Flat Acceleration at 3/4 Throttle and up

[stokes776]

Hi everyone!

Trying to validate my troubleshooting before I proceed.

History:
2004 GS500F, with a 1998 GS500 engine, 2004 carbs.
Bike put away 2014 no major problems.
Start of season 2015, bike out of storage would idle but died upon any throttle. Took opportunity to rejet to:
Main:  140
Mid:   62.5
Pilot: 20
Installed new jets, now rideable, worked pretty well. Still a bit rough.
Had issues with choke, always seemed to need some level of choke no matter what.
Checked spark plugs, normal, if a bit white.
Did valve check, all valves within clearance.
While sealing bike up from valve check found the restrictor ring on my K&N Filter was sitting on my work bench.
Reinstalled restrictor ring. Runs much better now, can finally take choke off after warmup.

Problem:
Bike responds fine at low and medium levels of throttle. Normal riding is fine. The problem occurs when, lets say I want to dart around some slow moving traffic and try to give it anything around 3/4 of maximum throttle.
Anything before 3/4 of throttle, the RPMS rise along with the acceleration of the bike.
As soon as I hit 3/4 throttle, RPMS remain FLAT, no increase, no decrease and the bike stops accelerating. It maintains its speed, but there is no acceleration.
If I roll back on the throttle to halfish, RPMS rise again and acceleration resumes.

First couple times it happened as a surprise when trying to manuever in traffic.
Since then I have tested it on purpose a few times intentionally, and can replicate it.
I continue to ride it like this, it rides fine on those lower level of throttles. I just worry that if I were to need that acceleration, it wouldn't be there.

Theory:
I am suspecting I have a clogged Main jet somewhere? As the Main handles that higher level of throttle fuel delivery?
OR
When I rejetted, I did not do anything with washers. Do I need to install washers on the needles? I did not research this but I remember it being mentioned.

[Slack]

So, you have a stock air box with K&N filter and restrictor plate, 20/62.5/140 jets, and a stock exhaust?

[stokes776]

That is correct!

[bombsquad83]

Did you accidentally switch the main and the mid jet?

[Weedy64]

A plug chop at the 3/4 throttle would tell you for real if you need to be richer or leaner.

What washers do you have and where? You/we need to know, in order to help.

I think 140 mains might be a bit big for stock exhaust  so lowering the needles (moving a washer or 2 up from below the circlip)
If the needles are all the way down then go one size down in main jets. 137.5 is whats in the table for you.

Above is based on the jetting table found in the GStwin.com Wiki - rejetting, it was very close for my 2009 with mods, I ended up moving moved one washer up for more 3/4 accel.  on for my bike.  A washer is considered to be ~0.020", newer stock bikes came with a double thick washer, and a single washer - tweak from there.

air screw caps removed? not that it affects 3/4 throttle but if your tweaking you need control of all and you already upjetted the pilots

[brownh7]

I had an issue similar to this on my '98. I turns out that the rubber cap that covers the vacuum port nipple on the top of my left carb had fallen off. So I'd check and make sure there are no vacuum leaks and everything is hooked up in the proper place. 

[stokes776]

Swapping the needles (MID to MAIN) to the wrong slot is a possibility, as it was my first time in the carbs, but I tried to do one set at a time.

I havent dont anything with Washers at all. No washers. I need to research and understand those a bit more.
Would having no washers cause a problem?
Yes, air screw caps removed when I rejetted. I believe I turned them both all the way tight and then out either 2.5 or 3 turns, whatever was specified as 'a good starting point'.

The rubber nipple cap, that is something that might be causing this. More than a year ago (wow, really two years now that I think about it) I did a carb sync procedure. After I pulled the bike apart to get it to the point where I could synch it, I put a leader tube from one of the nipples and capped it off, that would it would be easier to access I think?
Not entirely sure why, it was listed in whatever guide I was following as a good thing to do since it was a pain to get to in the first place. I wonder if that might be leaking a bit somewhere along the tube or at the very end.

[The Buddha]

So, you have a stock air box with K&N filter and restrictor plate, 20/62.5/140 jets, and a stock exhaust?

Yikes - k&N in the airbox ? try maybe 135 mains. 20/62.5 should be fine, but 140 is a bit much.
Cool.
Buddha.

[stokes776]

All right, got some time to work on it. Here is what I did.

Removed the carbs.
Removed my extension hose for one of the carbs for Carb Balancing procedure. Replaced it with a small capped off section that I cut from the same hose.
Idea being that the long hose may be causing small leaks and affecting the carbs somehow.

Opened up the bottom of the Carbs. All of the jets appear that they are in the correct spots.

I replaced the 140's I had for a main, with the original 130's. Didnt have time/money/patience yet to order new jets.

Buttoned everything back up.

Took her for a ride and it seems like the smaller jets solved the problem! I again have acceleration past 3/4. My idle is much smoother too, maybe that was the result of the vacuum capping.

There is a small barely noticeable stumble somewhere in the middle of the acceleration curve. As I am accelerating, going from presumable the mid jet to the main jet there is just this small little spot ( maybe 1 second at the most ) where it barely stops and then picks back up, and then it is fine at the high acceleration.

I'd call it good enough! Maybe after another ride or two I wont even notice it!

Thanks guys!

[The Buddha]

Stumble at 1/2 throttle ?
Try the original air filter in again, if it gets worse, you're rich try no filter at all, if it gets better ... well, leave it.
Cool.
Buddha.