Topic: gas mileage

[MikeJW]

I did a search for this topic and altho most of the posts I saw were for 500/550s, I got the impression that there are about a bajillion things that can affect mileage. I just checked my '72 750 today and calculated it to be 25.7 mpg. Granted most of my riding is in town but still....!?!  Not a real mechanic...can't even understand some of the really esoteric info that sometimes gets posted here. Anyone have any simple tips. I check my tires before every ride and I'm not heavy handed on the throttle. Wish I could at least get above 30 mpg so I could say my bike gets better mileage than my truck. Thanks
Mike in Memphis

[Gordon]

When was the last time it had a tune-up and new air filter?

[MikeJW]

Don't know about the tuneup. I bought it just a few months ago. PO seemed a nice guy but kind of a doofus. That is, he was even less mechanically able than me. I put in a new air filter about 150 miles ago. He was running pods and I put the stock filter box back on. He said he had a 'mechanic' friend of his install the pods but he didn't know if the guy re-jetted the carbs. I have checked the plugs and it looks to be running just a little rich but not ridiculously so.
Mike

[grumpy]

I have a pretty much stock K3 and I regularly get 48-50MPG while commuting.
But this is camped out in 5th at 5,500 RPM surrounded by(and in the draft of) pickups, SUVs, and big rigs for 40 minutes at a stretch.

[Gordon]

Don't know about the tuneup. I bought it just a few months ago.

That would be a good place to start.  Dragging brakes will also eat away at your fuel economy. 

It also would be a good idea to check the carbs to find out for sure what jets you have in there.  If it was running decently with the pods installed then chances are it was re-jetted.

[ev0lve]

Not a real mechanic either  Just for reference my bike is a bit out of tune and rich at the moment and I still see high 30's MPG. Previously mid 40's combined city/highway and 50+ all highway as above.

If you're getting below 30MPG something is seriously out of whack. I'd go with the dragging brake theory first. Easy way to know is get out on the road at speed for 10 minutes or so without using the front brake at all. Come to a stop using the rear then check your disk. If it's fry the spit on your finger hot then that's your issue.

On the jetting, if it was that rich I'd think you'd smell it in the exhaust and it would bog while accelerating - a lot. Is that the case?

Anyways, hope that's helpful and good luck!

[8 Track]

Yep, close to 50 mpg with my cb750, too.  Easiest things are the choke partially on or the main jets are too big or have been drilled.  Of course, there are about 37 other things to check, but those two are simple enough.

[MikeJW]

Thanks for all the responses. I am real suspicious of the PO. I think he put the pods on because they looked 'cool.' The dragging brake theory seems reasonable. Is it a fairly common occurrence? If you find it, how do you correct it? Replace the pads....or does it require a more complex fix?

[TwoTired]

Do you still have stock sprocket tooth count front and rear?
Are you running stock tire sizes?
Do you have the stock exhaust?
Brakes can drag front or rear.

You need to know when the last proper tune up occurred.  You may have to do one to establish that baseline.

If it ran well with pods, and you don't know that the carbs internal setting are stock, then you will have to open them up to find jet size (if drilled) and throttle valve needle setting.  You will have to synchronize the carbs after this.
You should also investigate where the air bleed screw is set.

On average, which gear to you spend the most time engaged?  And what is you ration of stopped running to moving running?

Another possibility is the the float height in the carbs is set too high or you have a sinking float.
Or, that the emulsion tube air bleed holes are plugged.

What altitude do you run at?

[MikeJW]

Two-tired,
I'll answer those of your questions I know now & check on the rest:
Tires are definitely stock and I'm pretty sure the sprockets are too.
I usually run around town in 3rd gear...4th between towns. Cops are tough around here. I think Memphis is like 100 feet above sea level. I don't idle for long periods. A couple minutes after start to warm up and then at red lights. When I arrive, I cut her off. Bike always starts easily and to my untutored mind, runs well...both before with pods and now, with the stock airbox and brand new filter. I understand and agree I should exam the carbs, but I'm afraid it could be beyond my skill level which is sort of low. Have tried to clean them by flushing carburetor cleaner in them with the bike running and putting Seafoam in the gas tank.

[Achmed]

Check the brakes, definitely. It is common for the brake piston to get all corroded and gummed. There's a pretty good thread somewhere around here that describes how to rebuild the front brake caliper assembly.

[Hasenkopf]

I get about 32-35 with my 75 550 f

[TwoTired]

I usually run around town in 3rd gear...4th between towns.

Certainly you must realize the best mileage will be achieved in top gear.

I understand and agree I should exam the carbs, but I'm afraid it could be beyond my skill level which is sort of low. Have tried to clean them by flushing carburetor cleaner in them with the bike running and putting Seafoam in the gas tank.

I don't know if Seafoam is effective at cleaning the emulsion tubes while the carbs are still assembled.
And, Seafoam won't correct drilled metering jets, too large metering jets, or Throttle valve incorrect settings.
I think you will have to live with the poor mileage or open the carbs.  IMO
Its your call, of course.

