Gas Milage

[5twins]

I can get near 50 highway, low to mid 40s around town with my '78 carb set, a few MPG better with my '76-'77 set. The '76-'77 set runs smaller mains which I think accounts for the better MPG. This set should give about the best mileage for the 38s.

If you haven't done so, check the condition of the needle jet o-rings and the needle jets themselves. Worn needle jets will just kill your MPG. They control the midrange and that's where you do most of your running. The needle jet should be a snug slip-fit into the carb body. If it's loose and wiggles in the hole, the o-ring is bad. That can allow fuel up past the sides of the jet instead of just through it. Also inspect the inside, the hole through the center that the needle sticks down through. That's the actual metering orifice and while quite large compared to the hole in a pilot or main, it can get worn from the needle rubbing or from gas residue eating away at the brass. If your carbs were badly gummed up at one time and sat that way for a long time, erosion of the jet is very possible. Examine it closely for a textured or sandpaper-like appearance. That's a sign it's been eaten away.

 

[retiredgentleman]

I took careful records of my gas consumption on a 2000 km trip a few years ago. My average was 55.8 mpg (american gallons), or 23.76 km/litre.

The gentleman that said he gets 60 to 70 mpg with hard driving, I find hard to believe. I know Pamcopete has recorded gas consumption in the 60 to 63 mpg IIRC, but I believe he was driving gently so as to minimize fuel burn.

I doubt any XS650 has ever got over 65 mpg.

 

[TwoManyXS1Bs]

Sorry, gotta jump-in here...
In the early `70s, my 750 got 70 mpg @ 70 mph, and still ran in the 12's, but that is a long story.
Check your plug colors, if dark and not tan insulators, too rich. If this mileage condition has been
worsening over time, you may be looking at a serious carb cleaning, maybe even engine OH. If this
is recent, you may look for particle/obstruction in an air bleed circuit (those little holes aside the bell
inlet area). Reverse blast/flush from the airbleed ports in the jet area into the carb body with good carb
cleaner. Look for spray emerging from these little air bleed inlets.
Also, while you have the carbs off, flashlight down the intake ports and see if the backside of the intake
valves are clean. I've seen bad mpg when the valves have 1/8 " or more of built-up coke there...

 

[tonyc]

I think Pete did get 67 mpg on one of his tests.

 

[retiredgentleman]

I think Pete did get 67 mpg on one of his tests.

You're probably right Tony, I didn't remember the exact numbers. Pete was going for the very best numbers.

In real world driving, I think most folks have to work hard to achieve mid 50's mpgs.

 

[5twins]

I never get mileage as good as some report on any bike I've ever owned. I've learned to accept this. I guess I suffer from the dreaded "lead wrist" syndrome, lol.

 

[retiredgentleman]

Yup, I suspect anyone that can get up in the 60's mpgs, is driving their bike like a 250 cc commuter bike....................maybe shifting from 2 nd gear right up to 5 th gear...........lugging the engine..................that's no way to treat your engine.

 

[tonyc]

I had a KLR before my xs650's and my friend had a single newer BMW. He was always getting 70 mpg. I changed my sprockets and did get 72 mpg on a flat stretch of highway doing 55mph. Not I lot fo fun so I switched back and lived with 60mpg.

 

[retiredgentleman]

For sure Tony. You can change the gearing on any bike to get maximum fuel mileage, but the fun factor goes out the window.

An XS650 with 18 front and 30 rear, driven gently at 55 mph, on level ground could potentially get 65 to 70 mpg, but that would be a dull experience. Aren't we driving these bikes to have fun

 

[XSLeo]

I got the 75 out for the state inspection and a good spring shake down run. Did 92 miles and used about 1.75 gallons. Some around town and some out on the better two lanes up to 80-85 mph. After a good warm up I didn't take it easy, just out for a fun ride. That's 52.5 mpg.
Out on the two lanes doing some leisurely cruising, just enjoying the day averaging 45 to 50 mph it does better, I think my best was 57 or 58. If I could stand it an average of 35-40 would do even better.
In the Clymer book it states in the 70-74 specs on flat level roads at 37 mph you get 82.5 mpg. That's a stock bike, probably in pertfect tune, running the tires rock hard and a 85 lb rider. Using some very miserly riding techniques.
Leo

 

[tonyc]

On my 78 with vm34's do 55-65 on the highway I get 55-60mpg. If I would stay at 55 I know I would get a little over 60. I just put in a new Sparx stator because i made a dumb mistake and did not tighten the stater bolts. A $175 dollar error.

