Latest Carburetor Mystery...

Ok, so, after imbibing much wisdom from this forum, I've reset (and reset again, carefully, using a micrometer) the floats in my XJ 550 carbs, and tweaked at the pilot screws until...at last...the bike runs like it should. Nice response at the bottom end, and decent power at the top end.

But of course, SOMEthing had to go wrong, so here's my next question:

The plugs are still getting carboned up after 10-15 kms of running the bike. The tangs on the floats are set at 2.95 mm. If I lift the floats to 3.00 mm, it runs too lean and chokes out at the top end like it's not getting enough gas, and I have to reach down and crank the petcock to prime for a few seconds, then it's good again for a few minutes, when I have to repeat the procedure (this is only on the highway -- in town, at lower speeds, it's fine). So I backed the float tangs back down to 2.95 mm and it runs good again, but carbons up, suggesting that it's too rich. It's bad enough that it's difficult to get it to start again after it sits for, say, a half hour after it's been run unless I pull the plugs and clean them.

Weird that 0.05 mm would make the difference between rich and lean.

I'm running the manual-recommended plugs in it at the moment. What if I ran a slightly hotter plug in it? Would it burn off the carbon?

 

I'm wondering just what you are measuring that you can read .05mm changes. Surely not the fuel level in the bowls.

Also

does not sound like a problem with the floats. Sounds like a problem with the petcock or possibly in tank filter. I'd pull apart, clean, inspect and repair if necessary.

 

I have to agree with MiCarl completely. If it was float height opening the petcock to prime would not help. The floats would keep the extra fuel out. So they must be open and allowing the fuel to fill the bowls. vacuum line on the petcock or petcock diaphraghm are very possible suspects. My money would be on a split vacuum line.

 

I'm with BlueMaxim.

It sound to me like your Petcock Vacuum Line isn't being a Vacuum Line.

If you replaced that line with Rubber Hose or (God forbid) ... the clear hose that comes with a New Battery ... switch back to hose that is Vacuum Rated.

Rubber Hose gets heated-up and folds or collapses-in on itself. Since it seals the Intake the Bike will run just fine ... but, there's NO Vacuum to the Petcock and the Fuel stops flowing.

The clear vinyl stuff just can't hack heat and vacuum without collapse.
Same story as Rubber Hose ... but worse. Sometimes the vinyl will seal itself closed and STAY CLOSED!

Then, you can't run the Bike unless you put the Petcock to PRIME.

 

.05 mm is about half the thickness of a sheet of post-it paper, are you sure about that ?

 

Hi guys,

Ok, egg on my face here. Replaced the in-line filter and it's much improved. Go figure. It had a tiny crack in it from pulling it out of the gas line so much lately what with all of my carb tinkering. Must've been defeating the vacuum.

The petcock has been rebuilt -- before that it just puked gas straight through. Now it runs on a vacuum like it should. The gas lines I'm using are actual gas lines, not plastic or clear pvc. Been there and learned that mistake all on my own. :>

The measurements I'm taking are between the tip of the float tang (that holds the needle valve in place) and the main metal frame of the float. I'd post a pic, but don't have the beast apart at the moment. Replaced the filter today and moved the tang up to 3.05 mm and have had some improvement re: carbon on the plugs. Gonna pull it again in the morning and lift the tangs to 3.15 and see what that does.

I started with the float tangs at 2.45 mm and have been raising them by .10 - .15 increments for a few days now. Each time the bike runs better and better. At 2.45 mm the plugs were positively fuzzy with carbon when I pulled them. At 3.05 there's just a thin coating of black on them. I'm just hunting for the sweet spot now -- I know it's close! Be nice if I could've found a reference in any of my manuals as to the "normal" range for float tang adjustment before I started this odyssey.

The last guy who had this bike must've monkeyed with the carbs something ugly to get them as buggered up as they were when I started. Argh!

 

That is not how fuel level is measured.

Fuel level is measured by attaching a piece of clear fuel line to the bowl drain and holding fuel line up alongside the carb body. Gasoline will rise in the hose to the level in the carb. You then measure the distance from the top of the fuel to the carb body. Or, if you're like me and don't have six hands you will scribe a mark on the bowl and use that to judge is floats are set correctly.

On your carbs that distance should be 1-3 mm.

 

True, you should use the clear tube to check the final outcome of float adjustment, but it is not a bad idea to use the method Fionnbarr is mentioning to help get the float there. I set my tangs up at 3.16mm and have measured out a good level afterwards. Just be sure to follow up with the float test afterward.

 

Seems to be pretty solid around 3.05 mm for me. Put it up to 3.25 and choked it to death. Thanks for the confirmation Robert!

Tried using the clear tube method, but it wasn't consistent enough. Read a posting here where a fellow used a micrometer and it's muuuuch better that way. No guesswork as to how consistent your floats are.

 

Last two sets of Carbs I've done, I used an old, HOLLEY Carb Float Level Ruler.

Put them right both all right in the groove.