Fuel oil furnace shudder.

My old "Sears 10" furnace has a strange shudder sound when the blower is running and it gets louder when the blower turns off.

Opening the "type R-C" draft control makes the sound louder after a 1 second delay.

The shuddering when the blower turns off "stops abruptly" before the blades stop completely.

I can eliminate the shuddering by increasing the combustion air flow by adjusting the air intake slider, but then it does not reliably light when cold.
Note: The electric eye trips the safety.

As far as I can tell I have the electrodes and nozzle adjusted to the correct placement based on a table I found on the Internet.

Q1: Could a dirty nozzle cause this?
I've taken it all apart and cleaned the tiny groves that make the fuel swirl. Then again I could have damaged it in the process...

Q2: Could this be a matter of dirty air passages?
There is some minor buildup on the fan blades and on the air swirl fins, but they aren't clogged.

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I am also trying to reduce the BTU's produced since I had to reduce the blower cutout temperature in order to protect that cheap insulated plastic ductwork I've had to switch to due to building structural reasons.

Q3: Can I put in a nozzle with a lower flow rate?
Right now I have a .75 GPH, 80 Deg, H (Hollow pattern)

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PS: Does anyone have a guess as to how old of a unit this thing is?
I know it does not have that many hours of run time due to being part of a "duel fuel" system. The heat exchanger for example is very clean.

It's a model number 867, but Sears has a policy of never printing dates on products to prevent people complaining about getting last years model.

I need to get this thing working perfectly or I have to replace it before I sell the place in a few years.

Is your fuel stored indoors, or outdoors where it is cold? Cold fuel is harder to ignite.

You can reduce the firing rate but you need to be sure you get enough heat there in MN. You would need to adjust the air to maintain efficiency.

You might need to do something about the ignition transformer. I have not had a problem with mine since the burner was replaced and I got a higher voltage transformer with it.

Increasing combustion air too much reduces efficiency. It should be adjusted by someone with the instruments necessary to measure compustion efficiency.

1. I can't find nozzles smaller than .75 GPH
2. The fuel was indoors earlier and it still did it.
3. I checked the transformer and the electrodes.
It gives a nice good spark across the electrodes.

I am also starting to wonder if the problem is in the pump.
Earlier the regulator piston was sticking due to all the rust.

I picked up a new nozzle and I plan on cleaning out the pump again.