Damn Gas-Hog!

Just got 69 miles on 2.25 gal of gas, or about 30.6 mpg. This is an XJ700N with about 11500 miles. Since I've had it--about 6 months--I have adjusted the valves, cleaned the carbs, wet set the carbs, bench sync'ed and running sync'ed them, used colortune to set pilot screws. I have the floats now set at 2 mm (specs call for 1 mm+/- 1), and the pilot screws are open between 1 1/4 and 1/3/4 turns. The plugs are still a little dark, but not sooty. I'm a cruiser, so I don't ever get on the gas, and cruise around 55-60 mph. I'd say about 1/2 of my riding is in town. there is no gas in the oil. Compression is good and even on all cylinders. I need some ideas and suggestions and review of what I've done. There has to be something I'm missing that causing such poor gas mileage. Let me hear your thoughts.

 

make sure the choke is shutting off all the way

 

As far as I can tell the "choke" system is closing fully. The plungers are in perfect shape and the seats look perfect.

 

How are the slide diaphrams? Any holes in them and are they free to move in the bore?
Have you checked the size of the jets? Are they the right size for the exhaust and intake setup you have?

 

The slides slide freely and the diaphragms are good with no holes. The jets are stock, and I actually measured them to be sure they had not been enlarged. As far as I can tell the only non-stock item on the bike is the air filter, and that is a UNI Foam filter.

As bad as this gas mileage is, it would seem that the plug would be sooty and even fouling out, but they are only slightly black with some brown. I almost think there is a leak, but I can't find any evidence of that.

 

You sure your neighbors aren't "borrowing" your gas? "smile"

 

I have the same bike but have not checked the mileage. Pretty sure I get better than that but it does burn more than both my kaws. Guess I need to check to see what I am getting.

 

My bike was getting 45mpg before the carb mod, 42 after. I was thinking about the choke/enrichment myself or the jets. What plugs are you running and what's the condition of the coils,plugs and plug wires?

 

Try going back to a stock paper filter, and putting the float heights back at spec. You'll probably need to re-ColorTune and re-sync.

Just a thought. The only thing you haven't done is to return it to bone-stock configuration; and that might be the answer.

I agree that it may seem counter-intuitive; it's the only thing left.

 

1 1/4 and 1/3/4 turns seems a little low, I thought pilot screws were set closer to 2.5 to 3 turns out?

Could he be idling on the next fuel circuit, instead of the pilot? i.e., the pilot circuit doesn't provide enough fuel for idle, so the butterflies are opened further via the idle speed screw until the next fuel circuit kicks in and allows for an idle?

 

The more I think about this, your suggestion of going back to the stock air filter and ManBots comments about the pilot screw setting fit together. If the UNI filter is restricting the air flow, then it is acting like a choke and causing a rich mixture. That would explain the very poor mileage and the need to lean out the pilot screws when colortuning and fine tuning the setting to get the appropriate color plugs. I'll bet if I run it for 3-4 miles at highway speed and then do a chop check of the plugs they will be sooty. In any case, I'm going to try the OEM paper filter.

 

Sounds like a plan. Sooty plugs aren't ideal even at all unless you are truly trying to run rich and somewhat cooler. They really shouldn't have any hints of black, and the brown should be light like heavily creamed coffee but slightly transparent. Anything else and you're burning more than you need to.

Are your bearings all good? Have you cut it at 30mph on a flat surface to see if you are rolling freely?

 

As far as I can tell it rolls very easily--at least when I'm pushing it. The front tire spins easily, so no dragging brake. The rear wheel turn easily with no brake drag. Brakes on front and rear are new.

Anyone ever had an instance of gas being sucked in from the petcock through the vacuum line? I rebuilt the petcock, but haven't checked for gas in the vacuum line.

Just a note of interest--I have an old UNI foam filter which I decided to split to about 2/3 of its original thickness and installed it. It doesn't affect the colortune at all. I may ride it a few miles and see if it affects the mpg.