At high RPM, bike sputters, and going through gas fast!!

[400BOBBER]

Hey gents! New here! Just picked up my first Yamaha! 1981 xs400! Bobbed a bit! Has stock carbs, with pod filters and mac 2 into 1 exhaust! As far as I know, the rest of motor is stock! Just today, found out from previous owner, that the guy before him, rebuilt carbs, and put washers over the needles to upgrade his jet setting??? I am pretty sure that wasn't supposed to work that way! May be the source of said problem! Onto that!
The bike starts, idles, and runs fine! Up until I hit the higher RPM's, then it's sputter, sputter, and I shift gears! I am also getting alot of backfiring. And the last question! I know it is an older bike, but what is everyone else seeing as far as gas mileage! I ran about 150 miles today, give or take, and ran clean out? Possibly part of the bigger picture here??
Any info would be helpful! Thanks

 

[JARichmond]

First off have a check at your carbs. Just because a PO cleaned out the carbs doesn't mean he did it well. Also, as far as I know, washers aren't going to do a darn thing for your jet settings but screw 'em up. I hope someone can chime in the proper jetting for your situation.
As far as mileage goes, I think you should be getting something like 55-65mpg from the bike. The stock tank is 4.5 gal so ideally you should be getting something near 250-300 miles from a tank. You jetting may be damaging your mileage significantly, but I don't really see how it could blow through twice the fuel unless you spent a ton of time up in the high revs (7-8k). Have a look for leaks all around the bike.
Perhaps pairing both problems together, your main jets (I believe) control your fuel system in the high rev range so have a look at those to see if they are in spec. Remember, the jet sizes are measurements in 100ths of a millimeter (a 125 jet would measure 1.25mm). It's possible that a PO drilled it out too wide and thus you are getting a sputter as your bike bogs down with fuel as this jet opens up.

 

[HoughMade]

My bike can get between 45 and 53 mpg, but with a small engine, it can drop rapidly if the bike is carrying a heavier load, ridden into the wind or spends all its time going up and down through the rev range rather than cruising at a steady state.

If it were me, I would take the carbs off and return them to stock. The washers under the needles is an old school means to get the carb to deliver more gas at earlier throttle openings, but it will not deliver any more at higher rpm because the jetting hasn't been changed.

After the carbs are surgically clean...and don't even say "the PO cleaned them" or "I cleaned them and I know they're clean". That's what I thought- 4 times. A true cleaning is very, very meticulous work. At that point, see how it runs. If it is still acting lean (which despite the washers, yours is), what you need is bigger main jets, and possibly idle jets. However, just pods alone may not require jetting, just careful carb adjustment.

This post has my suggestions for carb cleaning and adjustment. Just suggestions, but they certainly worked for me. I have not had to touch my carbs for over 2 years save for draining at the end of the season after doing this. Also, adding an inline filter helpes a lot.

http://www.xs400.com/forum/showpost.php?p=64401&postcount=17

 

[bcware]

Check the plugs; what do they look like?

Inspect the jets and report the sizes here. Measure your float height. Take a photo of the main jet so we can tell you if it's been drilled out.

 

[400BOBBER]

Alright guys! I am always one for advice, and following it! Especially ehrn it is something I know little about! I am mechanically inclined, which led to the fix, but not so bright when it comes to Japanese bikes!

I took the carbs back off today when I realized the idler screws were the recessed style! Took some working to get them out seeing as how the PO comletely wiped out the head of it! Finally got them out, and found that they were just gummed the hell up with carbon! With a little elbow grease, and some patience, got them and the springs cleaned up, eyeballed the butterfly openings, set the idler screws to 3 1/4 turns and fired her right up! Adjusted the throttle bar idle screw and she is smooth now! No more pops and bangs, no more hesitating on take off! Tomorrow I will let you know how the gas mileage goes! Headed out for a mountain ride with some friends!

Can't thank you guys enough for all the help and support! Really do appreciate it! Keep it up, and I will continue the trend and pass on what I have learned and know!

 

[xs400neophite]

Another happy ending....