Distributor bob weights? How do they modify the ignition timing?

I’m confused by a magazine article which says in centrifugal advance mode, the distributor bob weights are thrown out by centrifugal force, and this moves the baseplate, advancing the timing. Is this true? I thought the baseplate was static in centrifugal advance mode, and the weights moving out slightly rotate the shaft that opens and closes the points; i.e. the baseplate only moves in vacuum advance mode. Which is it?

It was common on the old mechanical distributors to use both the centrifugal weights and the vacuum advance.


The GM distributor moved the rotor. You could play with that one and see it move. I never took a Ford distributor apart to look, but if you can move the rotor a little, the weights might be underneath the plate.

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It’s been a long time since I’ve had to think about this but the centrifugal weights advance the timing as the engine accelerates. When you open the throttle to accelerate quickly, the reduction in vacuum retards the timing to prevent the risk of engine-damaging detonation.

For the full scope of the system…

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All I know is to check to make sure they weren’t stuck. Seems to me I changed the springs once to test oerformance but couldn’t swear to it. 50 years ago?

To check the centrifugal advance just clamp the vacuum line shut and hook up a timing light and rev the engine and see if the timoing mark moves

I would think that whatever mechanism alters timing in response to vacuum changes is the same mechanism used to alter timing based on rpm (the centrifugal advance mechanism).

Good project for you, George: go get an old distributor at a junkyard and figure out how it works.

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Haven’t had a timing light or distributor for prolly 40 years now. Barely remember them. Still have the tool though for setting the dwell somewhere in my tool chest. That’s why I liked Gm for setting the points.

I never had a GM with points, but used it when I worked at the service station:

Yep, that’s it. Mine is yellow though I think.

Good explanation, that’s the solution! On my Ford truck, I’m able to push on the ignition rotor (under the distributor cap) to rotate it maybe 3-5 degrees in the advance direction (when the engine is off of course). It feels like I’m pushing against springs, as it immediately moves back to the same place, so I’m quite sure the Ford distributor cent. advance moves the rotor shaft, not the baseplate. The rotor shaft contains the octagonal shaped distributor cam, the edges of which opens and closes the points as the cam rotates past the baseplate-attached points. The Ford’s baseplate does move and that also affects the timing for the opposite reason, but only by the vacuum advance/retard attachments to the distributor.

My guess is that all or almost all distributors cent advance work the same way, advancing the rotor to advance the timing, which is why I was surprised to read what the magazine said.

Thanks to all for the practical advice above, but I’m not actually trying to fix anything; just attempting to develop a better understanding of how the function is supposed to work.

Advancing the rotor alone won’t advance the timing, the cam must be advanced.

The distributor shaft is split, the upper portion of the shaft containing the cam is advanced by the weights.

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No dispute, but as shown in your informative diagram on the left (thanks for posting) , the rotor is attached to the upper part off the split-shaft that also contains the cam. So it appears from the diagrams the cam and rotor advance together. I mention “pushing on the rotor” above b/c that’s the easiest place to grab hold.

The amount of advance per increase in rpm must be controlled by the spring strength. It’s unclear in the diagrams, but there must also be a way designed-in to limit the maximum amount of advance.

Back in the mid '60s, I remember replacing the distributors with Dual-Point systems and to increase the performance even more, we also replaced the valve covers with cast aluminum covers… I do not remember any of us bragging about our faster ETs in the quarter mile, but I know the valve covers sure helped… L :grinning: L . . .

And we also experimented with the centrifugal weights. We drilled and threaded the weights and screwed an Allen Screw into the weight and the part of the screw that extended above the weight, we screwed on various size nuts to increase the weight in the hope that the increased weight caused the timing to advance quicker, and again the aluminum valve covers gave better performance… L :laughing: L . . .

Also I experimented and installed a “Cool Can” on the gas line between the fuel pump and the carburetor. The Cool Can was a Coffee Can with the gas line running up from the fuel pump into the coffee can and the fuel line was wrapped in a circular coil and then to the carb. The coffee can was filled with ice. The theory was the ice would cool the gas, making it more dense and you were getting more gas into the cylinder. We also dumped a bag of ice on the intake manifold between runs to cool it for the same effect of denser air getting into the engine…

Yeah, my aluminum valve covers were just as effective at reducing my ETs…

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In theory you were correct, the execution was not, with the engine on a dyno and using a very large cool can it makes more power, in a vehicle the extra weight of all that ice/water offsets of not makes the ET’s slower… lol… Engine Masters did a thing on it, I don’t remember all the details on it other than not worth it, running E85 does about the same thing as far as cooling the intake charge…

Timing curve change doesn’t do much for a stock engine (if set right from the get go), you need more compression and cam etc etc to make it worth it…

But them damn cool looking valve covers sure did drop the ET’s 3 tenths… :laughing:

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A prior gf decided that pouring cold water on her car’s overheating engine would help. It did. Sort of. Cracked the block. The good news is the engine never overheated again …the bad news, it never ran again either … lol …

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That is called “quenching”, a procedure used during the manufacturing process of steel and cast-iron parts. How hot was the engine? Two thousand degrees?

No idea, but she said lots of steam was pouring out. I think it was a Volvo.

I spent some time in the foundry casting wheels, caster parts etc. in cast iron and aluminum. They all cooled in the mold then air cooled. No quenching.

It depends on how the casted part is going to be used if it requires quenching.