'78 GS550 gearing and RPMs

[Ozymandias]

I've been a long time rider but I've been out of the saddle for about 7 years now. I recently found myself the new owner of a '78 gs550 and I'm quite happy with the bike. I'm the third owner and it's been exceptionally cared for though it's life. Only has 16k miles on it to boot.

However, I'm finding that this bike has plenty of power to handle me on the highway except it seems to be geared way too low. It's turning 6000 RPMs at 60 mph. Here in the SF bay area it's mostly interstate travel that's 65+ mph and to be honest, I don't feel comfortable taking this bike up 6500rpm or more just to keep from being a hood ornament. It's redline is 9k and 6500 for any length of time is just asking to play "swap the piston"

My question is this. Is this normal for this particular model/year? What would be the best thing to do to improve it's top end? While I'm a long time rider I never really did much in the way of mechanical repair (but I did go to school for automechanics so I'm sure it won't be a problem with the right info). I would like to be able to safely take this bike up to 70-80 and not have to shift out of 1st gear at 10-15 mph would be kind of nice as well but not critical. Rolling along at 35 in 5th gear turning 3500 rpm just seems wrong as most bikes I've ridden ... well, this is the smallest bike I've ever ridden. Either rate, I don't want anything drastic but I would prefer to have the rpms down around 5000 - 5500 at 65 for my own comfort level.

[Steve]

Let's face it...it takes a certain amount of work to move you down the highway at 65mph,
and that little engine only has so much to offer. 8-[

Now, that being said, if you were not insisting on winning any stop-light drag races, you could probably drop about 10% or so without too much damage, but I don't think I would go much past that. If you look at the power specs for your engine, it probably has a peak torque at about 6,000 or 6,500 rpm, and a peak horsepower at about 8,500. Peak torque happens because that is where the cylinder is filling and emptying most efficiently. Horsepower is a function of torque and engine speed. It seems that you are putting the most power available to the ground right at the speed you are wanting to run. If you move the engine speed too far away from that, it may not produce enough power to stay there without downshifting. I happen to know that even the freeways are not level in the SF bay area, so you might just be spending more time shifting than you do now.

How to change it? Count the teeth on your sprockets. Chances are that you will have 14-16 teeth on the engine sprocket. If you change to a sprocket with one more tooth, you will drop your engine speed about 7%. Two teeth will be about 15%. Personally, I would probably add one tooth to the front sprocket and drop a couple of teeth on the rear.

Even better...get a bigger bike with a shaft drive. Then you don't have to worry about lubing the chain every other day or so. \\/


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[tkent02]

It's redline is 9k and 6500 for any length of time is just asking to play "swap the piston"

It can run at the red line and beyond for years.
Wide open throttle at lower RPM is harder on it.
It's a little bitty engine that needs to spin.
Run it.

[Nerobro]

My 550's live at 6000rpm or more. when I"m "feeling it" I'll keep it at 8-10k as long as I like. I cruise at 70mph or so. Always have. That means 5500rpm on the 1980 GS550E, and 6000 rpm on the 1983 GS550ES.

Seriously, as long as you're not in the red, the motors will not just "hold togother" but they'll thrive and treat you well for it.

[Ozymandias]

Thanks so much for the information. As I mentioned, I'm used to much bigger bikes (smallest previous to now was a Honda 750four) so those high RPMs made me a little paranoid. I may still add a tooth to the engine sprocket just for my own peace of mind, but I'm not going to worry about it too much.

Either rate, I'm pretty happy with the bike. It's nice getting to ride again after a 7 year hiatus. I was driving me absolutely nuts going from riding every day (sometimes not even owning a car) to not having a bike at all.