fire it up.

Same for the 5-8 side except for step 2 rotate the crankshaft 90 deg (in the direction of normal rotation) to the ~7 deg BTDC from PM 5-8 and the #5 cyl ready to fire.

Right now either something is wrong with step 1 (i.e., the distributor really isn't set-up correctly) or step 2 (you're off a spline) -- Good Hunting!

 

Robert I seen your PM, sorry I didnt comment sooner.

As Steve was eluding, you more than likely have the points plate advanced or retarded in relationship to the rotor and its relationship to the distributor housing. Remember that the points assembly is slotted on its mounting screws, and can be "phased" to be in proper placement as the rotor swings past the tower inside the cap. Its takes a bit of fiddling to get it to the right setting, and once you do, make note of where the points plate sits in relation to the position of the rotor. IIRC, the rotor is just starting to go past the red alignment mark as the points break. Moving the points plate either direction will force you to rotate the body the same direction to place it in proper idle speed timing location. If you study pictures in the Bluemel book, you will notice many of the cars distributors are centered in thier adjustment slots so that the countour matches the cam cover, and thats what I believe we should strive for as the standard for setting.

 

Yes, you want the flywheel set on the idle speed ignition setting point to set initial static base timing.

The sleeve your refering to is used to keep total distributor advance to about 13 degrees IIRC. I assume its on the shaft instead of around the outside of the shell to keep the cam centered with where the points base needs to be?

You only need one condensor per points set, and the .25 mfd sounds correct for stock points and coils. If the condensor mfd rating is not correct you will have metal transfer on the points contact surfaces one way or the other.

Remember that maximum ignition advance is far above 5000 engine rpm. On the early carb cars its 3400 dist rpm/6800 engine rpm. After you set your advance curve on your machine correctly by shimming the springs etc., make sure that you adjust your ignition timing at 5000 rpm at the 5000 rpm flywheel timing mark. Idle speed timing should never take precedence over high speed timing. Correct your idle speed timing on your distributor machine if its out to far.

Those are very pretty distributors by the way.

 

Does this mean that you controlled both 1) how the housings are mounted to the stationary base (the easy part), and 2) how the rotor shaft is mounted to the rotating drive shaft (maybe not so easy?).

Otherwise, have you compared things like:

Is the angular relationship between the rotor arm and the square cam shape the same on all distributors?

or

Is the angular relationship between the mounting holes for the R1 points and the housing the same on all?

As Paul mentioned, the slots in the R1 base allow some adjustability, but a 39 deg difference is a lot, and it should be quite noticeable if one distributor has its R1 point shifted one direction (relative to the mounting holes) and the other distributor has its R1 point shifted the other way.

Just some thoughts -- keep chewing -- you've got no other choice

(Although I don't see the need for buying yet another set of distributors. You've got the machine -- did you try just resetting the rebuilt unit to where it should be? -- i.e., rotor pointing at red notch when R1 points open -- or did you run out of the slot adjustment?)

 

Robert, is it possible you have the R1 and R2 points positions confused? The R2 points are not 180 degrees opposed from R1, but are something like 176.5 or 183.5 ( I cant recall) from the R1, to retard 7 degrees crankshaft when the throttle switch is activated. If you run off the R2 points your rotor would point 3.5 degrees off with the points bases centered in there attachments. You should be able to swing the distributor on your machine by hand, and note the opening and closing points and the relative position of the rotor. The only real reason to have it spin up to speed is to check the advance rate.

 

So that's what they told you? Wow...

 

You do know that on your Sun machine you can rotate the degree dial to any position you choose?

 

Just an FYI for everyone trying to figure these cars out. On many other points ignition cars with mechanical advance mechanisms contained within the distributor, the rotor shaft is a seperate part from the drive shaft. On most cars you can grab the rotor and physically rotate it against the advance springs.

The Marelli unit does not work that way, the rotor shaft "IS" the drive shaft, and the rotor is directly connected to the camshaft, its all one piece. But the points cam is NOT part of the shaft, but is bushed onto it and can freely rotate around it, except being held by and controlled by the advance weights opening and closing via centrifugal force. As speed increases, force opens the advance, etc..The pins the weights attach to, the weights themselves and the points cam, are daubed with paint so they can be reassembled to original factory position. I dont believe rotating the cam 180 degrees, or swapping weights around could reposition the cam angles enough to matter, I believe the markings are simply to be able to re-match the original weighting and advance curve, and I mention it only as a possibilty and to help to explain how it works. I'll go take a look at a cam and see if would matter, but im sure it wont. I also counted my splines at 22 splines, so no matter if you installed the unit 180 degrees off you would still be centered.

There is really only one way to assemble the unit, and the only real adjustments once its assembled are the physical placement of the points within its very limited range of mounting position, and the placement of the distributor body to the cylinder head within the adjustment slots and the spline position in the camshaft in direct relation to the flywheel marks. That is "if" its assembled correctly. There are on some cars a bushing that the advance weights rest against that position the advance weights slightly outward for idle base timing. Altering its thickness or removing it will change the cams position relative to the ignition points in the low speed idle position. No bushing would retard the timing, and vice versa.

If the distributor is assembled correctly, and if the points are in the correct position with the correct dwell, then the distributor should sit against the head centered in its slots and aligned almost perfectly with the cam cover, not swung to the extremes of its range. If the points are swung to the limit of thier slotting, and you cannot achieve correct dwell AND be able to place the distributor against the head without it being rotated extremely one way or the other to time the ignition properly, I would inspect the inner bushing the advance weights rest against. If that is the case then as Steve suggested, something is wrong.

