Little stumble at low rpm.

I'm having this same problem on my XJ750. Did anyone find a solution to help point me in the right direction.

 

Is your bike also stock, including the air filter? When I mentioned that I had to "substantially' change the pilot screws, I meant that I have to close them a good bit after installing the stock filter. As k-moe said, the Uni foam filter allows a lot more air and therefore requires more fuel from the pilot screws. At least that was the story with my bike.

 

@JCH & Mukhtar,
If you've checked all things suggested by members in this thread the only thing I can think of right now is the float needles. If the float needles are aftermarket with a open clip they tend to spin around and get stuck on the front of the float tang which will cause this issue. Chacal has a write up in the carburetor section of "The Information Overload" about the orientation of the clip on aftermarket needles. I think he also still has the stock brass needles for sale. I hope this helps.

Gary H.

 

I'm in the process of going after that little issue currently and it will be a couple of months before the bike is running,i'm doing my winter tear down now because the stumble was driving me crazy,waiting on some parts that i'm getting refinished.My float levels were off and suspecting that was the cause ? and one of them was leaking intermittently.

 

I have noticed on my bike that even with a full tank I get some stumble with the fuel petcock on Run, but not as much on Reserve. Possibly there is a vacuum issue there contributing to the issue. Just another possibility.

 

I didn't have any success using the color tune ? i couldn't tell blue from white with that tool,i can't figure out what i was doing wrong if anything ?

Eric Did you have the stubble before the tear down, i'm suspecting that you did?

 

The bike is all stock including the air filter. I did test the vacuum in the tank. Only by opening the gas tank to relieve pressure when it was stumbling. That didn't help. I will look into the floats and heights. I also saw online that if the exhaust valve isn't set correct it can cause this issue. But that wasn't a model specific problem just older bikes.

 

Yeah I've had that since getting the bike a month or so ago and haven't done much tear down of it yet. Getting a handle on what might be causing what before I do much and that was one thing I noticed early. It starts nicely and I can feather the choke back to have it idling good and after about 10 minutes of warm up it will idle smooth no choke, after warming further or a short ride will start to stumble again. Cylinder 4 isn't quite right so I think it just takes forever to really get warm so it has been tuned to half warm previously. I'll be starting my own thread for this bike soon. Petcock rebuild kit and head bearings are on the way currently.

 

Hello Eric,so is this stumble like the one on my bike,if i rev the throttle at idle it go's away when i take off or is it after you take off from a stop,you can measure your float bowl volume with a small cup and see how much fuel is in each bowl or even better yet you can get some clear plastic 1/4 in line and measure them on the bike,watch the video on this site in the carb section and will tell you how you do it,but you can do it with the bike on the center stand too.

 

It runs fine on throttle, sitting at stop signs, etc.. I have to keep blipping the throttle to keep it alive or have the choke slightly on. I haven't measured the volume in the bowls yet. I have a plan to clean the carbs Just haven't yet. In your previous post it sounds that yours hesitates when you get on the throttle, maybe that it is too rich?

 

Maybe, i think it was lean but not sure,my floats were off and had one leaking at times,i won't have the bike back together for a couple of months or hopefully sooner for that i'm waiting for some other parts.
Your condition sounds lean if the choke seams to help the symptom, have you check for vacuum leaks with a propane canister with a hose attached.

 

Oh i'm sorry you already did the carbs and wet sync,did you do the electrical stuff that was mention ? another test is to spray some water on the plug wires,coils and check for a spark leakage,ohm test the coils too.

 

I do think mine is too lean. Did the vacuum test with carb cleaner and it seemed good but I’ll get a better look when the carbs come off. Plan on testing the coils, plug caps, valve clearances and determining if the carbs need cleaning or rebuild. That is all part of the plan when the tank comes off to rebuild the peacock. Will probably do some tank work too, as it has some paint bubbling near the seat and on the underside. That’s the plan anyway. Always tentative and subject to change.

 

Thanks for everyone's insight ill do some checking and get back.