What's wrong? 29 smoothbore vacuum issue maybe?

Thottle grip clean, Throttle cable free and clean, spring at throttle new. Slides rise and fall correctly when not running and same for quick revs at idle.
When held at 4k rpm, the slides are very slow to drop when throttle is released, as if there is a vacuum lock above them. WTF, anyone know what the culprit could be?

Hello, sometimes if a bike is running lean it will hesitate to return to idle. I'm not really sure what you're doing or have done but maybe go alittle richer on the air screws and see if it gets better. if not its easy to return to your current settings.

Kahuna360 wrote:

Thottle grip clean, Throttle cable free and clean, spring at throttle new. Slides rise and fall correctly when not running and same for quick revs at idle.
When held at 4k rpm, the slides are very slow to drop when throttle is released, as if there is a vacuum lock above them. WTF, anyone know what the culprit could be?

Disconnect your throttle cables from the carbs.

Manually lift the throttle lever with your finger and hold the bike at 4K RPM. Let go of the lever.

If the problem is solved, it's your cables.

If it still there, the carbs need to be disassembled, cleaned and the slides and slide bores inspected for grit or burrs...

If the problem is in the cables, a lot of times it's simply not enough slack in the adjusters. Back some slack into the opening cable adjuster so the closing cable can do it's job.

I've run lots of 29 smoothbores without a closing cable and had no sticking throttle issues.

This is a common sense problem / solution. It's really not complicated if you just rule out the potential problems one by one.

Kahuna360 wrote:

Thottle grip clean, Throttle cable free and clean, spring at throttle new. Slides rise and fall correctly when not running and same for quick revs at idle.
When held at 4k rpm, the slides are very slow to drop when throttle is released, as if there is a vacuum lock above them. WTF, anyone know what the culprit could be?

If not already done, would check for air leak in carb holders, or possibly a loose clamp or mis-fitment into carb holder.

With engine at idle, could spray WD40 or carb cleaner spray or propane around carb holders to see whether rpm varies or exhaust starts smoking. As this will indicate intrusion of outside air into the fuel mixture as it enters the intake port.

Also check for loose caps (plugs) on the vacuum nipples.

If a vacuum petcock is fitted, assure integrity of the vacuum tube and snug fitment onto carb and petcock nipples.

Good Fortune!

the slides are mechanical no vacuam involved that im aware of.
have you tried running it manually with cable unhooked while running?
are you sure the advance weights on ignition arent sticking?

Dyna ignition, coils, and wires. Carbs were dipped 2 days ago and slides are fine without power. Not sure about fogging the boots or bodies, I guess I could try. I just hate wd-40. Gums up.
What if the float levels are off? I used a cheap caliper.
Set them at 23mm. Would 2mm too high cause it?

Kahuna360 wrote:

Dyna ignition, coils, and wires. Carbs were dipped 2 days ago and slides are fine without power. Not sure about fogging the boots or bodies, I guess I could try. I just hate wd-40. Gums up.
What if the float levels are off? I used a cheap caliper.
Set them at 23mm. Would 2mm too high cause it?

Brake Klean or carb cleaner will work just as good to find leaking boots.

Sounds like a worn out return spring to me
Remove and clean all actuators oil em and make sure all moving parts are free.
If that dont work get a new return spring the one that closes the round slides.If that dont work you are gonna have to remove and possibly dress the surfaces of the roundslide bores.
Ive never run a return cable maybe you have to do this.
I prefer to hear the carbs SNAP shut on spring pressure.
Just my 2cents worth

PLUMMEN wrote:

the slides are mechanical no vacuam involved that im aware of.
have you tried running it manually with cable unhooked while running?
are you sure the advance weights on ignition arent sticking?

i would be looking there also....

I agree as well check advancer behind points plate

The advance weights in back of the Dyna S sound like a solid thought. I just went out and sprayed the springs and the pins the weights are mounted on, then blew out excess lube with air. I will let you guys know tomorrow if that was it.
Neighbors were patient with my 8 valve GS1100 restore so I don't want to push it.

my neighbours didnt like this one