Light sputtering at 5000 to 6000 rpm

I need help with getting my carbs set just right. I have a 1981 seca 750 (Canadian model). Everything is stock except for the jets and mufflers. I have 122 Dynojet main jets with new washers. That helped with the power at 6500 rpm to redline a lot. I still have the light hesitation/sputter at 5000 to 6000 rpm though. It's really annoying! I tried shimming the needles to see what it would to. It made the problem a lot worse, so I took the shims out again. The needles are bone stock and everything in the carbs working like they should. I'm wondering if it's the float levels. I talked to the PO and he said I should look at the floats as well.
Just FYI. I was having a problem with the petcock leaking fuel into the carbs (and engine) if left for a few days (yes i changed the oil after). This is now fixed, but I was thinking the float needles should hold the fuel back in such a case?
Specs: 1981 seca xj750 (Canadian), pilot jets stock 43, main jets 122 (dyno jet), float level is not known yet, New OEM filter in stock air box.
Please give me your thoughts on this!

 

Too rich, move the needle down, if you can't go lower try a smaller main.

 

You are correct in thinking that the float needles should hold fuel back regardless of petcock status.

Float levels are critical to how the bike runs; so if you know they need attending to start there.

 

OK, thanks Fitz and Streetbrawler. I will start there. If that does not do it. I will try moving the needle down if possible. I suspect the float tangs need to be adjusted properly. The carbs are super clean inside and out. Looks like I'm off to Home Depot to get some PVC hose for float adjustment.
I hope it's off, because that would not be too hard to fix. I will keep you posted on the outcome

 

Sure enough number 1 float bowl is a little high, and number 4 just fills the engine with gas when on prime! I tried cleaning the needle and seat (already looked very clean!), and I even tried adjusting the tang. It still floods like an old basement! I'm just scratching my head at this point.

 

Suspect the seal between the float valve seat and the carb body. If that has failed, fuel will flow right around the needle/valve assembly.

 

what about the float hanging too low in a empty bowl and getting stuck

 

To BigFitz posts: That was something I thought of too. It was not it though. I tried holding the float up with my finger for a long time. I didn't see any leak. I replaced the seat, needle, and float with various ones from the other Seca 750 that i have for parts. Same results. It looks like I may have to get professional help on this one.

 

You can use a pyrex measuring cup in place of a float bowl so you can see what's happening as fuel enters. The float could be hanging up when it's free to float, but not enough to notice when you're moving the float by hand.

Dumb question: the floats aren't in upside down are they? It's happened before, and is pretty easy to flip them.

 

http://xjbikes.com/Forums/viewtopic/p=4 ... tml#416841

 

The floats are right side up. Good call though. It's easy to mix it up. I will use the Pyrex glass trick. That sounds like a great idea!

 

I used a clear glass to see what was going on. the needle was still letting out a very small amount of gas into the bowl when fully closed. I remembered that the PO gave me a bunch of extra stuff including a rebuild kit for the engine with matched pistons, rings, gaskets, plus a few other things. Sure enough there was a NOS needle and seat with all the stuff. I installed it and it seems to be holding just fine.
I will keep you guys posted on how it is over the next few days. I hope this gets rid of the sputter. At least the carbs don't flood now. Thanks again for the help!

 

What about the other three?

 

The other 3 work perfect and are clean. The fourth one must have had a flawed seat or needle. I used the NOS unit I found and the carb has been great so far. The petcock rebuild kit came in today. I will install the original petcock in a while too.

 

The bike is running great now! All float levels hold and are in spec. It goes like a stabbed rat, especially past 6000 rpm.

 

Update: I installed 124 main jets, and so far it's a bit better. I'm taking the bike in on Monday to get the valves adjusted and the carbs synchronized. I'm hoping it does not rain. I will let you guys know how it goes.

 

Dont take it somehwhere!
Thats like a $4-500 job!
Depending on what shims you need its very cheap to do yourself!!

 

Egads! Is that what the normal charge is at most places? I'm taking it to a Yamaha mechanic that has been working on bikes for about 40 years. She is not only the best around these parts, but also the least expensive! It's about $60-80 to get the valve clearances done, and $20 to sync the carbs. That's in Canadian, so about $90 usd total. Not bad eh?

 

So i had the valves re-shimmed and the carbs ballanced. It runs better just cruising around and has better manners in town. cold starts are nice and easy every time now too. The mechanic said the pilot mixture is not quite right. It would not set right. At 2 turns out it would not run, at 3 turns it was too rich. She got it as close as she could to proper tune, but not bang on. Her advise is to sink the carbs in cleaner for a while and see what that does. The other idea is to try smaller pilot jets. My Canadian model came with 43 pilots. I'm thinking 41 pilots may help out. First I need to make sure that the float level is dead on though (what is the proper level?). What do you guys think?

 

Step one. Valves. Got that done right. Which ones were changed?
Clean the carbs. Don't just dip them completely take them apart and replace a rubber parts and MAKE sure everything is clean.

Float levels
http://www.xj4ever.com/setting%20fuel%20levels.pdf

Sync. Colortune sync.

Brakes...

Let's see what happens.

 

The brakes are all good. I rebuilt the calipers and the shoes are brand new in the back (going to get stainless lines soon). The only valve that was tight was #4 exhaust. I Really need to get a colortune plug set. It's so wierd the carbs look sooooo clean inside, but I know there could be something inside the channels. I suspect it's just a pain to tune without the colortune, considering all 4 carbs are doing the same thing. This is also why I'm thinking of checking the float levels again. There is a tool made for doing this that I would like to get my hands on. It would be better than using a pvc hose