Poor fuel mileage on my 1982 XJ550 Maxim

I finally got my 82 XJ550 Maxim running and after driving it around town for about 2mo now, I have noticed my fuel mileage is only between 26-30mpg at best. Original specs have this bike advertised at 50+mpg. I have proper air in tires, air filter is in good shape, there are no air leaks or vacuum leaks at intake or air box fittings, and carbs were only bench synched after a thorough cleaning and rebuild kits installed. I have not checked the valve shims yet since owning this bike. Seeing what anyone's thoughts are on what could be robbing my fuel mileage and recommendations to fix. Thanks.

 

Fuel mileage as advertised was based on the EPA mileage loop, which was somewhat unrealistic and incorporated the then 55MPH national speed limit. Even so your mileage is on the low side. Describe how you ride and what conditions you ride in.

 

Right now most of my riding is on and around the military post no faster than 35mph for multiple 3mile trips back and forth to work. Rpms are usually in the 3K range in 6th gear during ride. Even if my carbs were vacuumed synched I cannot imagine that making much difference.

 

the 3 mile trips 6 minutes of running you are barely getting warmed up. with short trips your fuel is being used warming the bike before you start riding and while shifting up and down.
you would have low mpg in a car as well with just a bunch of three mile trips

 

Good point, that does sound reasonable. I usually warm the bike up to where it comfortably idles at 1/2 choke at the house and then take off. By the time I hit the first stop light 1/2mi down the road doing 20mph, I take the choke off the rest of the way. I am going to do a highway experiment this weekend and take bike on a 50mi hwy round trip and see how it does. Most length of hwy on bike at any given time has only been 5mi one way.

 

Why are you riding @3k rpms in 6th gear?

Gary H.

 

Starting circuit (chock) dumps extra gas while it is on into carbs to start bike so riding with it on cuts your gas milage

 

That's just what the tac is reading at 35mph in 6th

 

6th is a highway gear. At the speeds you are riding you should not get out of third gear, maybe 4th if you need to keep the engine quiet.

 

Look at your plugs and see if they are black and sooty which would indicate rich. I had a similar problem, though not as bad, and I used a colortune plug to find a lean setting and then opened the pilot screws to get a blue flame. I installed new plugs so that I could easily see the color progression as I tweaked the pilot screws until I got the correct color plugs. My bike runs better and my mpg went up about 6 mpg. At low throttle settings you are mostly using the pilot system, so if it is set rich it will really impact mpg--at least that's what I've read and has been my experience.

 

Engines are most efficient at their TORQUE PEAK rpm's, although I don't have the books in front of me, I'd guess that it's somewhere around 5500 - 6500 rpms. Riding at 35mph in 6th gear, while it can be done, will not result in the best fuel efficiency.

 

Another good point, I did not think about it in that fashion, I will try riding around in 4th while on post and see how that does. I have been riding for over 30yrs, was a Harley rider for the longest time, this is my first Japanese transverse inline 4 cylinder I have owned so I am still learning the quirks of it Lol.

 

I've thinking about this for four days now, and it still doesn't make sense to my brain. I'm not saying you're wrong, I just don't get it... To me it seems that the engine may not be making the most efficient use of the gas at low rpm, but it would still be using less gas due to less explosions per minute. Am I over-simplifying? Or am I dmub?
PS I am used to thinking in terms of fuel injection, so if it's a carb thing, can somebody explain it to me? Slowly? Please?

 

Fuel and air contain energy (many ways of expressing it, BTU's, etc.) and the engine extracts that energy and uses it to push down on a piston(s) to produce rotary motion (which ends up turning the rear wheel and propelling you a certain number of miles). The point at which an engine is most efficient at converting the energy in the fuel into the greatest push on the piston(s) is where the engine will get the most miles-per-gallon, since you are getting the most usage of the energy available in the fuel.

I tried looking up the torque rates and can't easily find them.......I guess it was somewhat of a "Yamaha secret", but I could find a few horsepower ratings, and the max HP is typically produced at about 8500 - 9500 rpms. Torque peaks are generally far lower than HP peaks, so let's say the torque peak is 6500 rpms. This rpm will use fuel most efficiently. Of course, the rate and manner by which you ARRIVE to 6500 rpms will effect fuel mileage, as any rpm's lower than that will be a less efficient use of fuel. And heavy handed use of the throttle in arriving at 6500 rpms will waste a tremendous amount of fuel.

It's not a fuel-injection thing, it's an engine/energy/physics thing. There's only so much blood you can squeeze out of a rock (or energy out of a gallon of fuel), and the more energy you squeeze out of it, the more "FUEL efficient" you will be.

My estimates of the torque peak may be too high. The torque peak of an engine can be somewhat accurately estimated by where in the rpm range the engine feels most "snappy" or responsive, and don't confuse greater acceleration (due to greater gear multiplication in lower gears) with engine "snappiness".

"Snappy" and "snappiness" are hard-core technical terms, BTW.

 

just to over simplify it think of it as city mpg vs highway mpg, and the different driving patterns that go with each.

 

I'm also having the same problem, 1981 XJ650 just had the valve clearances done <100 miles ago. I have a 3.4 gallon tank and im only getting 80 miles before it starts to chug hard and I have to go to reserve. My usual ride is to work and I'm usually in the 65 mph/5500 rpm range for most of it. First year or two I had the bike I was getting much better fuel range, any help on how to improve it?

 

Another thing to consider: when the throttle plates are more closed, the engine is always less efficient. This is a major reason you'll get better fuel economy when you're riding in a gear that allows you to have the throttle relatively open.

Riding at a given speed, the power requirement to go that speed is constant. The only thing you can change is how efficiently you generate that amount of power...

 

I guess I could try and vacuum synch the carbs and see if that helps any.