Huge fuel usage. 40 MPG on a GS400?

[duaneage]

My 400 Yamaha gets 50+ in town and 60+ on the highway. It has two 34 mm Mikunis (just like a GS) so you should see at least 50 out of that bike.

Check the tire pressure, chain lube, brake drag, wheel bearings after you check the carbs,

 

[67fire]

From the topic "What mileage does your bike get?" on another motorcycle forum:

"GS400 87mpg"

Eric

 

[Turtleface]

I must be less reckless with the throttle than I thought I was being. Best I've got on a tank was 46 mpg. Average is around 37-39 mpg. My coils are slowly crapping out on me, and mileage dropped dramatically. Down to about 31-30 mpg. I have the same sort of symptoms you do. I can ease around on 1/4 throttle or so, but quick rolls result in bucking and sputtering. Make sure your ignition system is working properly. Spark at the plug doesn't necessarily mean it's hot enough for a complete burn.

 

[gearhead13]

Seeing you have a mix of engine and frame, what carbs are on it, because those needle holders dont look like anything I have ever seen on cv carbs.

 

[Calvin Blackmore]

anyone ask him if his valves are set properly?
cos that would be my first thing to check before fiddling with carbs.

 

[Charlie G]

I bought a new1977 GS400 and for the 4 years I owned it It averaged 48mpg. The speed limit was 55 then and I rarely went over 60 on my way to work in San Francisco. My GS450 gets around 45 in town and 50mpg when I'm on a longer trip, going speeds between 55 and 65mph. GS 650 gets 45 or so. I weight less than 150lbs.

cg

 

[Steve]

There is an awful LOT of guessing going on here:

I think that I should be getting 50-60 mpg, shouldn't I? ... Too rich?

... What could cause such a huge drop in mileage?
I havn't synced the carbs, could that do it? The pilot screw is 2 turns out like I said - that couldn't effect the mileage THAT bad could it?
... If it's running rich on a circuit, could I tell from a plug chop?

I would synch the carbs first and see if that changes anything, probably won't make that big a difference in mpg. ...

That bike should get 50 mph easy. Sounds like something is wrong. Did you adjust the valves and check compression? What do the plugs look like? Dark brown? Maybe the petcock is leaking?

... I don't know what to do here.. Maybe it's running extremely lean?

Do you think my float level could be too high, and flooding too much gas? ... I'm not sure, but it could be related.

First, you don't know if it's running rich, then with no adjustment you think it's running lean? The easiest way to tell what's happening inside the cylinder is to do plug chops. But, do them correctly. The different circuits in the carb are throttle position related, not engine speed. Example: you can run 2000 rpm in top gear up a slight hill and need wide-open throttle to maintain speed. Or, you can use second gear to do the same thing in second gear and barely crack the throttle. Definitely different circuits in use.

Along with all the other questions and guesses, I am going to ask for a bit of clarification. You have a '78 bike with an '82 engine. Are you using the CV carbs from the '82? If so, you technically don't have "pilot screws". At first, I had not caught on to the fact that you had a newer engine in there and I was going to ask "which pilot screws are turned out 2 turns?" (fuel or air), then I realized you might have carbs without "pilot screws". What you have are "idle mixture adjustment screws". When you cleaned the carbs, you should have started with them about 3 turns out, then tweaked them for highest idle speed. They might have ended up at 2 turns, but you would at least know that's what the bike needed. If you just blindly set them at 2 turns and are guessing about everything else, you will never know.

Do the plug chops. Find out if you are lean or rich, and on which circuits. Then you can stop guessing.
This thread has been going for 4 1/2 months, now. One hour of testing and you will have your answer. :-\\\

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