Why is my 1981 KZ 550 underpowered?

[KahnSingh]

Recently acquired 1981 KZ 550 has top speed of 75 mph and very poor fuel economy (17 mpg) with no fuel leaks. Runs well otherwise. Any insight appreciated.

[OKC_Kent]

Runs well otherwise.

Wait until you get her straightened out, this will be the understatement of the week. 550's are great engines.

It could be one or a combination of things, your bike is almost 30 years old and who knows what previous owners have done (or not done) to her.

You probably have fuel issues, some electrical issues and maybe mechanical issues that limit the performance and gas mileage. That's pretty much every system on a bike. A 550 should hit 110 every day of the week, and get over 50 mpg.

Get a shop manual and start checking things out. Clymers Manuals are good for beginners, and the Factory Shop Manual (FSM) is a good addition. You can find FSM on eBay, and eBay and other vendors carry the Clymers.

The best thing to do is start with a complete by-the book tune up. Also start with a complete fuel system inspection and/or cleaning.

Write back and let us know what you find out.

[Andy the gringo]

With the nunbers you posted you can't possibly be running on all 4 cylinders. The 550's are little screamers when running properly. Kent gave you good advice, follow what he suggested.

[StarGate]

Sounds like he's running a 3 cylinder 550 to me. Or maybe he needs to cut the rope and stop towing his car while he's checking his performance number.

[MFolks]

Pull the sparkplugs and see what the center electrode's color is:
Black is very rich, White is lean, and a light tan color is about right.

Check the spark intensity; a hot fat spark across the sparkplug electrode should be almost blue in color, anything less has me think you have a weak ignition system due to corrosion in/on the electrical connectors.

Remove the gas tank and with the ignition turned on check the input voltage to the ignition coils; it should be battery voltage or a little less. If it's down around 8-10 volts you have some resistance lowering the voltage to the coils contributing to the poor gas mileage.

[apbling]

I second OKC kent. I have an 82 that I can twist the speedo around and get roughly 60 MPG.

[Mak-90]

My '83 gets around 50-60MPG, but the 110mph seems a bit optomistic to me. 80mph is about 7500-8000rpm. I don't like pushing a 26 yo bike!

[StarGate]

I've never owned a 550 but that may change in the future since I'm on the lookout for a project bike. 8k rpm at 80 seems a bit high. Is that with the original size rear tire and sprockets? BTW, I push my 28 yr old bike to redline quite frequently as I like to keep the cobwebs out of it and it seems to run all the better for it.

[Mak-90]

Yep, factory size ( 16/38 ) sprockets and tires. I tried to put a 18-tooth front sprocket, but it would not fit (sprocket fit, but there was not enough room between the sprocket/chain and the chain guard under the cover). I can't seem to find a smaller rear sprocket out there.

I have no issue with the bike and power, it pulls very stong on the streets. Also, I am 6'1" and 230lbs. I avoid the freeways anyway and stick to the streets.

[apbling]

I about 25 lbs heavier than you and I can twist the speedo around no problem...and still get great mileage. I have a 38 tooth rear and 17 front. I also have not been able to find a smaller rear one...it would be nice though.

[StarGate]

Have either of you contacted Z1enterprises to see what they could do in the way of sprockets for changing the ratio.

[9am53]

I have a 550 spectre (6 speeds and a shaft drive) not sure of the 6th gear is much taller than 5th on another 550, but I weigh 316 pounds and I have had it as fast as 160 km/hour which is 100 MPH. At this speed it was at about 7500-8000 rpm. I cruise to work on the highway at 80 mph at about 6000 rpm everyday...again, not sure about teh 6th speed or how the shaft affects this.

[Mak-90]

As stated previously, I tried an 18-tooth front, but it's diameter was to large and would not allow the sprocket/chain to fit past the little chain guard in front of the shaft under the sprocket cover. I might have to pick-up a 17-tooth as "apbling" said it fit his bike.
I have checked all over, including Z1, and there does not seem to be a rear sprocket smaller than 38-tooth. If you got a source, please post! I would love to change the shifting of this bike and increase speed while lowering RPM (reduce the factory 2.38 ratio). I would love to stay in gears longer.

I believe that it was the orginal poster "KahnSingh" who expressed issues about mileage and top speed concerns. I only chimed in to report that my 83 gets around 50-60mpg (around 180 miles +/- per tank). I just stated that I don't see my 26year old, 15400 miles on the clock, 553cc doing 110mph without a stiff tail wind and some NO2. Could I rev it high enouhg to get to 110mph? Probaly at redline and beyond. But, if I want to do those speeds, I'll borrow a friend's Hayabusa. I am happy zipping through town and dodging cagers with the low end torque this bike has in spades.

[ironyankee]

Those are nice,peppy bikes.Jeff at Z1 will get you what you need for sprockets.If it's not online just call and talk with Rob or Jeff.They'll fix you up.I had a 550 a few years back and I liked it,good power and balance.It ran great and never failed me.Just stick around and let us know what kind of problems you're having.Lots of knowlegible people here to help get the bugs out.

[KahnSingh]

First, I would like to thank you all for your input. Much appreciated. Today it was determined that the #2 and #3 spark plugs were not firing. Further examination revealed that those spark plugs were not plugged into the coil! Acceleration has doubled and today I pulled 85 mph with power to spare. What a fun bike! Thanks folks.

[StarGate]

Wow a 550 twin cyl, even worse than I thought. I'm impressed you were able to get those numbers on just 2 cylinders.

[King David 72]

Carbs are dirty bro. Use Berryman CHEMTOOL B-12. 2 ounces per gallon. do not refill until mixture is almost gone. 1 ounce per gallon every 3,000 miles from now on, and you'll never have dirty carb problems again. I don't know where you live, but this product works like magic on this corn mix fuel we have up here in Wisconsin. Try it. It's only $5 at your local auto parts store. If they don't have it, they can get it.

[apbling]

2 dead cylinders will have that effect... good to hear you found the issue.
Mak-90, I bought my 17 tooth sprocket from Dennis Kirk. I believe it was JT sprocket....but that was also about 5 yrs ago.

[bandit354]

I had two KZ550 an 82 LTD and a straight bar 83 550 the 82 had a super trapp exhaust with uni air filters and stage 1 dynojet jet kit and the 83 had a supermoto exhaust with K@N airfilters and K@N stage 2 jet kit and they both topped out at around 115mph with stock gearing the 83 I ended up porting polishing blue printing the motor hot cam intake and exhaust cam and wiesco 13 to 1 compression pistone .20 over after all the mods could pull the front end off the ground with the power band without dumping the clutch ended up selling the bike.

[isarr]

Originally Posted by apbling

I about 25 lbs heavier than you and I can twist the speedo around no problem...and still get great mileage. I have a 38 tooth rear and 17 front. I also have not been able to find a smaller rear one...it would be nice though.

Great news for a good friend of mine, and it seems like a lot of other 550 owners. I located a web site that will machine the sprocket you want at a very very reasonable price for the rear. No other bike dealer or company manfactures less than a 38 tooth rear . We went with a 17 tooth front and a 30 tooth rear, my buddy can't wait to try it out . The web site is (REBEL GEARS MANUFACTURERS MOTORCYCLE SPROCKETS) ASK For Brad tell him Scott told you to give them a call. I'll repost the RPM AND MPH results. MY friend bike is a 1982 Kawasaki 550 LTD.