Running Fuel Levels

[BillyRok]

Just for kicks today, I decided to check the fuel levels on my carbs with the bike running as the FSM instructs on the XJ. This spring I had the bank completely apart to replace butterfly shaft seals and ultra sonic cleaning, and all the floats (8) were set at 23 mm / .905 inches. I get the bike set up so the carbs are level front to back and side to side, and set up a fan to cool the motor while I tinker. Here are the instructions per the FSM --

[IMG][/IMG]

So then I put a metric ruler up against the bowl as a point of reference --

[IMG][/IMG]

What has never been clear to me is that the FSM says fuel level should be:

3 +/- 1 mm below from the carburetor mixing body chamber edge

I interpret "the carburetor mixing body chamber edge" to be at the 20 cm mark on my ruler up next to the carb yet the pic in the FSM clearly shows that point to be at the 20.5 cm mark on the ruler. That's a difference of 5 mm in where the level would be dead nuts on. If it is truly important for all 4 fuel levels to be exact (and the debate rages on) and you're shooting for 2 mm to 4 mm below the mixing body as your target, then defining the point where the level is perfect is crucial. I'm not trying to reopen the discussion about how critical exactness is to peak performance, I'm just prepping for XSSE and thought why not set things properly. Here are pics of the fuel levels, carb #1 through #4 --

#1 [IMG][/IMG]

#2[IMG][/IMG]

#3[IMG][/IMG]

#4[IMG][/IMG]

So does anybody know without question or doubt whether the perfect, target fuel level would be at the 20.3 cm or 20.8 cm mark on my ruler? I would say my levels on #1 and #2 are at about the 20.6 cm mark and #3 and #4 are at about the 20.7 cm mark. Now that Brant is back on the site I know he'll take a swing at this one, any other carb gurus want to chime in?

Hope you all had a good Labor Day weekend!

[Ken Talbot]

I would suggest not trying to anticipate what the perfect level is.

Rather, get all four as close as possible to the same, say the level in the first two photos. Then, sync the carbs, set the pilot screws, sync the carbs again, and put in a fresh set of plugs. Take it for a run and do a chop test.

Then, and only then, will you know if the setting you started with is correct, too high, or too low. If the levels need to be corrected, change them by 2mm and repeat the paragraph above. I suggest 2 mm so you should be reasonably certain of seeing a change. You could try 4mm to be sure you will see a difference, then maybe have to do it again to come back to the middle, i.e. 2mm

A couple of iterations and you should have it.

[madmax-im]

fascinating question Billy...to me it looks like you have it correct according to the FSM diagram...good info Ken...

[Bonz]

I am gonna get hammered on this! The XJ is no different than my 80SG as far as I know. Set them at the factory mm spec for 80+ model year carbs and ride on!

Here's the bugger with setting fuel levels... If the bike has anything less than 100% sealing of the needle seat O-rings or needle tips, you will be setting fuel levels by adjusting float heights for the wrong reasons. If the carbs are sealing well, etc, then set by the measurement and be done.

However, if you must, measure and if the actual fuel levels are so far off from each other with floats set within spec height, find out why before chasing them. A single mm in float height equals 2-3mm of fuel level. No way in heck a 1/3mm - 1/2 mm adjustment in tang height (for 1mm of fuel level) will do beans for this bike.

It came from the factory set up within specs (float height measurement, not fuel level) as THE fastest production bike in the world in 1978. Changes were made in 1980 and still dang fast, but float height measurement was still mm spec'd, not fuel level spec'd. The XJ wasn't faster, better running or anything more than what the XS had always been before it, despite the fancy fuel level measurement.

Hammer away...

[mack]

This IS a contentious issue for sure. I spent most of last summer doing testing to establish this very point. I haven’t finished the thread yet for a few reasons, one of which is that with the XS’s the mixing chamber body profiles are different from 78/79(same), 80 differs from them and then 81 was different again. I’m not sure about the XJ’s.
To be as salient as possible, everyone makes good points. The only way to be sure is with Mr. Talbot’s observation. The fact that the term, “running fuel level” is used, muddies the water again, in that this is difficult to do even with an 81. First the bike needs to be off the stands sitting plumb and level as you would be riding it, and then the hoses need to be attached, opened and compared to each other. It took three people to do it properly when I tried it. This procedure of course is impossible with any bike earlier than 1981; unless, you’re very brave and have lots of fire extinguishers handy. And that's why I did all the testing to determine what the design static fuel level was.
Bonz makes a good point as well. The only problem is that the hardware has changed over the years. Many don’t have genuine mikuni, floats, inlet valves needles etc. so the performance is different today than when the bikes came off the line, factory adjusted. Even if you do have all your genuine mikini hardware from the factory, these components wear out with usage. I read a big thing on the mikuni site concerning performance curves for jets, jet needles and emulsion tubes. They start to wear the first time the bike is started.
So having said all that Billy, what I believe to be the intended static level for all years is what you show in your first picture. That would be the desired level with a +/- fudge factor of 1mm either way.

After I did my "running test", I transfered the levels to a carb body for reference. The first line is even with the mixing chamber body, the next line down is the desired static level, then next line is at idle, next is 3K next is 5.5 k and last is 8.5 k ( this might be a bit arbitrary because I didn't want to hold it at the red line for too long so I was a bit hasty marking that level on the hose)
If you look at the cutaway image that TC provided, you'll notice that even at red line, the inlet for the pilot tower is still well submerged with a 3-4 mm to spare.

[motoman]

Just a bit overthunked there Mack. That 3-4mm S from bottom of upper carb body lip, not as you have done. 3mm of fuel is fine at sea level(remember, all measurements fuel related IS sea level measurements.......always, always, same with initial timing) 4mm down from upper carb boby lip works perfect here at 5000ft., but is a 'bump' rich here at 5000ft. elev.)o different than old school automotive csrbs3-3.5mm fuel levels identical in all four will be fine, but if a constant elev. gets abo velocityve 4-5000ft. gonna be a tad rich, even though top-side constant velocity assembly better controls the meterig rods location. Compensation from less vacuum does the job well.
I measured a neighbors fuel with bike dead level (81Special)which have NEVER been apart, and fuel level with bike psrfectly ;evel for and aft. Fuel levels while idling is spoy-on all four being 4mm down from upper carb body housing lip. Have ridden that bike several times and not so much as a hic-up no matter if low rpm or a constant mid0range and high rpm. That told me all I needed to know Hope that all helped others! BTW,since theres movment from having the added teeny spring in the float needle, no way are all fuel levels gonna be the same. A stagnet(motor or carbs off)check of fuel levels IS also a waste of time. WHY. cause stopped fuel levels WILL differ a bit cause the teeny springs in the float needles vary in compression rate shutting off fuel.........Absolutely HAS to be done with motor idleing.

[MPittma100]

Fsm

The way I read it is that the measured fuel level can be .080"-.160" (0.12 +/- 0.04 in) down from the chamber edge. If you measure the furthest point below the chamber edge (.160"), that is not quite to the end of the bowl screw head. To obtain this fuel level, the floats need to be set at 24.5-25.0mm.

Your float height setting of 23.0mm (.905") has yielded the highest (in the bowl) fuel level. 24.5-25.0mm will yield the lowest (in the bowl) fuel level as stated in the FSM.

In short, the level can be from the edge of the bowl down almost to the end of the bowl screw head.

Just a matter of preference as to where you may want your level to be. I prefer the lowest (leaner) level of fuel in the bowl allowed. "Lean is Mean".