I'm trying to resurrect an XJ550 that has been sitting for quite some time. First bike, and not much of a motorcycle mechanic (yet), but so far I have sealed the tank, put on a new battery and cleaned the carbs and replaced the needles/floats. It starts right up but then revs to about 5k rpm's then dies. I can rev it a bit but if I let go of the throttle, it dies. I have the airbox off, does this make any difference? Are the pilot jets the ones that are used for idling? I just want to see it run a bit before I decide to put more money into it as it needs a lot. I know you've all read lots of posts like this, but I appreciate any help you can offer.

Sounds like fuel starvation. Try setting the fuel tap to PRI. That bypasses the vacuum diaphragm and allows straight flow. If it cures the problem then replace the tap. Screw the idle up a bit more to keep her running.

If it starts up okay then that usually rules out air leaks.

When you cleaned the carbys did you blow out all the tubes/jets/holes in the carby?

Did you change the sync or mixture settings at all?

An awful lot of trial and error sorting out carby probs.

Replaced my inlet gaskets on my XJ550 the other day as I had idle problems. One gasket had a 2mm wide crack in it. two others were loose. Runs like a hummer now.

HI, I too had(have)problems with the over revving. The problem alas is still dirty carbs. May I suggest the following check ups. On the float needle, there is a very small pin that is spring loaded. After the carbs sit for awhile, that spring loaded pin sticks and drastically changes the float position. If you're real lucky like I was, it was stuck in the full open position causing that piston to take off and rev the bike to redline. Problem two: the idle circuit is unbelievably sensitive to varnish buildup. I had to finally take my carbs and "boil" them in SeaFoam for over an hour. ( use a bucket and a fish tank aerator). Problem three. On top of the float needle is a teeny tiny screen for filtering out more junk. They get clogged and need to be cleaned. Problem four: I am becoming somewhat of an expert at prying off the carb set, BUT . . . it's also messing up the rubbers between the carbs and the head. Those intakes are noted for getting cracks and leaking air. My solution was one I found here in tech talk. I took some clear silicon tub caulk and wiped it onto the suspect areas. I let it dry for a full day and put everything back together. It worked. Problem 5: that pesky selector valve under the tank gets gunk too. It can be cleaned but that's another story. Hope some of this helps

OK, same thing happens on PRI, and while its revving high, if I give it gas, it revs.I can hold it at 4-5k rpms for as long as I like, but as soon as I let go of the throttle, it dies. I've already replaced the needles and seats, and cleaned out the filter screens on the seats, and I also got the boots with rtv silicone. When I pulled apart the carbs, I soaked the main jets and pilot jets in carb cleaner. It was easy to tell that the main jets were clean (you could see through them) but the pilot jets are much smaller. Isn't it the pilot jets that feed the fuel during idle? If I can rev it, to me that means that its getting gas at high rpm, but if it won't idle,then maybe its gas starved because the pilot jets are still blocked. How wrong am I to want to replace the pilot jets?

Make sure you pull the brass jet that the tapered needle slides in and out of,It sits in the center of the carb throat. it sits under the main jet on the bottom and from the top the needle sits in there. It must be tapped out from the main jet side to the diapragm side.It has 16 tiny holes that can not be drill unless you have the correct dia. drill.Even then its extremely doubtable.(can you tell that the name of the part escapes my mind?) What I have done in the past with those jets is soak them in carb cleaner and try to push on the clogged areas with some safety wire or even a needle that you would use to sew on a button. Just make sure that you do not remove any metal from the sides of the holes or you will change the jetting. Make sure there are no holes in the diaphragms. Pull them out of the slide and hold them up to the sun, if there are any holes in them they should show up by doing that, unless you are doing it at night :).
These motors are pretty bulletproof, they can built and still be fairly reliable, besides lots of other Yamaha engines will work. Any fj600, Fz600 or Radian motor will easily fit without any mod's at all. Lots and lots of possobilities including carbs if you decide to go with the K&N filters.
Sean

OK, I took the carbs off, again, and took out the pilot jets and soaked them in carb cleaner for a day and a half, and now the bike idles nice and smooth. but doesn't rev. starts right up and runs great with the choke on but if I give it gas or take the choke off, it dies. I'm thinking along the lines of fuel starvation now. I had a filter on the line between tank and carb, but it had a tight curve, so I put a smaller one in, but while its idling, I notice that the petcock is wet and dripping gas. GGRR! time to get that petcock rebuild kit. Could it be that the petcock's not letting enough gas through for anything more than an idle? Or is it totally something else?

On a totally different note, how tough is it to change tires? Any tricks to make it easier? My front tire is dead, and its a real bitch pushing this thing outside everytime I need to start it up with a flat. By reading the Clymers, it doesn't seem like its much to take the wheel off the bike, but the changing of the tire looks iffy.Is it worth the trouble or should I just pay the bucks (how much is it?) and take it to a shop?

I'm still thinking starvation. However, if the carbi float level is full it should rev straight away till the carbi empties. Running on choke makes the fuel mixture richer. Have you adjusted the mixture screws correctly. SOunds like it may be running too lean.

Screw in the mixture screw(clockwise) all the way and then back them off(counterclockwise) three full turns. While this is not the correct tuning it should run and rev quite happily at this setting.

ANother quick check is to check the approx quantity in the float bowls.

Basically, put the bike on the sidestand and place a flat dish under the plate on the bottom of the carbs. Undo the drain screw of each carb and allow the fuel to run down the plate into the dish. Compare quantities from each carb to make sure they are roughly the same. This will give a rough idea of whether each carbi is getting enough fuel. If there are major differences then it can be either bad float adjustment or a blocked fuel line between carbs.

When you cleaned the carbs did you just take off the top and bottom or did you seperate the carbs. Whole new set of instructions if you broke them up ;)

Definately pull the petcock out and check the filter in there for blockages too.

I didn't separate the carbs, only took off the tops and bottoms. Are the mixture screws the ones that are sealed and preset at the factory? I figure the next thing I'll do is take the tank, petcock, and filter off and put a big funnel on the fuel line, then start it up and see how it goes with a good gas supply. Hopefully, that will isolate it, if not, then I'll use your method of checking the level in the float bowls, which I'm hoping is ok, because I really, really, don't want to take these carbs off and apart again.

Your mixture screws still have the factory seals???????? Thats gotta be rare as hens teeth. Factory preset is a bit of a myth IMO. Any changes to exhaust etc over the years requires retuning mainly by mixture. Even changes to fuel quality over the last twentyfive years will deem adjustment neccesary. :huh:

If yours are still at the factory settings I'd be interested to know how many turns it takes to screw them right in.

If you did not take the mixture screws out when you cleaned the carbs then that in itself may be your problem. Could be varnish in the air galleries. Try removing the screws, pour a bit of petrol down them and blow them out. Replace screws as per previous post and see if there is any improvement. Factory seals should just pop out.

If you get some improvement but not perfect then I'm afraid it may be a remove and clean again giving particular attention to mixture galleries.

If your factory seals are still intact then I'd put money on that area being your problem. ;)

Persistent, lousy weather has been a hindrance, but I was finally able to drag this outside again and give it another try. Put a funnel on the fuel line to give it as much gas as it needs, but it did the same thing- started right up and idled and even idled with the choke off after a while, but died when any throttle was given. Sounds like fuel starvation, except that while it was idling, it was sucking down gas rather quickly, about a quart in 20 minutes of idling, which seems a bit rich. The mixture screw still has its plastic plug, so any ideas why this is running so rich? And when its stalling when revved, is it flooding? I think its inevitable that the carbs are coming off, again.