Thought I would review a bike I haven't had in about four years. This is one of those bikes that never gained much popularity (back then, or now), but in my opinion is quite excellent.
About the bike: it's a "standard" bike with middle controls. Yamaha also made the Maxim which I believe is more of a cruiser. The Maxim has a 16" rear and the Seca has a 18" rear. It's your standard UJM inline 4. This one is an aircooled 8 valve. Most commonly you'll see 8 valve and 16 valve inline 4's. Assuming a regular firing order, you can usually hear the two apart. The 16 valve sounds like a race bike, 8 valve sounds more mild mannered. This particular 8 valve has shim bucket adjustments, which can be more tricky but hold longer.
Inline 4's already make smooth motorcycle motors. For perspective, the typical standard is that under 2 liters, an inline 4 does not need a balance shaft. We're talking about a motor that's nearly a quarter that size. Further, it's rubber mounted. This bike is ridiculously smooth when tuned properly.
It has cast wheels intended for tubeless use (which, you should if you can. Tubeless is a huge advantage on a street bike). These aren't the Yamaha seven H-spoke which are incredibly strong, but these still look reasonable. The 550, unlike the 650 and up, had a chain drive. Lighter, better handling, and with today's chains, not a big deal maintenance wise.
The powerband is mild and well mannered until you really start to rev it. I can't recall correctly, but it might be about 6,000 RPM that the rather utilitarian motor comes alive and actually pulls impressively well. Guessing the bike makes about 50hp at the wheel. Not high nor low for its class/era.
One clever aspect to the bike is the motor's compactness. It's a "stacked" configuration, I believe with the ignition and power (cannot remember if magneto or alternator) over the transmission instead of adding more width. Some inline 4 motors on 70's and 80's bikes can be ridiculously wide, this one is not.
The handling is what you would expect for the era in terms of suspension compliance. It's okay. It is very neutral handling, striking a near perfect balance. It's very, very predictable. I had a 90/90-19 Bridgestone S11 in the front, 100/90-18 in the rear, if I remember correctly. The rear was a front tire mounted in reverse. This is one of the few bikes I could get the back end out around corners. It would do it on throttle alone and quite easily.
The underside of the motorcycle is cleverly flat. Some bikes have the pipes in such a way that if you were to ground it ou on a curb or something, the point is far off to one side or the other and more prone to tip you over. This is not. The exhaust is also very nice and quiet.
It also has a centerstand. The shocks were marginal, I replaced mine with some that I found on an SR-500. They fit perfectly and were a notable improvement in damping.
In the US, they come with high bars. In time, I've come to like and appreciate them for around town riding. Maybe not ideal on the highway, but a tank bag may offer a reasonable improvement. Stock, they had a steering mounted fairing. I've never ridden one with it. Without it, the chrome headlight is exposed and can be blinding in the sun, so something to keep in mind if you take it off. Generous steering, lock to lock for parking lot maneuvers.
Weight seems to be in the lower 400lb range if I can remember correctly. Gas tank was maybe 4 gallons and nicely shaped for grip at the knees. Interestingly, I had an RD-350 at the same time and found this one to have better low speed balance and driveability, likely largely due to clutch and engine characteristics. Even though probably a 70lb heavier bike. I'm 6'1" and remember this bike fitting me exceptionally well. Not awkward or forced, just right. Will obviously vary with your geometry, though.
I remember the bike being fine on the highway and fine two-up. Obviously, there are better bikes for both cases. It was fun on twisty roads and in town. Sufficiently quick. Economical, too. 50 MPG (US gallons).
My biggest gripe is the front brake. Single piston and always had issues with it flopping around a bit. It was not held in place all that well. There should be possible fixes or alternative calipers (with some work).
Despite that flaw, terrific all around bike. Very, very underrated. 80's UJM's in general, when dialed in, have some of the nicest responding motors around, even today. Today's motors may be faster, but unlikely to be as smooth as a good Japanese 80's inline 4.
This is one of the bikes that got away, so to speak. One of the few I miss. If you have one that's stock and you'd like to sell, I'd consider picking up another at some point.