 

I can kind-of get a picture of what you're saying with that above statement and in my mind, I see that maybe your R1 points are accidentally mounted in the area of the distributor where the R2 points would normally reside, thus, throwing off the timing by that much (?...).

My Allen machine works in the same manner, the arrow fires with each dizzy firing and that would make four arrow points on that timing disc (see pic below). So what you're saying is that when you chuck the rebuilt dizzy onto your machine, all four of those arrow points are 39 degrees off from when the first dizzy was on there?

(Note: the pic is a bit out of focus and shaky because I had to take it without the camera flash. Previous attempts with the flash drowned-out the lights in the arrow points...)
Image Unavailable, Please Login

 

Peter, I assume that your degree scale is moveable also? It looks like it rotates to any position much like mine does, and I assume the Sun units do the same?

Robert, the picture above shows advance weights that are missing two of the four springs and pins. If there is not enough spring tension on the weights, they will fly out to full advance at very low rpm, perhaps far below 500 (dist., rpm). Try physically locking the weights in thier inner, low rpm idle speed position, and try running it again. Also, it may be possible that with pins missing, the weights could fly out farther than they otherwise would causing greater advance, so try testing it with the weights locked fully out. I looked at a points cam and it appears to be symetrical, so I dont believe assembling it one way or other would make any difference in timing position.

The OWM shows the R2 points 185 +/- 30' degrees opposed from R1. So even mistakenly running the points at position R2 instead of R1, would only shift your timing position 5 degrees. If you are truly seeing 39 degrees difference, I would definetly lock the weights in both the fully in and fully out positions and check it on your machine. You only need turn the machine on and rotate it by hand to observe the firing points by the strobe. Maximum advance from low to high should only be 13 degrees. IOW, block the weights closed, rotate by hand with test machine on to observe firing points and place dial at zero degrees to the strobe. Then lock weights fully open and repeat. The firing points should advance 13 degrees on the dial. Or whatever advance curve for your particular engine your looking at (or creating). Always remember that the distributor turns half engine speed, so that 13 +/-1 degrees at 3400, is actually 26 +/-2 degrees crankshaft at 6800 engine rpm.

 

Yes, it does rotate to any position.

 

I notice in those pictures that between the two cams, the squarer one seems rotated more to the right. Maybe its an illusion?

 

I was refering to was the "indexing" of the high points of the two cams relative to their arms. The one on the right (with the more squarish cam shape) seems more rotated to the right (clockwise). This would make it open the points in a more retarded or advanced condition, depending on distributor rotation.

As to the inner bushing, it would achieve the same effect. You would generally have your dwell set to factory specs with the points slid up or down to phase the opening point of the points to the rotor pointing at the red mark. As we know that moving the points up and down will move the mark, so will a larger or smaller inner bushing that the weights fall against.

 

I modified one of my distributors to run both banks coils. Basically I took a dual distributor and converted it to a single (in operation), by relocating the other set of points. By the sounds of it you have a standard single distributor that someone removed one set of points from and your attempting to use it as a dual distributor.

And not to confuse anyone, most (but not all) US cars run dual distributors, most (but not all) euros run a single distributor, though primarily because the oil resirvoir for dry sump cars leaves no space. The duals operate like a 4 cylinder distributor, and thats basically what they are. The single has a dual plane rotor, two sets of points (one for each bank), and two planes of 4 plug wires. Its basically two four cylinder distributors combined into a single unit with an 8 plug wire cap and two coil wires.

The points cam shape, or profile, would effect dwell based on points gap. Given the same points gap, the squarer shaped cam would provide more dwell period (time the points are closed) against the rounder shaped cam. With the square cam you would have to set the points gap wider to achieve the same dwell period as the rounded cam. All things being equal I think the squarer cam would induce wear more aggressively if you tried to maintain proper dwell.

 

Those straight-line advance curves from the 308GT4 WSM Section L are for the S159A distributor operating with a static R1 advance of 16 deg BTDC (see the specs given in the first pages of section L). The bent advance curves from Section O are for the S159B distributor operating with a static R1 advance of 6 deg BTDC. If you calculate the total advance at 5000 RPM you'll see that it is about equal (i.e., on the S159B system they reduced the advance in low RPM range relative to the S159A system).

My guess on those cam lobe shape changes would be that it was just the natural evolution to an improved design as the manufacturing (grinding) equipment got better (although that doesn't explain why they changed the relative angles). The round-looking one is the easier one to make where you start with a round shaft and grind on some flats (note that the centers of the curved areas on the corners coincide with the center of the shaft); however, this has some downsides:

1. Where the flat meets the curve there is a slight discontinuity (which they try to post-polish out a bit), and

2. to keep this discontinuity to a minimum, the depth of the flat is kept to the minimum, but this results in a more "sluggish" opening/closing action for the points.

The square shaped cam lobe on the S159C has no discontinuity at the junctions between the flats and the curves (note that the center of the curves on the corners are not coincident with the center of the shaft). Consequently, the flats can be deeper, and this "sharpens" the opening/closing action of the points (which helps with things like the timing not changing so much with wear).

You've confirmed that the F engineers were smart enough to make sure that the adjustment range covered an angle slightly greater than 1-spline , but I still don't see why you are having so much trouble. You've got all the bits and the Sun machine -- just make it happen as it should on the bench (rotor points at red mark when R1 point opens) and mount it on the engine the same way with #1 cyl ready to fire, and (operationally) it doesn't matter if you are centered in the slots or not.