why the crappy gas mileage?

Another point to consider, would be proper tire size/inflation. I have an '82 750 SECA with stock size Metzler Lasertecs running 30# and 42# front rear inflation. I am getting over 150 miles to reserve. Once I get her tuned, I will do actual fuel mileage calculations. I believe I am over 50 mpg though.

Nachoman

 

Man, I sould bring my bike down to you have you tune it. Although the seca tank is over 4 gallons, isn't it about 4 gallons to reserve meaning you around 40mpg?


I run Bridgestone Spitfires with 34/36 psi front/rear and up it about 2 psi in the rear when carrying luggage. I get 40-42 mpg around town and 37-39 mpg on the hi-way. lol and about 27mpg when riding in the Rocky Mountains (yeah it doesn't like elevation).

What I don't get is the Honda CB750 gets in the 50's and aren't the XJ's Yamaha's answer to the CB? My brother has a CB750F Supersport and it would get mid 50's around town and low 50's on the hi-way. He just put a 900 engine in it and now it still gets better gas mileage then me, low 50's around town and high 40's on the hi-way

 

I was getting in the mid thirties. Got the carbs rebuilt. Now getting 50+.

 

I've checked with my doctor and she recommends I get my carbs jetted as opposed to losing 30 lbs for increased performance.

I guess 135lbs wouldn't be the optimum weight for a guy thats 6' 2"


I keep reading all these posts about pod airfilters and carb jets....makes me want to do it more and more. Maybe I'll be able to afford it before winter settles in.

 

My bike get between 43 & 49 miles per gallon depending how I run'er.
Once I got 52 mpg but I don't know if I figured it right because I can't do it again!!??
I was cooler weather then?!

 

On my first 81 Seca 750, I could get 45-50MPG depending on riding conditions. It wasn't in tip-top shape by any means, but it only had 18,000 miles on it.

Then I got an 82 Seca 750. It was in GREAT shape, was well cared for with only 9500 on the clock and I could only get around 40-42 MPG out of it!

Now I presently have another 81 Seca 750, in great shape and well cared for, with 11,000 miles on the clock and I can get 45-50MPG out of it!

Maybe the fuel economy is just better in the 81's for some reason? Were the carbs or the jetting changed on the 82's?


Chad

 

I get about 45 - 50 MPG on my 82 xj 550, all depending on how I drive. Just put a windshield on it, so will have to see it it improves mileage or makes it worse. Oh Its only got about 13000 miles on it, and I am running Michelin Macadam tires on it and using Mobil Super gas.

 

Last time I checked, I was getting low 40s. But the bike is in dire need of a valve adjust(shims on the way), and a carb sync. So after that, I'll have to recheck the fuel economy.

Of course, my 40 might be off, as I later had a speedometer failure(the 2 teeth on the gear were broken), so I might have been recording lower mileage than actual. And my bike only has 8000 miles on it too.

 

The reason the Honda might get more gas mileage is because we have shaft drive and I believe the Honda is chain driven. The power loss from driving the rear wheel at a 90 degree angle (hypoid), as opposed to direct drive (chain) is tremendous! I want to say around 10%. This would decrease our gas mileage accordingly because our wheel power/cc ratio would be down on the Honda.

 

Speedwagon, where did you get those shims?
How much did they cost?
I may need to be looking at those questions as my bike can use a valve adjustment too.
Mike

 

YICS was added maybe? Yamaha claimed a 10% improvement in mileage.

 

YICS was on all the 750 Secas, 81-83. But you did get me thinking... maybe the YICS passages were clogged up a bit on my 82??

 

YICS was on all the 750 Secas, 81-83. But you did get me thinking... maybe the YICS passages were clogged up a bit on my 82??

 

Keep in mind that the Maxim 650 was an 80's performance machine. See what kind of mileage an '83 Trans-Am with a 350 gets. Here is a list of some things kill the Maxim's MPG.

