Suzuki 2 Strokes

I made my first attempt to time my GT550 with a dial indicator...unfortunately, my indicator doesn't measure in Metric...and I'm just generally confused as to how to work it. I did left, center, then right...but I can't be sure if I did it right. I had to mess with the points gap, because moving the plate had no effect on when I got power to the volt meter.

I did my best, then started her up. It idled MUCH lower than it did before, and smoked about 2 times as bad.

Sounds like you missed it by a little...
According to my book that I used to set mine, the spec is 3.37mm, or 0.132in. for the GT550.
I set mine up on the bench when I had the motor out, then once I got it running I checked the marks with a timing light. They were a little off so I painted new ones and now don't fool with a dial indicator when the engine is in the bike.
You might find that using a test light rather than a meter works better also. I was getting some funky readings the first time I did it in the bike with a meter. I believe this was due to the condenser soaking up some of the power. I made a tester for mine out of a pigtail socket for a 194 side marker bulb (available at most parts stores). It comes with 2 six inch wires attached and I just put a couple cheap alligator clips on them and keep it in my bike tool bag.

I'm gonna stress this one more time. You CAN NOT accurately set the timing with a dial indicator, UNLESS the indicator is at a right angle to the piston crown. But think about it. If you use the plug holes, the indicator is at an angle. Think aboout how much off it would be at a 45!. Of course this isn't that severe as the plug angle is only about 10 or 15 degrees. Plenty of lost motion. Coupled with that, as the piston rises the indicator point is travelling down the piston crown. I reality, you don't know where the hell you are at.
I always strobe mine in to the the factory marks. Bike runs best there and no more back breaking hours fooling round with an indicator. Once you have all 3 strobed in, If you want to advance the spark a little, turn the entire points plate counter clockwise. To retard the spark a little, turn the points plate clockwise a tad.
But I repeat, indicators are worthless when working at an angle. I don't know about you, but I misplaced my sky hook years ago.

I haven't owned a 550 for around 30 years, but I'm guessing the points set up is similar to the 500 twin.....just an extra set of points to adjust.

I use a similar method to Coyote to set the points, but even more simple !
With the ignition on, and the plug that matches the set of points you are adjusting earthed on the head, put a ring spanner on the end of the crank and turn it slowly until you hear the plug spark, (a nice quiet garage will be needed). It's then simple to adjust the points back or forth around the cam until the sound of the spark occurs when the factory scribe marks are aligned.
I have set points this way since my first bike in 1975, and it is accurate enough to have your bike running perfectly, with no special equipment required.

I seem to remember that the 550 head has the plugs in the top centre of the combustion chamber (or VERY close to), so no worries about angulation of the dial gauge.....at least I never noticed any pronounced angle on the gauge placement. (The Buffalo, however, is another story. )
In fact, I've occasionally used my dial gauge as a double check for how close the points are set with the factory marks on the "propellor".
The differences are not worth the bother to correct.
On a stock engine with original exhaust, the factory timing marks are plenty good enough for 99% of bikes I've seen.
My .02 worth.....