 

[TwoManyXS1Bs]

A friend of mine drives a Suzuki Bergman 650 (scooter, yuk). He gets 70 on his trips.
I believe, in the late 60s thru early 70s, that our gasoline had a higher energy density (btu per lb).
Also, I've read about oxygen levels in our atmosphere dropping to about 18%, it was closer to 20%
40 years ago. Scientists, analyzing ice core samples of trapped air pockets, report that they believe
oxygen levels may have been as high as 35% just a few hundred years ago. Imagine the HP and
MPG you could've gotten back then. Maybe that explains how people could build castles, ships and
pyramids back then. Right now, I'm exhausted just from this typing...

 

[aldo5468]

Yup - a gallon of E10, which is what most regular and premium are today, has 97% of the energy of pure gasoline.

 

[motorshag]

New Issues have surfaced. After carb sync, needle re-positioning (leaner set at #2 Slot), new air filters, new manifold boot clamps, and air box clamps, hole in muffler welded, has giving me another 50 miles a tank full. So I road 120 miles, and filled up and road another 50 miles....(not all in the same day), before I started having problems. The bike does this thing when getting on the throttle. Like a blap blap blap, then kicks in and runs fine till after the next gear. Almost resembles the same situation when bike has been started and petcock is no on...So, I took the carbs off, put the needles back to the original position (standard). Then I took the bowls off, checked them, blew them out with air. No plugged holes. Took off jets and blew them out. Checked the floats, they are fine, and in the correct postion. Checked the diaphrams and no holes so they seem fine. I put the carbs back on and ran it last night, about 20 miles or so, ran fine the hole time. Then.... All of a sudden blap blap blap. Thought it was fixed, and maybe I was running out of gas again. I filled up. Same problem..I am at a loss on this. I have a Pamco Ignition I put on it about a year ago. I figured I would change the plugs, adjust the timing chain, valves, and then check the timing again... But seriously I am at a loss. I have 42 days to get this bike right for the EDR. Any suggestions are more than appreciated.

 

[gggGary]

motorshag; Pictures of the bike and or a good description of everything you are using for fuel and exhaust.

 

[motorshag]

@gggGary, it's just stock pipes, air boxes, carbs on a 1977. I've used 87 gas mostly, and mostly ethanol free gas. I had a crack in the left side exhaust, which is now welded closed. But that was cracked by the PO, and never caused the problems I am having now with the stuttering while accelerating. Thx

 

[motorshag]

The left plug is from the left side of the bike, and the right, the right side. Both needles are set to the center slot. I guess the stock position. Thoughts?

 

[gggGary]

It's either gas or oil fouled, kinda looks like oil fouled causing gas fouled to me, first guess would be an intake valve seal if it's not smoking too much. Is the spark nice and bright on that side? What coil are you running? Did you dip test the floats in hot water, looking for bubbles? Did you put in new float valves and seats? When you have the carbs off, shine a light in the intake can you see oil on the intake valve?

 

[motorshag]

I am assuming this could be causing problems? Looks cracked. Only a year old, from Mikes.

 

[5twins]

Looks like you're running without the original metal shrouds over the manifolds. That can happen with any manifold if you do that. That's the right side if I'm not mistaken and may be why that plug looks so lean (air leak). There's something definitely wrong with the left side though, too. It's running way too rich. Since you have a Pamco, good spark on one side should mean good spark on both sides. That single dual output coil fires both plugs together. That would only leave spark plug, cap, or wire on the left as far as ignition things that could be wrong. If that checks out, it could point to that carb or maybe internal engine problems with the left cylinder. Have you done a compression check? That would be my next test.