I get about 32-35 with my 75 550 f

In stock trim and adjustment, that model get about 10 more MPG than you get.  FYI

[MikeJW]

Thanks again for all the good advice....especially you, Two Tired. For what it's worth, I did a road test this morning...riding around only using the rear brake. Apparently the front brake is not dragging. I don't know how to test the rear brake. So, I guess I'll have to get up the nerve to open the carbs after I study all the FAQs & manuals. Wish one of you old timers lived in Memphis. I would gladly buy the beer for some hands-on pointers.

[Brown Bomber]

     Last year my 750K2 managed 48mpg when I was commuting 40mi round trip, (mostly hwy). That's with a new chain/sprockets/fork rebuild, and Kenda Challenger tires. I haven't checked around town mileage, but it is obviously much thirstier in stop-n-go traffic.
     I'm not an old timer, just old , but I'd take you up on your beer offer if you lived closer These carbs are pretty easy to clean up, lots of info on that in the forum.
http://forums.sohc4.net/index.php?topic=5410.0

[Gordon]

Thanks again for all the good advice....especially you, Two Tired. For what it's worth, I did a road test this morning...riding around only using the rear brake. Apparently the front brake is not dragging. I don't know how to test the rear brake. So, I guess I'll have to get up the nerve to open the carbs after I study all the FAQs & manuals. Wish one of you old timers lived in Memphis. I would gladly buy the beer for some hands-on pointers.

You check for brake drag on the rear wheel by putting the bike on the center stand and spinning the wheel to see if it spins freely.  Same for the front except you either need a jack to lift the front wheel up or have a friend push down on the back of the bike to lift the front up. 

How did you determine that the front brake isn't dragging?   

[MikeJW]

I rode it about 10 miles without ever once touching the front brake lever, stopping with rear brake only. Then, I felt the front rotor to see if it felt warm. That's what someone said to do. They said if the brakes were dragging, it would feel hot. Is that not correct??
Mike

[Gordon]

That works, too.

[Ayrity]

mine seems a bit low as well, subscribed, just in case there is a genius idea put forth...

[TwoTired]

mine seems a bit low as well, subscribed, just in case there is a genius idea put forth...

Return the bike to stock configuration?  How's that for genius?


 

[phudeb]

I have a '78 750F SS and it's getting 31-35 mpg and I ride it medium to hard.
I am using PODS and did not rejet.   Just some info.

[wdhewson]

Most of the time low mpg or high fuel consumption is due to air/fuel richness.  Rich mixtures use a lot of fuel, they burn cool, and the engine runs cool.  A plug check will tell you if you are rich.  Also the engine is kind of lumpy if rich. 

These Honda engine can take a pretty lean mixture if most of your work is below WOT.  Do your carb tuning to shoot for plugs that have center electrodes that are light beige.  Bright white is too hot or too lean.  The color of a brown paper bag is about right for a perfect mixture, but I usually go leaner. 

Most of my 750 cm3 bikes get 50 to 60 mpg unless I'm having some fun with the engine in the upper reaches.

[scottly]

What is your technique for checking your mileage? It should be done over a period of time, not just from one fill-up to the next, to minimize error. 

[Ayrity]

mine seems a bit low as well, subscribed, just in case there is a genius idea put forth...

Return the bike to stock configuration?  How's that for genius?


 

Mean! Don't have the cash for that sort of thing haha

[scottly]

mine seems a bit low as well, subscribed, just in case there is a genius idea put forth...

Return the bike to stock configuration?  How's that for genius?


 

Mean! Don't have the cash for that sort of thing haha

Don't take it personally, Ayrity, some people just can't think outside the box. These bikes ran lean stock, and anything that improves air-flow exaggerates the condition, and sometimes leads to over correction with one jet or the other. A genius would be able to tell you off the top of his head that you needed this idle, that main, with these pods, etc. BTW, if I was to return my bike to stock, I would need almost a whole new bike! I think I'll keep what I have.....

[Spanner 1]

wdhewson... 50/60 mpg on a 750 is cruel and unusual punishment.... lean to the point of askin' fer trouble... IMO.

[scottly]

I got 60+ MPG cruising at a steady, heavily law-enforced 55 MPH. It was cruel and unusual punishment on ME to go so slow!!!

[wdhewson]

Perhaps another thing to add about fuel consumption is the seasonal affect, which comes with the density of the atmosphere and the blending of gasoline.

Motorcycles are an aerodynamic nightmare with drag coefficients worse than the standard barn door.  So in cool weather when the atmosphere gets dense, our engines work a bit harder to push us through the air.  This is usually a matter of about 5%.  So if you're getting 60 mpg, then 5% is 3 mpg.

Also, gasoline for cold winter months is much more volatile than midsummer blends.  A liter of  high volatility butane for winter has a lot less combustion energy than a liter of trimethyl xylene for summer.  And our carbs are more volumetric devices than mass flow.  This is probably another 5% or so.

So the overall season affect is about 10% from dense atmospheres and low energy gasoline for the colder season.

Cold weather also makes our air-cooled engines run cooler and thus less thermodynamically efficient burning more fuel.  And less mechanically efficient with more viscous cold oil robbing power.

Lots of variables at play in addition to the cock angle of the right wrist...........

Don