My Mpg just turned into Yards per gallon

Have had my 83 xj550 seca for 5 years now. Cleaned rebuilt and replaced a whole bunch of stuff. Engine has been running fine (or what I assume is fine) the whole time. Actually shelled out the money to have my carbs professionally cleaned last year (not worth it). So all of the sudden this year (on mile 3000 or so this season), I have dropped from 140miles to a tank (about 40 mpg) to 100 miles to a tank (about 28-30 mpg).
I even went and leaned out (plugs were a bit blacker than normal) 1/4 turn on all 4 carbs, and after two tanks of gas I can say it didn't touch my problem.
Air filter is brand new, new oil, new plugs. Wiring good, fuses good. The idle has been finicky, one day it's at 1500 the next it's 1050, the next it's 1200.
Any thoughts? Anything I'm not checking? I'm gonna tear down the carbs this weekend and see if maybe I have a float problem, last time I did a bench test on gas level in the bowl and all was good.

 

sounds like a leak somewhere.

 

No visible leaks anywhere. Even the air box (where I've had fuel dump out before) is dry.

 

The Mikuni carbs have to be squeaky perfect... especially the FLOAT levels. I have found the perfectionist point to work on the float levels for hours upon hours with my XJ550 Maxim (same carb-rack as Secas). In the end, it was worth it. I got 'em perfect using wet-fuel method finally and as a result, I get 50~52 mpg now!!!

All along I never believed bigfitz was getting 51~53 mpg, until I started getting around same fuel consumption on my 550 maxim. And I spent the most amount of time getting the float levels perfect.

That would be my starting point. Now, I don't understand why you would have such a drastic drop in fuel consumption but somebody else will hopefully have an idea about that. While you have it apart be sure to just clean every passage, including the tiny holes for enrichment circuit. Obviously its a "carb" problem with erratic idling symptoms

 

the o-rings on the float seats shrink and leak gas by. very common problem on those carbs.

MN

 

I'm in the same boat, but my idle is fine. I'm getting about 33mpg. My carbs are squeaky clean, so I've been advised to check the floats again, check the pilot screw adjustment again, and check and re-clean the pistons, bore, and diaphragms. The drastic and sudden change in yours would make me think that a float valve may be leaking and/or the petcock may be leaking through the vacuum line. Good luck, and I hope you find the problem. I'm going to watch your thread to see if I can pick up any suggestions.

 

How about a valve lash check ?? Recent??

+1 on floats or faulty petcock.

 

check your engine oil with a sniff test. if it smells like gas, you know then the float valve(s) are leaking and that would be the path of where all your gas is going.

 

When WAS the last time you checked the valve clearances? Been doing it every 5K like you're supposed to?

It does sound like it's time for the basic carb service as outlined in both the factory and aftermarket (Clymer) manuals. You very well may have a stuck float. Are you running an inline fuel filter?

Also; have you checked the condition of this thing? http://xjbikes.com/Forums/viewtopic/t=31872.html

 

Two things: I don't make stuff up. And having your fuel economy come up to match what the magazine road tests got back in the day is the sure-fire sign that you've "nailed" the state of tune. Float levels were what finally did the trick for me as well.

 

I openly admit my stupidity.
Have recently measured and adjusted valve clearance (thanks to Hogfiddles), took the carbs off and blasted all the passages,swapped coils,and cleaned electrical connections.
Mileage jumped up from 31-32. Then took a dive last week. WTF???
Pulled away from the ATM last Sat. and got a big whiff of gas.Looked down and it was dripping steadily onto the side of the motor.
Turned out that with all the taking off and putting back on I'd cracked the fuel filter(90 degree bend type) between the stem and body AND worn out the fuel line.
Course with a working petcock it only leaked when running,so no puddle under the bike when parked.
Cut the line by a half inch,finished shopping,went home and replaced the filter and line.
I think I recently ran across a post by BigFitz about replacing filters periodically,due to clogging.Here's another reason to replace them,and also the short section of fuel line.
Now that my wrists have healed from the last time I put the carbs back on (G-D airbox built for small hands),guess I'll check the float levels.

 

You can check the float levels on-the-bike by running the front tire up a ramp, then get clever with stabilizing it level.

You can't actually ADJUST them without pulling them.

 

Based on your erratic idle I would go after air leaks as well. Spray some starting fluid all around the carbs and boots while the engine is running at idle. The engine will suddenly rev up when/if you find an air leak. You will never get your idle stable unless you get rid of air leaks.

 

Okay, carbs were a bit dirty. Balanced my floats to - 3mm from top of bowl thanks to the write up from xj4ever.com. Now I am at about 38mpg from 30ish. But I am running funny, got it up to 65 on the highway and started going up hill, pulled on the throttle and it didn't do anything, just seemed to be flooding the engine or something. Also I will sit at idle at a red light, then green , give it gas and it sounds like I'm flooding it till about 2000rpms and then I take off like a shot. Vacuum has also been leveled on carbs with my homemade yics tool.
I'm about to give up and shell out another $400 to have someone else go through these carbs again.
How much does a new set cost?

 

a "new set" of Mikunis? Oh, about $1200.00 if you can find them.

Float level spec on the Mikunis is 2mm, +/- 1mm.

How apart did you take the carbs? http://xjbikes.com/Forums/viewtopic/t=31061.html

Emulsion tubes out, clean inside and out?

One possibility for the uneven running and low fuel economy is one or more enrichment plungers not fully shutting off. Did you have them out, and clean their seats? Inspect the rubber seal in the end of each plunger to be sure it wasn't damaged?

Another is that you really aren't in sync. Did you get a good accurate bench sync before you put the carbs back on?

Are you also experiencing a "hanging idle" when the throttle is released?

So you blocked the YICS how and used what type of manometer set-up to sync?

 

Yup, that far apart. Soaked and cleaned every port and tube and hole. As far as a "hanging idle" only do much that when I pull in the clutch and let off the throttle it may hang at 1300rpm for 10 seconds or so before dropping to 1200 (idle speed). I hate to ask if it's a timing issue, cause that is a whole other monkey to deal with (that I have never done before). I leveled off the carbs with a murcury manometer.