what the @!#!

[boediddly]

okay , been trying to figure out this problem for 2 months now bike(79 xs special) has been breaking up at approximately 4-5 grand i say approximately because the tach is broken bike idles just fine ,and pulls hard up until that certain rpm and then just breaks up and stutters like hell ,it does the same at highway speeds around 65 or 70 then no-go i have been up and down and up and down on the jet needle clips and same on the main jets all the way up to 167 's i have tried it with the filters on and off and with the gas cap open to check for vent restriction it has pingel petcocks that pour fuel and ive tried to raise the floats up to 23 mm all that did was make them leak. Ive also checked to make sure the diaphrams are fully opening.Ive changed the vacuum advance and the pickup coils even though i dont think the originals were bad ive tested the advance with my timing light observed also with my eyes that it works well ,nice and smooth.I took a degree wheel and made sure it was at tdc and that the cams lined up(spot on)i have the rpm function on my multimeter so today I started it up on the centerstand and it revs at least up to 10 grand and also in gear but not under a load ive exhausted all the options I can think of .any ideas from you 11 junkies?

[Ken Talbot]

Plugged exhaust?

[XS1100 Newbie]

Coils breaking down? My MNS done that as well, and it was the float level, but it sounds like you got that. Did you wet check the carbs after setting the floats, and did you synch itafter you were done?

[WMarshy]

Sounds like its time (lol) to start checking your timing...tried that yet?

[slow mo]

cracked pick up coil wire? ride safe ..slow mo!

[prometheus578]

A few more things:

Just to eliminate and narrow things down...
What do the plugs look like?
What's the cylinder compression readings?
What are the valve clearances?

[boediddly]

compression is 170 across the board pickup coil wires are good and timing has been checked and checked just went out and checked the exhaust- seems to be coming out real strong

[Geezer]

Fuel starvation? Check to see that it can flow gas well enough to feed it under that much load...

Geezer

[BigRed]

170 on a compression test is above spec. You may be getting detonation due to carbon build up in the combustion chambers. I can't remember what the spec is, but it's in the 150's. Do you run premium? It may help. If it doesn't, you might need to pull your head and clean it.

[esw1104]

Opps re read he did check them

[81xsproject]

cracked pick up coil wire? ride safe ..slow mo!

Yep. Even if this isn't the problem your actually suffering from, you should just do it. The stock pickup wires are like ticking time bombs. Sooner or later (most likely sooner) they are going to start breaking down. If you do it now and get it out of the way, it will make troubleshooting now, and down the road much simpler.

[DiverRay]

If it WILL run ok for the first 1/4 mile or so, and then no longer rev up, check the fuel lines. I didn't watch the lines on my Special, and had a crimp on both sides. bike ran well at low speeds, but would not climb a hill, or get above 70. Moved the lines so they were open, and it started to run correctly once again. Also, what are you using for filters? if you have pods, and they are the "cheap" ones, there is a lip that blocks the air jet, and can cause the same problem.

[dbeardslee]

The only time I've seen them hit a wall in the 4-5k range was from an over rich condition. I just went around and around with a set of '79SF carbs, and among other things the old brass floats were leaking causing it to load up. Might check yours while you're at it.