1) Shaft drive is less effiecient (But better in every other way)
2) No fairing causes poor wind resistance.
3) High performance 2-valve engine means big ineffecient vavles.
4) All riders of mid-80's rice-burners are really fat; causing increased wind resistance and rear tire wear. Also, nacho and twinky debri often gum up the carbs. Switching to a full-face helmet may alleviate this problem.
5) Valve lash is often neglected in older bikes, and can result in poor fuel economy.
6) Comfortable rear seat often results in the wife coming along for the ride (see reason #4)

 

I wonder what the math is on weight and horsepower, cuz I need to lose about 30 pounds before August 2nd if I even hope to do the trip I have planned out. 235 pounds down to 205 pounds ought to save me quite a bit of gas... and Cheeto money to.

The only problem is my trip is planned around brewery tours of Eastern Europe so my mileage will go down as the trip progresses.

 

Horsepower won't change as you shed pounds, but city economy will. Your bike's engine can really only generate a fixed amount of power given tuning and other things, but every time you accelerate, the bike needs to get your keester moving. Take this:

Force = Mass X Acceleration (Newton)
-Mass is you plus bike

Force X Distance = Work

So the more your weigh, the more you bike has to work given a consistant acceleration, the more gas the bike will use to get up to speed. Same goes with cars and junk in the real trunk.

Generally speaking, pods and exhaust will decrease milage. An engine is basically an air pump. The freer it can breathe, the more air it can move, the more fuel it needs to mix with that air; the more power it makes. But there are exceptions of course.

Hypoid bevel gears are generally about 97.5% efficient when not submerged in oil. Unfortunately ours are half submerged which reduces their efficiency do to oil churning (which is why the oil gets hot). Our bikes have two sets of bevel gears so assuming 96.5% efficiency, per set, we get 3.5% losses, squared which equals 87.75% . Chains are generally around 98%+ efficient IIRC which may help explain the economy differences between the XJ and the CB.

 

also for all you people who have with in the last year or two noticed a decline in the MPGs of your bike car truck ect must remember that the gas is different now with the ethanol in our gas. i only know this cause we have people bringing their bikes in left and right saying the carbs are screwy or their truck isn't getting the fuel economy it use to. the new gas is much less efficient no matter what the govt tells you i see it first hand. also it can eat away at fuel lines and add water to your tank which reduces fuel economy even more. ethanol will actually extract moisture from the air and deposit it into your fuel tank. many a time i have had to drain fuel tanks on equipment and vehicle's because of large amounts of water in the gas. also NEVER leave the new fuel in the bike for an extended period of time, like over the winter. old timers would tell you to top off your tank for winter storage DO NOT do that anymore cause you will clog and even get rust in your carbs from the water.

it was really nice of the govt to tell us that but then again it is the US govt they tell you only what you want to hear

 

Well said. All part of the plan my man, all part of the plan. Just when fuel prices are getting unbearable, the government steps in an worsens the problem through policy. Not only does the ethanol harm fuel, but it has made food pricier.

I bought a bike precisely for the reason of rising fuel prices. I can feel the pinch now, and it's nowhere near the $5+ a gallon the globalists are hoping for.

And if you think this is all by coincidence...I have a bridge to sell you...

Now, back on topic.

Duane

 

A lot of good info here.
I'm concerned with the milage that I'm getting, the manual says 47 mpg.
So for a 26 year old bike I figure 40 would be good.
It seems most 650 owners are realizing between 35 and 40 mpg.
I'll keep my fingures crossed.
Still got half a tank, by eyeball, and just reached 100 miles, looking good.

 

Yes, ethanol has about 70% of the energy per gallon of gas. So if you have a 10% ethanol blend, your new blend has 97% of the energy of straight gasoline. Also, reformulated gasoline has also been "cracked" into better burning short chain hydrocarbons which reduces the energy density. Gas of today isn't like the gas that was available when our bikes were new.