Lubricating carb slides??

[Mahalaleel]

The throttle on my 78 GS750 is getting stuck, and won't return all the way down to idle. If i reach in and push down on the pulley it returns to a normal idle. The throttle cables are loose enough. When I use the idle knob to bring the idle up and then turn the knob to turn it down the pulley stays as far out as I pushed it.

I just cleaned the carbs completely so I didn't think this would be a problem. Is there some place I should have used some lube? What could I use to lubricate?

[Easy Rider 2]

I just cleaned the carbs completely so I didn't think this would be a problem. Is there some place I should have used some lube? What could I use to lubricate?

Seems like I remember that it is common to get the slides back in incorrectly and the result is much like what you describe. Also having the throttle return spring missing, stretched or improperly installed could do it......as can a binding of the twist grip.

Nothing in or on a carb is really intended to be lubricated.....except maybe the points where external cables attach.

[WintrSol]

I once got a 305 Super Hawk because it only ran on one cylinder. The slides had been swapped, and one was stuck; the knob on the end of the cable had pulled off, too. I reversed the slides and re-soldered the cable end, and put over 20K miles on it, before selling to a friend.

[Mahalaleel]

That is extremely likely...should I swap the slides and re-synch?

I lubed up the cables and they're turning more smoothly now, but it still doesn't return to a normal idle unless i push down on the pulley with my hand (not extremely convenient).

[WintrSol]

If it's at all possible that you swapped them, I think putting them back is the first thing. A re-sync may not be needed, but it's worth checking, anyway.

[Mahalaleel]

ARgh!

I'm starting to get frustrated and confused.

So, I switched the slides around until it felt like they were all as smooth as possible in there. Then I resynched them.

I reinstalled everything, started her up...no change. If I give any throttle it won't come back under 3000 RPM without a manual push on the pulley. However, if I push the pulley down the idle will sit around 1300-1400 RPM and sound really good.

I tried taking off the throttle cables and just moving the pulley by hand to test if they were keeping it from returning to a good idle position. The idle got stuck at 2000 instead of 3000. After a push by hand it stays at 1400.

I have no idea what to try next. I'm losing patience after a month of fiddling with this bike.

[benny hedges]

i take it they are cv carbs with diaphragms moving the slides?
there should be an airhole in the slide.
if it's blocked, no air can get in / out under the slides and their movement will be hindered.
check also that the linkage isn't damaged - there should be a circular spring under the linkage...
even if the cables feel free, check for damage and lube them by forcing oil through the outer with a foodbag with oil in, held in place with tape.
dont ride it until you are 100% positive the return action is sorted - a bud on vtr1000.org recently flipped his storm due to the return sticking.

[Mahalaleel]

This one is actually not a diaphragm set of carbs. It's a 1978 Suzuki GS750, if that helps. They are mikuni carbs. One of the cables has a kink in it, and a hole in the outer plastic coating. I will end up replacing the cables, but even with them off I can't get it to return to a healthy idle without a push.

I just can't think of anything else to do to fix it. The pulley spring still feels pretty strong, but maybe it's not strong enough?

[WintrSol]

So, the throttle is a butterfly which is turned by a pulley, which itself is rotated by the cable, and the CV pistons do not have diaphragms, correct? And, the throttle won't close unless you force them closed. Is there just one pulley that turns the butterfly of each carb? Did you disturb either butterfly, loosen the screws that hold them to the shaft?

[Mahalaleel]

The throttle is a bar that goes through arms connected to each of the slides (they are cylindrical they go up and down) as well as a pulley attached in the middle of the bar actuated by throttle cables on either side of the pulley (it's like two wings of the pulley).

Does that make sense?

When I push down on the pulley they close all the way (descend until the space between the slide and the carb body itself is extremely tiny). When I let the throttle go it doesn't close all the way, and thus leaves it idling very high.

I'm not sure I can post pics yet, but would that help?

[WintrSol]

So it appears to be the bar (and arms) that won't return, and is keeping the slides open. If the carburetor bodies, or the support for that bar if not the bodies, got out of parallel when you cleaned them, the bar could be binding. It wouldn't take much to make the bar stick. A photo would help, even a diagram in an on-line manual.

[bdavison]

http://www.thegsresources.com/gs_carbrebuild.htm

That should help.

[WintrSol]

Thanks, bdavison, but those photos show butterfly valves on a common shaft. The OP said the common shaft pushed the CV pistons directly. I can't see that happening with butterfly throttles.

@Mahalaleel: are those your carburetors?

[Mahalaleel]

Yes, those are them. They aren't butterfly valves, they don't rotate around a center axis. If you look closely they are slides that go up and down when the bar is moved.

Thanks Bdavison

[WintrSol]

Photo 005 shows the butterfly throttle valves, which are turned by the common shaft; 006 shows the CV pistons, one of which is up. In carburetors with butterfly throttles, the CV pistons are not mechanically connected to the throttle, but rise with increasing air flow.

[bdavison]

With those carbs, the slide has no mechanical linkage that controls them. They are manipulated in the carb by the diaphragm using air pressure changes from the intake port above the venturi.

As you open the butterfly valves, the air pressure changes in the intake. The pressure change causes the diaphragm to inflate lifting the slide, and subsequently the needle.

If the slide is not returning to closed position when you close the throttle, there are a couple of possible reasons.

1. The throttle slide return spring that rides under the top cover and down inside the slide is missing, broken, or not aligned properly.
2. The air vents holes on the slide are blocked off...so the air in the diaphragm cannot deflate.
3. The needle is bent or malformed or misaligned and is not seating properly into the jet. If the needle is not perfectly centered in the jet hole, and it hits the side of the jet when the slide trys to return...it will jam, and prevent it from closing.
Ive seen this happen with shimmed needles, because they used too many, or junk washers to shim it, and the shims knock the alignment of the needle off.

[Mahalaleel]

Oi!

Upon closer inspection of picture 5, those are NOT my carbs, sorry!

I'll get pics and post them up later.