Waugggghhhh! Why does my mpg suck?

I'm only getting about 25-30mpg from my '81 KZ550. Most of my riding is at 55 mph and below and I always try to keep it in top gear once I'm up to speed. I understood these bikes got double that kind of fuel economy or better.:blink:

My scoot only has 9500 miles on it and the previous owner had gone through the carbs. The bike had set for years and the brac's were full o' nasties (that's a tekinuckle term). Is there any chance that they got thrown together improperly? He had a factory service manual, and refurbing old bikes is his hobby, so it's not my first assumption, but I'm not familiar enough with these carbs to know if it is possible to get jets/orifices mixed up or if excessive pitting could be allowing fuel to bypass its normal path.

Suggestions from you experienced types is very welcome and appreciated. I'm a former marine mechanic and know my way around engines, but I'm smart enough to get some ideas before I tear into something I don't have personal experience with.

1.) Stock airbox or aftermarket pods?
2.) Stock jets or aftermarket Jet Kit?
3.) Stock exhaust or aftermarket?
4.) Leaking gas anywhere under the tank from a line?

1.) Aftermarket pods can do this if you don't have the carbs synched properly and adjusted correctly.
2.) Aftermarket jets can do this, once again, if the carbs are not adjusted correctly.
3.) Quite a few aftermarket exhausts have baffles in them and when they get clogged, they can do this.
4.) Well, leaking gas can do this because, umm, it's leaking. I didn't really know how I was actually going to explain that one at the end...

I guess it would have helped if I had mentioned the state of the bike and the things I already checked...duh! (Forgot some of the most basic rules of posting tech questions...):S :laugh:

It's a bone stock ride. The air filter looks great. There are no obvious external leaks and it's not "making oil", so gas isn't bleeding into the crankcase. It has new spark plugs. It starts OK and runs strong, at least to me (I'm a newb rider, so for all I know this thing is doggy compared to others). About the only oddity while running is that there seems to be a bit of a hesitation when I roll hard on the throttle at speed.

Thanks for the reply, Scubas'!

It's ok, sorry about those responses then. Well, if it has the stock exhaust, one thing I would do is just see if it's clogged or not. The hesitation is kind of normal when rolling hard on the throttle. Mine does it too, too much fuel at the low starting rpms (rpms you are at when you first full throttle it). Another thing you can check is your sprockets. See what size rear sprocket you have (how many teeth?) and if it's higher, you're going to lose gas mileage. Check your chain, how much slack is there? There should be about a half-three-fourths inch of slack.

You know, I just thought of something. Check your spark for all four cylinders and then check compression for all four cylinders. Pull the plugs and look at them, I know you said they're new, which could make this hard, but is there oil on anyof them?

if the bike is running good, as statedprior look at the sprockets, also what rpm are you turning at 70mph

Thankee much for the additional recommendations!

Well, this sucker turns some high RPM's...about 4800@55 and 7000@75 mph. But even on tanks of fuel that I never go over 45 mph, the economy is about the same.

I always run premium fuel and have dosed it with Seafoam to see if it would free up anything that might be stuck in the carbs.

I pulled the pipes loose and blew through them with compressed air. There didn't seem to be any noticeable back pressure.

Chain slack is good.

Tire pressure is where it should be.

I will pull the plugs and do a compression and spark check tomorrow. (The mosquitos were trying to carry me off this evening!) A running compression check, though, showed no major dicrepancies. RPM's dropped the same with each pulled plug wire.

I'm seriously thinking of running some Engine Tuner through this four-popper to make sure the valves and pistons are immaculate. For those who may not be familiar with Engine Tuner, it is a decarboning agent made by OMC...it emulsifies carbon and flushes it through the engine. Makes a heck of a mess on the ground underneath the exhaust and occasions an oil change, but it's usually worth it.

Thanks again!

Since you have a mechanical background, you must understand that the purpose of additional octane in gasoline is to retard combustion. Higher octane fuel is intended for machines with high compression so that these engines don't pre-detonate. Having said that, the 550 is a low compression bike that runs best on 87 octane fuel. "Premium" tends to carbon the combustion chamber, valves and plugs.

If I had that kind of mileage on a 550, given your sedate riding habits, I would purchase a Kaw Factory Service Manual and tune the bike up and ensure that the carburetors are brought to factory spec; that is, service fuel level is set, carbs are synchronized and the pilot fuel mixture is correctly adjusted.

Please read your spark plug color. THEY WILL TELL YOU EVERYTHING.
If you just purchased the bike and it was sitting for a while, run some gas treatment through a few tanks of gas. My 1266cc zzuke that is modded up the bazoo gets 35 mpg and i wack the throttle open from time to time.

BTW how does it idle??

Post edited by: slowpokedom, at: 2007/06/29 22:21

Run some 87 octane. What oil is in it? Does the clutch feel fine? Even when wide open in 2nd or 3rd, doesn't slip? Oil look burned at all? Motor getting too hot? Carbs feel overheated? That's about all I can think of without the bike being right in front of me to troubleshoot. Kinda like telling a friend how to install subwoofers over the phone.

I would suggest that you use a product I buy called Gumout injector/carb cleaner. It cleans the carbs and your intake valves increasing compression and fuel milage and you feel more power, I did!

This sort of thing has just happened to me.

The last 2 tankfuls of petrol have only taken me less thn 200kms where previously I was getting 270kms.

I hit reserve and there was nothing there too! Had to was the lump to the servo.

What gives?

Thanks for the fuel leak prob suggestion. T

hink I'll check the airbox for fuel and give the carbs a whack with the heel of my boot.

I'll also drop in some metho to mop up any water and check my petc0ck to see why reserve decided to take a hike.