XJ 650 Carb problems?

Hi I am hoping someone will be good enough to give me some advice.

I have just purchased a 1980 XJ650.

The bike starts fine and runs on the choke to warm up as normal. The problem begins after about 5-10 minutes on the road. The bike starts holding up in the revs at about 2,500-3,000rpm as if the choke is still 1/2 on. If I am at lights with the clutch held in it will slowly come back to an idle.

To try and fix this I have checked float levels, sealed the manifold as best as possible. They have some signs of wear but no holes that can be found. I have even replaced the carbs and manifolds with a set from a wreckers that appeared to be in a better condition but I still have the same problem.

Have also given carbs a good clean. checked throttle cable is returning and am going nuts!!

Please help, any advice would be appreciated. Thanks. Fred.

 

I would check the routing of your throttle cable for starters. Secondly, have you checked for leaks at the manifolds with it running and a can of WD40? It is possible that the carbs could also use a very through cleaning and syncronization. Good luck and post more details as you remember/encounter them.

 

Okay will let you know how we go. Thanks for your suggestions. Have already checked that the throttle returns and sprayed manifolds with WD40 to check for leaks, but will try these again just to be sure. Thanks.

 

Check out some of the other thread topics on renewing the manifold units and how to clean out the carbs. There is plenty here to help out. Use the search function on the top of the page in the forums. I suppose you might just want to sit down and clean the carbs out next. By the by, what did you do with your old carbeurators? (Yes, I'm looking for a spare set).

 

don't forget about the butterfly shaft seals, replacing these solved my problem with high RPM out of nowhere stints!

 

This was my problem, try to sync it if possible, and if as you sync it the revs go higher uncontrollably to about 4K even with idle screw out, i would almost garuntee its your throttle shaft seals

 

Not to ask a dumb question, but what are the Butterfly shaft seals and where are they?

Maze

 

Not a dumb question at all.

The butterfly valves are on the engine-side of the carb throats. These "valves" are what are opened & closed when the throttle is turned. Each carb has a "throttle shaft" that the valve pivots on, and each shaft (on each carb) has a seal on either side of the carb body to prevent air from entering the mixture via the shaft opening, and thusly screwing up the mixtures (i.e. making it lean from too much air).

If the problem is not the seals themselves, the it could be that one or more of the butterfly valves are not fully closing - and thusly would also let too much air in, and cause a lean mixture - which would cause the engine idle to race. You can test this theory by pushing down on the synch screws with a screwdriver or your finger while at idle. If the idle speed suddenly drops to a normal idle speed (appx 1050 rpm) then it's likely that a butterfly valve needs some fine adjustment - so that they all close off at the same time (and completely close off), and open up at the same time.

Rex S.

 

Well said Rex, I think mine need a little attention if your advice is accurate. I'll poke at mine once she's back together. Maze, there is no such thing as a dumb question, only dumb answers. Feel free to throw questions out at any point. We love questions!