Little stumble at low rpm.

OK, I've been to the Church of Clean, took great care to rebuild the carb rack, adjusted the valves, new plugs, set the float levels (with great care), synched the carbs, and done pretty much everything I can think of to do everything right. I'm very pleased with how all of that stuff turned out.

On to my issue, after all of this work, and after the bike warms up, whenever the engine is at lower rpm, sometimes I get a little "stumble" from it when accelerating from a low RPM.

I kind of get the feeling that the problem is in the transition circuit of the carburetor or something like that.

I don't have a colortune, so I've been trying to get the pilot circuit dialed in the best that I can.

I don't think I've missed anything in all of this wealth of information, but for the life of me, I just can't seem to get it "perfect".

Am I going to have to get a colortune plug to get it right?
BTW, when it comes to my vehicles, I'm sort of OCD about it. I can't stand rattles or anything that doesn't work as it came from the factory. In other words, it has to be damn near perfect, or it bugs me to death.

I mean other than the before mentioned problem, it runs great, and it's not even a big "stumble", it's very small.
Am I just expecting too much from a 34 year old machine?

 

what is your idle rpm? it is listed on the left side cover
have you synced the carbs?
wet set floats?
2-5/8 turns on mix screws is working for me

are you running pods?
if you are not using the stock air box that is what happens with the carbs when you get every thing tuned they have a flat spot then you rejet to put the flat spot where you can live with it

 

Tried adjusting the throttle cable slack?

Gary H.

 

No you aren't. Check into the idle mixture screw adjustment as mentioned above.
When you cleaned the carbs did you also add an inline fuel filter?

 

Wow, guess I left a few questions unanswered.
OK, the floats where wet set, I spent a lot of time trying to get them just perfect, I feel like I'm good there.
Everything is stock, and I did add an inline fuel filter. I got it from chacal, it is the good one with the brass or bronze or whatever that stuff is.
I did synch the carbs with a set of vacuum gauges, and again, I took my time and feel like it's as close to perfect as I could ever get.
I haven't done anything with the throttle cable other than hook it and unhook it when I take the carb rack out.
Oh, and the valves have been adjusted and within spec.

I think that it's in the idle mixture adjustment, I started at 2 1/2 turns out from seated and it ran OK, I turned in about a 1/4 turn and it seemed to get a little better. I might try another 1/8 turn and see if it improves. If not, I'll go the other way a little at a time.

 

+1. Does it stumble when you twist the grip at idle?

Gary H.

 

What have you done on the electrical side of the combustion trifecta?

 

Not all the time. But sometimes, yes.


Idle RPM is like 1200...Hard to tell exactly with the factory tach, but it's less than 1500.

Electrical is all stock. Everything is stock on this bike.

 

Did you do any testing to confirm that the ignition system is in good working order. Plug cap resistance? Coil resistances? Plug wires not arcing to ground? Plug cap to plug wire connections not corroded? Spark plugs gapped correctly? All of these thing can cause a stumble. It's easier to check the electrics then it is to revisit the carbs, so I'd sart there to rule out an electrical misfire.

 

I'll check into that stuff. That's a good area to start from. I'm sending the tank and side panels out for paint sometime soon and that would be a good time to check it.

 

Thench, pardon the redundancy, you have answered questions rather direct except for one rather important one. You have mentioned twice that 'everything is stock'.

For a complete understanding of your situation and to add a punctuation mark to how important one piece of this puzzle is . . . .

Does your bike have the factory original airbox and air filter on it?

 

Good point on both factory air box AND air filter. Mine came with the red foam UNI filter in there... A stock paper filter is on the "soon to be purchased"list.

 

the uni filter is treated with oil and needs to be cleaned and re-oiled , supposed to be equal to the stock filter. does it run better with out the filter

 

Yes, factory airbox and filter.

 

Uni actually tend to run a little lean, but not as lean as the K&N.

 

HURRAY!!!! Keep it that way. Makes things much easier.

 

I plan to keep it that way, I don't need to be adding any more gremlins to chase on a 34 year old bike.

 

My XJ700 was bone stock except for the Uni-filter. Changing to the OEM paper filter made a BIG difference and I had to adjust the pilot screws substantially to get it correctly tuned. A colortune plug will make it much easier to get the proper pilot settings. You can see exactly where the flame changes from white to blue and then install new plugs and ride, check, and adjust until the plugs look good.

 

I'm curious... more or less than 2.5 rotations (on the pilot screws) from lightly seated once you put the stock filter in?

 

Hello Gary,yes my bike doe's exactly that every time, unless i rev the motor a few times before i take off,i'm thinking a pilot jet issue,every thing else has been done.

 

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.