Since you have a rat engine and want to get the timing just right here is what I do (been doing since the race days) to get my 2 cylinder point ignition race engines timed perfect so both cylinders fire at the same BTDC position:
Tools: I use the magneto buzz box Travis24 posted (very accurate based on inductance not continuity) and a 0.001" accurate analog (not digital) dial gage (Harbor Freight). I took a spark plug and busted out the center, removed the tip L ground electrode, filled case with JB weld, drilled it out for the dial gage shank OD, tapped the side for a set screw. I made one for 14mm and 18mm plug threads. Makes dial gage set up very quick and easy.
Timing process:
1. set point gap accurately on 1st cylinder to be timed (pick a cylinder arbitrary), the other cylinder point can be set approximate – step 3 will adjust it;
2. place dial gage in 1st cylinder, find TDC, lock dial gage at zero, rotate engine CCW to desired BTDC timing (I actually go beyond desired BTDC a little bit and then CW back to desired – a slack thing), rotate mag plate till points break with buzz box signal, adjust mag plate to stop there. Check with slight CW and CCW rotation and note dial gage needle to be at your desired setting. On the race engines I lock the mag for no movement to prevent any slack/wear in the mag plate that changes the timing. Thus I start and run at full advance, no problem since running at high revs 99% of time. Starting could be an issue with kick back, develop a technique to start;
3. place dial gage in 2nd cylinder, find TDC, lock dial gage, rotate engine CCW (and CW as I note in 2.) to same BTDC setting at 1st cylinder, adjust point gap for that cylinder so they just break with buzz box signal . This is a touchy thing because when you tighten the point holding screws they affect the setting, need to sneak up on it. Note the point gap will be slightly different than 1st cylinder, not an issue just a slight difference in coil dwell voltage – plenty there to fire the plugs. Check with slight CW and CCW rotation and note dial gage needle to be at your desired setting.
For my setups I check ignition with a timing light with timing marks made for both cylinders. You will be surprised on timing fluctuations changes when running, if all well the fluctuations will be minimal. If a lot of fluctuations there is mag plate slack/wear, point bounce, etc in the mag. A lot of erratic fluctuations happen if the points do not have full parallel contact when closed. This needs to be fixed for top performance and reduced point contact wear. Even new points are not aligned correctly and need to be fixed.
Once you do this a few times it gets very easy to repeat. Check often to keep top performance.
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