carb question... or is it?

hi, first off great site, tons of great info and members!

'81, basically stock with 4100 miles with pods (not the ones that block air holes)
newly rebuilt petcock and cleaned out tank
new fuel lines & intake boots & spark plugs
cleaned carb and replaced float needle and bowl gaskets
adjusted float levels to spec (27mm)
tested the diaphragms with my thumbs over the air holes- all good there
all other seals seem to be in good shape
mix screw 3 turns out

issue: from idle to about 4k rpms the bike runs, pulls and idles fine. after about 4.5k the engine stumbles like it's running lean and it's difficult to get it much past 6.5k. changed the main jet from stock 132.5 to 135 and now 137.5 and still getting the same results without any real noticeable difference. i'm currently waiting for 140 mains to show up and test but i'm starting to wonder if they will make a difference. unfortunately i don't have the stock airbox or filters to use as a control test (actually i have them but they were the originals and they were disintegrating). the plugs don't tell me much since i haven't put many miles on them yet.

i have checked and adjusted cam chain tension and valves. all good.

basically just throwing it out there for another set of eyes to look over and suggest something i might be missing. and yes, i've read, studied and tattooed the carb guide to my arms.

thanks!

This may be totally irrelevant but I once had the exact same problem with a Guzzi 850T and after cleaning and rejetting my carbs for a couple days I found my ignition advance to be stuck.

willyguzzi53 said:

This may be totally irrelevant but I once had the exact same problem with a Guzzi 850T and after cleaning and rejetting my carbs for a couple days I found my ignition advance to be stuck.

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that's a thought but i have the black box so i don't think my advance is adjustable...

Did ya sync the carbs ? Check 'em at idle and under way..

Do you have plastic or brass floats? That 27mm setting is for the brass ones. Plastic ones should be set at 22mm. I know the factory shop manual lists 27mm for '81, it's a mistake. The manual wasn't updated to reflect the change until like '82 or '83.

5twins said:

Do you have plastic or brass floats? That 27mm setting is for the brass ones. Plastic ones should be set at 22mm. I know the factory shop manual lists 27mm for '81, it's a mistake. The manual wasn't updated to reflect the change until like '82 or '83.

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well that's an interesting tid bit of information. seems that the ol' clymer book followed suit with passing on bad info. gonna go give this a try right now...

ok, adjusted the floats to 22mm. now it's stuttery from 2k on. idles and starts fine though. also, tried the petcock at on, res and prime positions with no effect.

i kept with the 137.5 mains with this run. i can try 135's or stock but i'm not thoroughly convinced it's a main jet issue. especially now that it's stumbling at just over 2k.

i gotta say though, i'm now a whiz at removing those carbs!

I would go back to the 135 main jet and see how it runs the fact that you lowered the float height and raised the fuel level in the bowel and now it stumbles at 2000 rpm you are probably too ritch with your jetting not lean.

I seem to be having the same problem. Mine peters out at 5,000 RPM. That's just barely 70 MPH. Does great out on the two lane, but hit the Freeway and traffic gets all screwed up. I'm pretty good on carbs. I've been thru everything I can think of including everything on this thread. I've come to the conclusion that it IS a fuel delivery problem, but before the carburetors. I got a Mikes manual petcock to replace the old leaky vacuum style petcock. Something I noticed leads me to think that the problem is located here. The pic-up tubes in this aftermarket seem to be a little too small to carry enough fuel out of the tank. Did you happen to do the same?

my issue ended up being more severe than i first realized. here is a link to the culprit -> http://www.xs650.com/forum/showthread.php?t=20439

your issue doesn't sound quite the same as mine. maybe carb issue?

ducktoo1 said:

I seem to be having the same problem. Mine peters out at 5,000 RPM. That's just barely 70 MPH. Does great out on the two lane, but hit the Freeway and traffic gets all screwed up. I'm pretty good on carbs. I've been thru everything I can think of including everything on this thread. I've come to the conclusion that it IS a fuel delivery problem, but before the carburetors. I got a Mikes manual petcock to replace the old leaky vacuum style petcock. Something I noticed leads me to think that the problem is located here. The pic-up tubes in this aftermarket seem to be a little too small to carry enough fuel out of the tank. Did you happen to do the same?

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Your problem is not with Mikesxs manual petcock. They will pass the required amount of fuel, unless you have rust from the tank partially blocking them.

I suggest you start a new thread. Include as much information as possible.............year of bike, year of engine, are the carbs stock, is the ignition stock, are the carbs clean inside, are floats set correctly, what size jets are in the carbs? Have you checked that the carb diaphrams are in good condition?

Yes, not being able to exceed a higher speed (like 60 or 70) is a classic sign of holed diaphragms.

When my bs38s were out of sync it would act similar to what you are describing...