Low end stutter

[OneHand]

Cant seem to get the low end sorted out. Still got a stutter under 3K under load. After that runs like the afterburner was lit. Feels like when you're running out of fuel, but goes away at 3.5K. Only really happens when you want to accelerate quickly i.e heavy hand.

1. Carbs are properly cleaned and balanced. I bought a 4 gauge vacuum sync, and it works beautifully.
2. replaced mains with 142.5's. Man! What a difference it makes to the power. Set the mixture screw by ear, plugs look OK.
3. My concern - There is a 3 by 1.5in hole in the airbox, and K+N filter. Too much?
4. 4 in 1 pipe.
5. Starts first swing, with full choke, but does not want to idle untill fully warmed up, thus I got to keep my hand on hte throttle. When warm, idles smoothly at 1K

A mate suggested that I close the hole in the airbox, or that I lift the needles a notch. What do you suggest?

Apart from the above, Vader runs really well

[petejw]

hey onehand,
mucking around with the needle height will only
effect middle throttle range, how are your float
levels?

[81xsproject]

Sounds to me more like those "by ear" idle settings . Check out the tech tips and there is one on using propane to set the mixtures along with some other methods that might give you better results for that low end pull. I personally don't recommend the Color Tune, but others swear by it. The floats are a good thing to check to, too.

[skids]

I know that if the float levels are too low you can get some studder. I think it has something to do with opening the throttle plates earlier than you normally would, causing more fuel from the mains and not enough from the pilots for that rmp.

[79XS11F]

I also agree that this is likely a carb /jetting issue. pilot jets are not on off and have an ongoing effect on all other jetting changs. If carb float adjustments don't fix it try different pilot jetting including the air jets
Rob

[Ken Talbot]

Re: Low end stutter

Originally posted by OneHand
3. My concern - There is a 3 by 1.5in hole in the airbox, and K+N filter. Too much?

A piece of duct tape will answer this question real fast.

[The Lorax]

Sounds like the idle circuit is lean, fatten it up a bit and see how she runs... please anyone feel free to correct me if I am mistaken, but it sounds like a lean stumble.

[OneHand]

Floats are set at exactly 25.7mm. I checked them in boiling water, No leaks, and they all float about the same depth. Set the idle mix as per the tech tips for each cylinder, ie revs (by ear)

What I'll try tonight is closing the hole in the airbox, and new caps on the 1 and 4 vacuum nipples - They are fairly loose and could easily be leaking Only thought about it late last night. But definitely fixing only 1 thing at a time to isolate the problem.

[OneHand]

Not Lean, but RICH!!!

Fiddled around last night, and accidently pulled off the vacuum pipe on the petcock. within a couple of seconds the idling started smooting out and increased nicely. DOH!!! My logic suggested that the level in the bowl is 2 high, when it started running dry it leaned out. The floats must be floating too deep, probably beacause of age. Pulled the plugs again for interest sake - black and sooty!

Off with hte carbs again, set the floats to 31mm (basically just on/off) Starts up on choke, idle goes up. Move choke to first click, still fine, idle goes to 2K. Close the choke, then it dies. Too low now!

Looks like I'll have to play with it, thinking 29mm. Damn - carbs to come off again. Getting quite good with it now!

Will probably have to buy some new plastic floats sometime.

Report back after the weekend...

[skids]

29mm? That seems a bit low for the levels... You might want to go back to specs from the manual.

[OneHand]

Well, I set the floats to 29mm from gasket surface to top of the float when it is just resting against the fuel needle.

Seems to work OK. Definitely better at low revs with minimal/no stutter in the lower gears, althought 4th and 5th still does not like to be under 3K. I think I might have lost some top end power, but is willing to sacrifice that for smooth pulling at lower speeds. No-one rides at 7K + the whole time anyway.

I'll ride it like that for the time being, and will check plugs etc in a couple days.

Thx for the help and suggestions

[DiverRay]

When trying to set the carbs, go in SMALL STEPS. I usually adjust 1mm at a time on the floats. It doesn't look like much, but can make a huge difference in the way the bike runs.
And yes, I will be going into my carbs once again this weekend. It's been almost a year, and 15K miles. I'm starting to smell gas when I park it, so time to adjust once again.

[79XS11F]

I'm NO CARB guru by any means but I have done some research on the Internet and from what I gather the float height is suppose to be set with the float needle just in light contact with the valve. This is what I have read at a few sites that get into carb tuning. This may not however be what Yamaha wants done when setting up to their specs. I'm sure many will have valuable input about this?
Rob

[concours]

Ahhhh.... I ran into that 27 years ago... it's the vacuum advance diaphragm oscillating back and forth. When you pulled the line from the petcock it created a vacuum leak which squelched the oscillation. Perhaps a drop of moisture in the advance line/can. Pinch off the advance hose and test to verify my theory....