3500 RPM Idle

Hi, all,
Cleaned my XJ750 carbs today to correct a small problem, now have a bigger problem. Bike started but climbed to about 3500 rpm and stayed there. Have confirmed that the throttle closes fully, as do the enrichment plungers (not touched, by the way) and that the idle adjustment is backed way off. Boots are in good shape and can detect no obvious air leaks, but it has to be sucking a ton of air to idle that fast. Any carb gurus able to tell me the most likely culprit? Expecting to have to remove them again tomorrow - which is my second favorite things. My first favorite thing is going down to the junkyard barefoot in January and kicking bricks.

 

Did you maybe hook the choke cable up to the throttle arm?

 

Thanks but no. Visually confirmed that and even disconnected the throttle cable at the carbs to be sure the throttle was closing fully and that the cable wasn't binding. No, it is something internal.

 

you put the vacuum caps on, right. try to do a running sync, there's nothing inside them that does that sort of thing unless you had left over parts

 

Yes, caps are on, hose to petcock good and attached, manifolds good. beginning to think I have a slide hung up somehow. Carbs have been carefully synched and never disconnected from the rack since then.

 

Vacuum slides pass the clunk test?

 

They seemed to but I am wondering if the were good enough. What concerns me is, even if one of the slides is sticking, the butterfly is still closed, making me wonder how that much air could be getting through.

 

could be shaft seals, could be butterflys not seated correctly. If you soaked the carbs in carb cleaner, you may have ruined all the seals and O-rings. Do a propane test around the front of the carbs. If rpm's increase, probably shaft seals, if not, probably butterflies are not seated correctly.

 

Vacuum leak somewhere...... Boots, gaskets, vacuum caps that are on but are brittle and loose enough to let air through, enricher linkage sticking open a bit, or a linkage issue on the rack holding things open and you just can't see it---- hafta start narrowing the field. The propane test is a good way to begin

 

After following the advice given pull the vac hose off the petcock and check for cracks. If fuel is getting past the diaphram and being sucked into the carb it'll raise the idle and the vac hose will harden and crack.

Gary H.

 

My KZ750 did this too after rebuilding the carbs. After pulling the carbs three times, I found that the slides were sticking. What would happen is it would start and idle fine, but once I revved it the slides would stick up and cause the RPM's to race and hang. If I backed the idle knob out, it would die. Turn it back in and start it would race again. Turns out my slides had some varnish on them that the cleaner started eating away at. I carefully wet sanded and polished the slides and has been good to go ever since.

I attached some pics, when they were like that they did not even come close to passing the clunk test. Remember these also block airflow, once you have air rushing in from the airbox they will lift and if they can't drop back down the motor will run at that higher RPM. The other symptom I had was inability to vac sync. I would get them set and a quick hit of the throttle would throw it out of whack. Mixture screws on some cylinders didn't make a difference too (as expected).

Don't get any carb cleaner on the diaphragms if you pull to clean!

 

You could also check that the pilot screws are at the correct setting.
Too rich can cause the symptoms described. I don't know the settings offhand but start with say 2 turns out from the base.

 

2.5 turns out from soft-seat.

If you're talking carb dip, yeah don't get the diaphragms in that. If your talking the carb spray can, I spray into a cloth and the wipe the diaphragms down with that. It cleans them quick and dries before it has time to even think about degrading anything.

 

A lot of very good recommendations. No dipping. Leaving the carbs on the rack.

One comment has me a bit concerned - regarding pilot screw setting. I had set mine with a Colortune plug and they are all set a 5 1/4 turns out and the bike was always a real runner but that does seem like a lot. I need to check the plugs for carbon buildup.

I have made some progress after the 5th or 6th cleaning. All four pipes are getting hot but the running is rough whereas it was always very smooth. Accelerates well but has a miss at light throttle. This all started from letting the bike set unused for over a year. prior to that, carbs had been synched, valves adjusted and ran like new. I truly believe there is some port, jet, orifice, etc. that I am overlooking or not getting clean enough. I was reading through the xj carb cleaning guide and may have a clue. In the bottom edge of the float bowl there is a very tiny jet. These were plugged and have been cleaned. But, the guide indicates that carb cleaner forced thru this jet should shoot a thin stream out of the upper edge of the bowl, right where that thin brass tube protrudes from the carb body. I got a little fluid out of the port, but a LONG way from a stream. However, he says this would cause hard or no starting, where mine starts pretty easily. Could it have other effects?

Also, you can hear those slides "clunk" across the room. Feel those are good.

At this point I may want to change spark plugs. Don't think those go bad setting but you never know. Will do one more leak check with starting fluid before the next cleaning effort. All boots, manifolds, air box rubbers, caps good and solid and still supple. Really getting sick of taking these carbs off and on.

There must be an engineer is this group that could devise fuel injection for these bikes!

Thanks all. I have just begun to fight.

 

There are several who are working on/ or have already done EFI conversions with Microsquirt and/ or other setups

 

As mentioned by hogfiddles and myself, start with the pilot screws themselves. That's your problem at 5
¼ turns out.

Personally I have never used the Colortune gizmo and never will.

 

Idle screws should be around 2.5 turns---- you're waaaay to far out.

I've read back through, and and now wonder ( didn't see it mentioned, if I missed it I apologize)..... Are you sure you didn't inadvertently mix up some of the air jets--- there are some that will fit in multiple locations---

 

I agree with Hogfiddles. Air jets would do it, and the Haynes manual has them mislabeled.

 

I had one carb that required 5 1/2 out using colortune and it turned out to be a badly torn intake manifold gasket for that carb. Installed a new gasket and that took care of the problem.

 

Either some of the other issues outlined above, and/or the pilot circuit is clogged (partially or almost fully).....typical condition, especially if the starter jets in the bowl were also plugged.

Reversing the main and fuel AIR jets will typically result in black, sooty plugs almost immediately, like within about 5 minutes of operation.

 

Was very careful with the jets and am aware of the issue with the manual, and will try adjusting the mixture screws. That would be wonderful since I wouldn't have to pull the carbs again.