Low end stumble

[planedick]

I have been dealing with this low end stumble for a long time. Top end and WOT it runs like an XS11 ought to, scarey at least. Out of the hole at a signal runs like s**t until I get 3500 on it then it really gets with the program. I have had the best luck with stock jets and it is a little better with the needle clips on the top leaving the needle in furthest, but still stumbles at low RPM. Any ideas on improving the bottom end?

[cywelchjr]

I have been dealing with this low end stumble for a long time. Top end and WOT it runs like an XS11 ought to, scarey at least. Out of the hole at a signal runs like s**t until I get 3500 on it then it really gets with the program. I have had the best luck with stock jets and it is a little better with the needle clips on the top leaving the needle in furthest, but still stumbles at low RPM. Any ideas on improving the bottom end?

Well, sounds like my 400 before I replaced the o-rings under the mixture screw springs. Have you replaced those or are they original/almost gone?

[planedick]

Oh

I never changed those "O" rings or even inspected them. I guess I should do that now. Forgot about them even.

[planedick]

No "O" rings

There are no "O" rings there nor is there a place for them. I think you must be talking about another type of carb as there is no reference to them in the manual either.

[CatatonicBug]

I think the o-rings are for the '80+ carbs. The '78 has a different mixture screw.

Have you inspected the pilot jets? Until 2500-3000RPM, the pilot jets are what feed the carbs. If they are clogged, even a little, they will affect performance.

[LuckyEddie]

Low End Stumble

Here is something I posted some time back.........Hope it helps.

Solved!!!!!
________________________________________
Greetings,

This past Saturday I finaly found my problem. The bracket on the end of the #3 carb butterfly shaft that hooks to #4 was loose on the shaft. I pulled it out peened it over to tighten it up and every thing is golden.
The carbs would not remain in synch because every time I would move the throttle 3&4 would open just a hair before 1&2. It would run, but it ran like crap.
Yeah, I know it took me long enough to nail it down. The riding season is now pretty much shot. Now I look very much forward to spring!!!
Thanks for all your help. Add this to the knowlage base.
__________________
LuckyEddie
1980 XS1100LG Midnight special

[dbeardslee]

I had a low rpm stumble last year that looked for the world like a carb problem. As the motor warmed up, the problem got worse. After pulling the carbs 15 times, I decided to look elsewhere for the gremlin. Wound up being the ignition coils. Diagnosing it was a real booger, as the resistance tests on the coils looked good. I finally got to the point where about the only thing left was the coils - after replacing carb boots, and every electrical connector on the bike. So I took a chance and replaced the coils. No more stumble. Sometimes ignition problems can do a pretty passable impersonation of a carb problem. JAT

[Larrym]

Square One

I have been dealing with this low end stumble for a long time. Top end and WOT it runs like an XS11 ought to, scarey at least. Out of the hole at a signal runs like s**t until I get 3500 on it then it really gets with the program. I have had the best luck with stock jets and it is a little better with the needle clips on the top leaving the needle in furthest, but still stumbles at low RPM. Any ideas on improving the bottom end?

I'd say that in the "hunt" to find what's causing the stumble you'll likely have had those carbs off/on a few times.

If at any point you get lost, frustrated, or just need to re-establish a reference point, the best choice is to put those carbs on the bench and walk away. Put them aside and see what happens when you put on a set of carbs with original jets, original settings, etc. (Known good performers...)

I have just a set. If I recall correctly, you were instrumental in making this set possible. Was way back when but you did graciously sell me a much needed carb bank that is now a dedicated "test" bank. See this post:

http://www.xs11.com/forum/showpost.p...7&postcount=25

[Cobia]

Stumble

Planedick,
I experienced something similar with my J. BikerPhil and I tried everything under the sun, multiple carb cleanings, multiple float adjustments, multiple idle jet adjustments, etc. to no avail. My stumble happened between 1.6k and 2.5k rpms. I finally decided to try larger pilot jets and, voila!! Problem went away and it pulled strong through the entire power band.
Good luck.

[planedick]

Bigger idle jets

That's the only thing I haven't tried. Reading the plugs at idle shows white, but chopped throttle under load test showed orange. I tend to think that Cobia is right and it's lean at idle because it also pops and backfires on decel especially in the higher RPM range like when using 2nd to slow down. I have checked the throttle plate movement one against another and there doesn't appear to be any play anywhere and all throttle plates move together so I don't know why the balance goes all off when the throttle is cracked. I have also carefully checked the amount of gas in the bowls when I drained them and found that #1 was considerably more than the other three and was causing #1 to show rich at idle and I adjusted #1 float lower and things are better, but low end is now OK after 2500. I don't have any jets other than stock so I am looking for them now.

On another post I noticed some one here has the Harley stock pipes like mine and was having a somewhat similar problem, but I have lost the thread and can't find it again. I was gonna watch it and see what he found.

[buggrin]

I've got the same situation on my 78. Replaced the pilot jets, cleaned carbs, and the stumble got a teeny bit better and decel backfire went away. Replaced plug caps and the plug color is good, but the stumble is still there. My carb guy thinks that it may be turbulence in the carb caused by worn surfaces. I'll run it a couple months to see if deposits fill in the pits and smooth out the low end.

If you pull the choke one notch does the stumble go away? Mine does. The suggestion of bigger pilot jets sounds like a good fix except the stumble started all of a sudden after a hard run. Seems like something (coil wire) broke

If you find the problem, please let me know. I'm so frustrated those FJRs are looking mighty good---and that ain't right after 32 years................

[Toolmaker Tim]

I have the same problem with my low end. Right now my pilot jets are #45's. Stock is #42.5 if my memory is right. The reason I went to the #45's was my middle pipes would glow when I choked it when I cold started it. The worst part of my stumble happens at 3000-3500 when I am running around on side streets. I have been told this is the transition zone from the pilot circuit to the main jets. I am beginning to wonder if I should drop one size in my pilot jets.

[3Phase]

I have checked the throttle plate movement one against another and there doesn't appear to be any play anywhere and all throttle plates move together so I don't know why the balance goes all off when the throttle is cracked.

Do you have another set of carburetors? I can bring you a spare set or just pull the one's off my bike so you can test.

If there's no play anywhere and the carbs won't stay in synch you may have bad throttle shaft seals but that's a complete pain to check. Once you've torn down everything down that far you might as well replace the stupid seals even if they look good.

[planedick]

Spare set

I've thought about a spare set test and Larrym has offered to send me a set to test, but it's not something that came on suddenly and probably becuase of the Harley mufflers. I went on a ride to the mountains and it really ran like s**t so I decided to get rid of the problem. I had ignored it for a long time as down here on the "flatlands" it wasn't much of a problem. I must admit that it wasn't as bad as it is now and in the process of trying to fix it I made it worse. These carbs are supposed to compensate for atmospheric pressure changes, but only when the diaphram is activated which is above 3500 RPM. The low end is richer at higher elevations.

I had spare sets of carbs myself and sold them because I figured I'd never really need them. After I sold them I discovered I did so way cheaper than I should have.

[planedick]

Idle jets

Does anyone know where i can get a set of #45 idle jets?