Carbs again

[Hobbit]

I installed the main and pilot jets that are recommended for my xs11 Special with pod filters and 4 into 1 exhaust.
The bike starts and idles real nice after synchronizing the carbs. BUT at 4000 rpm it starts to break up something terrible. At full throttle it comes alive again.
The floats are set to factory specs. The idle mixture screws are out 1 turn.
I disconnected the vacuum advance to prove that the pick up wires are not broken. I also applied vacuum to the advance at idle and this did not cause the engine to miss, so I'm sure that the problem is not ignition.
The spark plug on cylinder 1 is a lot whiter than the other cylinders (new plugs). I cleaned the passages in carb 1 with carb cleaner and compressed air with no change. Any ideas?
Suches is coming up this week and I've got to get it going soon.

[fxrer]

"at 4000 rpm it starts to break up"

Walt,

To me, the term “break up” implies an electrical misfiring (even though you’ve eliminated that as a cause of your problem). Hesitation, or stumble, sounds more accurate to describe a problem with loss of power, especially at given rpm range, caused by fuel/carbs problems. (I’m not trying to nitpick here)

That said, there’s several recent threads that discuss midrange (4000 rpm) stumble, hesitation, or whatever you call it, most notably the one started by “Corey”, on 10-03-05. Concurring with you, the consensus in that thread is that this stumble is carburetor related, not electrical.

I tend to agree because by design, the 4k rpm range is where the main jet/nozzle/needle take over fuel
regulation from the idle circuit. Along with others, I'd focus on these as a cause, as well as other related fuel flow items like slide vacuum (diaphragms), intake filters, etc.

You said you have "recommended" jets for your '80 SG. What size? My SG has 115's, and I have no stumble at all (with stock air filter).

With so many recurring carb problems reported in this forum, it would almost make sense for us to start a “collection fund”, and hire a Yamaha carburetor expert to troubleshoot and explain problems, and solutions, to everyone!?

[GNEPIG]

.. take the "new pod filters" off then try it, it's an easy check ..chances are you got the ones that cover up the air inlet holes in the ventury
.. took me two days to figure that one out on my bike..
if thats not the problem then we'll think up something else

[Hobbit]

carbs

First fxrer, your right, I'm not probably not describing the problem correctly. It feels as if one cylinder is not firing, that's why I went after the ignition wires. But I feel pretty sure that they are okay.

Gnepig, I tried the pods first, all of this is occuring without the air filters installed. Also, the bike has been running for a while with these filters.

One thing that I did find out, I had installed small fuel filters in the bowl vents just to keep the dirt out. When I removed the carbs, the filters got wet from the fuel in the floats bowls and became VERY restrictive. Removing the wet vent filters helped with some of my problems.

[Pat Kelly]

Needle setting affects mid-range I believe.

[Ken Talbot]

What brand of jets did you install?

[Hobbit]

Carbs

Pat, I have tried several needle positions with little or no change.
Ken, I took out the jets that came with the K&L kit and replaced them with jets from motorcyclecarbs.com. I went up two sizes over the originals.

[Ken Talbot]

The breaking-up that happens around 4000 rpm - does it happen all the time at the same engine speed, no matter what gear, etc?

[Hobbit]

carb

Yes, but only at light throttle. When I open the throttle a lot, it will start to come alive.

[DiverRay]

double check the float level. You may be getting too much fuel, and it's running rich just at the transition point. You can try setting the floats 1 mm lower, make sure they are ALL at the same setting, and try again.
Ray

[Hobbit]

the answer

I found the problem(s). The previous owner had apperently changed the floats to a different style. When I set the floats to the correct measurment the fuel level was way to high. I set the level using the manometer method.

Also the needles were aftermarket and when I found a set of Mikuni originals (came with my spare engine) I could actually see the difference in the taper of the needles. The xs now runs better than ever, and I think that with some fine tuning I can still make some improvements.

Thanks for all of your replies, sometimes I just need a sounding board to help organize my thoughts.