Magneti Marelli S159B Distributor Help Needed

I answered your question above. The sharper lobes would have a different dwell period if the gaps were equal.

As to the marks not aligning, same deal. The square cam you show in your pictures is rotated further to the right than the "normal" cam. Because the distributors rotate clockwise, the square cam would advance timing even further.

Marelli distributors like these date back many decades using many of the same basic parts, and cover cars from Lancia, Fiat, Alfa, Maserati, Lamborghini, Ferrari, and possibly others. Those square cams could virtually be out of almost anything, from almost any era.

 

I'm confused...

You've got a nice machine that was designed to give you the exact answer that you're seeking here. Why not take the time to spin them up yourself?

Yes, I've got the info you want. PM me. But why not use that pretty machine Mark sold you?

Rob

 

Rob,
I have a V6 Dino but have spent a lot of time on the dis. First, if two distributors have to play together, they should be identical except if there is an angular difference of cams as noted in the book (not probable.) Second, if you want a two stage advance, there must be two weights with two springs. One in each distributor does not count. If you want same dwell, cam profiles must match. Static setting of each distributor begins the process. TDC of the timing cylinder should have the rotor aiming at the right cylinder. On both banks in turn. If you have that, the points should have the right gap and should open within the adjustment range of the dis. slots. Sounds like you have one each of too many distributor parts. All distributors have vaguely interchangeable parts. Start slow and easy. See you in a few weeks.
John

 

Robert:
I've been following this thread for some time, but up to now didn't have anything to contribute. If you remember correctly, a couple of years ago you sent me one of your distributors and I checked it on my Allen Synchrograph. It was the one with only 2 pin/spring sets which I annotated in the picture you provided (I didn't have a handy pic of one with all 4 pin/spring sets in it). I was looking for something in common in all three distributors I had to test which would explain how the advance curve was implemented. I measured everything and finally came to the conclusion that the clearance between the pin and the inside wall of the distributor determined when in the RPM range that particular pin/spring came into play. That's what I've annotated in the picture.

When I tested and mapped the advance curves of my distributors I found that one was advancing more rapidly than the other in (I think) the 2000 RPM range. I identified the pin associated with that rpm range and placed a small brass washer at the base of the spring where it enters the swing arm, thus increasing the height of the pin and causing it to come in contact with the inner surface of the distributor case earlier, applying the spring pressure to decrease the rate of advance. When I tested it back on the Allen it had, indeed decreased the rate of advance in the specified RPM range - but too much. I disassembled the distributor again, lapped a few thousandths off the washer and retested - and got the results I wanted. This procedure confirmed (to me at least) that I had correctly identified the mechanism of the advance.

I agree with John and Moku. Assemble the distributors and map the advance curves, checking the advance every 500 distributor RPM. Make the adjustments as suggested above, matching 2 distributors exactly.

 

Rob

I didn't have the equipment to properly test the spring tensions, but I did notice the lower RPM springs were made of a lighter gauge wire, hence would have less resistance. My theory is that as the RPM increases the lighter weight springs compress, then as centrifugal (centripital?) force increases the heavier weight springs are engaged. I did notice my springs were all about the same length (which gave me the idea to measure the clearance to the inside of the distributor body). I think the geometry causes the weights to slide out along the slots in the slotted rotor, rather than pivot on it. The pivot point seems to be the spot on the arm where it contacts the inner surface of the distributor body.

The distributors you sent me were missing 2 spring/pin sets. Clearly, someone had "tuned" them, and the advance curve didn't look anything like what's in the shop manual.The other ones you acquired seem to have been the victims of the same kind of "tuning." #5 and #6 are each missing a spring in what I think is the 1000-1700 RPM range (2000-3400 crank RPM). I'm willing to bet the advance curve jumps up (advances) in that range outside the published tolerances. Map their curves and let me know.

#4 has an awful short spring in the upper left- I can't tell the RPM range it affects because it doesn't have the yellow paint dab. I'm going to guess it is in the 2300-3000 RPM range (4600-6000 crank RPM) and was done to increase the max advance. #7 is missing that pin/spring combo altogether and probably advances even more than that. I don't think its a good idea to mess with the factory advance curves, because they are matched to the cam profiles. Others on this board, especially those with programmable electronic ignition, may have other ideas and experience.

I'm assuming you just want to get two good, reliable distributors with advance curves exactly matched. you've got quite a collection of springs, pins and cups. I would pick out the two least worn distributors, and build the advance mechanisms up from scratch. Pick the best looking (least worn) pins and cups. Your #4 might be a good place to start. Map the curves and make adjustments with washers until you get the profile in the manual. Then do a second one.

I found the graduations on my Allen were not discrete enough to accurately map small differences in the advance. So I made a paper arc segment graduated in 1/2 degree and pasted it on the degree wheel. I also used an external tach/dwell because I don't trust the 50-year old tube electronics in the Allen. When I get the chance I'm going to upgrade the electronics to digital - but that's just one more item on my list.