XJ550H Sudden High Idle Problem

Hey all,

It's been a hot minute since I've been on the forums, I've put roughly 2000km on my bike since my last posting:

https://xjbikes.com/forums/threads/xj550h-turn-signal-woes.129531/

The bike's been running strong since, but I've recently run into a problem that has me stumped. It's taken to revving to 4k rpm whenever there's no load on the engine (once hot), with no changes made from when it ran like a new bike. It was vaccum synched at the start of the season, tuned, plug readings were a nice tan colour, and float levels were within spec. All diaphragms passed the clunk test, and no air leaks have been found in the system.

When I rebuilt the carbs the second time (I didn't break the rack the first time), I changed the throttle shaft seals, fuel rail o-rings, and fitted an inline fuel filter. All rubber components except the diaphragms have been replaced. When I checked the plugs yesterday, they were on the rich side, so I reset the idle mix screws to 2.5 turns out and went for a ride today. Tomorrow when the engine is cool I am planning on checking the plugs again.

Anyone have any shot in the dark ideas on what to check next? It's rideable as is right now, it's just frustrating having to modulate the clutch so much and I don't wanna burn out the clutch plates. I have some theories (stuck float/float which drifted out of adjustment, stuck diaphragm, air leak I didn't spot), but any input is welcome.

 

What happened after you adjusted the idle mixture screws? Did the problem start after you fitted the throttle shaft seals etc. What kind of inline fuel filter do you have? The small lawnmower ones are useless.

 

What Franz said. Usually, high revs when warm are, in my experienced, caused by either carbs out of sync or false air/lean mixture.

 

Yes if the fuel filter is not delivering enough fuel it will be lean. The fuel filter is the cause I think.

 

I checked the plugs yesterday, they were all on the rich side with #2 being extremely sooty. Tailpipe is also fairly sooty. When I took it for a ride after adjusting the mix screws it ran slightly better but still had a consistently high idle.

As for the carbs being synched, that was my first thought. Resynched them last week and they were very close to what the end result was at my start of season sync.

The problem started after I rode through fairly heavy rain (like, couldn't see the lines on the road, everyone has their hazard lights on level heavy rain) but it ran fine with the new throttle shaft seals and all the new parts after the rebuild I did last fall. The filter is one I got from chacal, and I've had it on for the length of time I've had the bike running with no issues until now. I do have a second filter, I could replace it and see if it helps?

 

Unlikely to be the filter but easy to change to check before further work.

 

Took a look at the plugs today, and took the bike for a ride after making a discovery and adjustment as a potential band aid fix. Plug #2 was definitely the rich one out of the set, and out of a sneaking suspicion I had, I checked my fuel levels in the float bowls.

#1: 4mm below carb body lip
#2:0mm
#3:3mm
#4:2mm

When I initially rebuilt my carbs (2 years ago), I set my float levels so they were all at 2mm (I allowed .5mm tolerance). Do float levels ever change, or was I just a bad mechanic previously, or...? I'm glad I at least now know the likely culprit of my issue, but I'm still annoyed by it nonetheless.

 

While float levels can change slightly over time, it does not seem likely they will change 2 mm in 2000 miles. But if you have/had air bubbles in the hoses, readings can be off. Water in the fuel, readings can be off. If the carbs tilted differently, same thing. It is not easy - for me, at least - to get an absolute correct reading time after time. Still, 2 mm is quite a bit.

It would, however, surprise me of the differences in float level would cause an idle speed of 4000 rpm, but I hope for you that is the cause - would be nice if setting the levels right will give you back a trouble free ride.

If the float levels should prove not to be the cure we hope for - are you absolutely sure that the throttles are closing fully when you let go of the gas? It takes a minute amount of extra throttle to bring the revs to 4000 rpm in neutral. Water could have made its way into the throttle cables and cause just a little bit extra friction.

Also, what happens if you reset the idle speed to 1100 rpm or so when the engine is hot? Usually, I would have expected this to be an issue to look into if a bike acted like this after a service, when someone set the idle speed cold, as that would definitely cause a high idle after the engine warms. However, this does not appear to be the case here. Still, knowing how the engine runs when you drop the idle speed to the correct level when hot can give us good clues on what to test next.

Other potential issues would be to check for water in the fuel, since you said it appeared after riding in the rain. Fuel can get past the lid. Again, I would rather expect poor running than high idle, but could be a final thing to check if all else fail.

 

Make sure the springs on the throttle shaft are pulling correctly. When the engine is racing try pushing down on one of the adjustment screws.

 

I would reset the floats. When you do it be sure to let them sit for a while and recheck them. Might even bump them a few times. I found that the initial reading often changed and it took several efforts to get them to stay steady.

While you have the carbs off I also check the throttle valves (butterfly valves) and make sure that they are installed correctly and close smoothly. Just a little bit off would cause a problem.

Since you are running rich and do not have a colortune plug try to set the pilot screws by starting low--around 1 1/2 open, with new plugs (easier to read) Ride about 10-15 miles and look at the plugs--you are looking for a light brown or gray color. I still white, open the screw about the width of a dime and ride and read again. Continue this until you get the light brown or gray color on the plugs. Each carb will probably be different and the screws may range from well below 2 1/2 to almost 3. Mine are all below 2 1/2.

 

Hey all, got an update for ya!!

I double checked the fuel levels on the bike, sure enough carb #2 was level with the lip of the main body, if not a little higher.... Which had me worried. Combined with the rich state of plug #2, I figured I'd take the carbs off and reset the float levels. Funny that it started happening now, but whatever I'll fix it.

I put them on my patented carb stand (a bench vice with towels wrapped around it) to recheck all 4 float levels before jumping in to changing anything..... And all float levels were exactly within spec. Huh. Okay....

So I double checked that, put then back on the bike (on it's centerstand) and checked then all again. Same issue, #1 was low, #2 super high, 3 and 4 are reasonable. Well, what the heck.

I took them off the bike again, checked them on the anvil, same result as the first time. I bent the tangs so they would fix the problem seen on the bike, took it for a good 45 minute ride and ZERO trouble. Ran like a top and no high idle issue. Had more power down low, too. It was an easy fix for my high idle problem, but now I'm curious as to what could be causing that difference in float level from bench to bike.

 

I've never checked mine on the bike, always on a stand where I can be absolutely sure they are level in all directions. But from what you say, I'd say that your "patented carb stand" needs some work. Glad you got it fixed.

 

Once in the vise make sure you are level left to right and front to back (with a level).