uncontrolled speed due to gass mixure?

[Guus de Jong]

I got a GS400 which has not been running for years. I Cleaned the carbs but now when I start the motor it first idles with about the right speed but once I pull the throttle it does not slow down anymore and keeps running with very high speed. I have inspected the carb over and over again but could not find a cause. I checked the petrol level in the carb bowls and it was according the spec. Can it be that eht gass mixture gets poor once the throttle is pulled which makes the speed go up uncontrolled? Can it be that the diaphragm is not working? Is there a way to check it?

[chef1366]

Sounds to me like stuck slides.

[Guus de Jong]

I can move the slides easely against the diaphragm spring pressure

[chef1366]

Check for intake leaks. Spray a little WD40 on your intakes and see if the idle dives.

[Guus de Jong]

Thanks I wil try it out and buy some WD40 tomorrow. You advice me to spray the WD40 on the connecting pipe between the cilinder and de carb? and if there is a leak on that part the high idle speed will go down? BTW do you know if the diaphragm works with the inlet air filter removed?

[kalessin]

Yes, spray on the rubber pipe that goes between the carb and the engine. The idle speed should change if there's a leak.

Which diaphragm do you mean?

[shortlid]

Did you check the throdle cable for sticking??

[Guus de Jong]

I checked the trottle calble and that one is working fine.
With the diaphragm I mean the piston-jet needle-diphragm part which I suppose will move up against the piston spring pressure when motor speed goes up to let more fuel and air flow to the engine.
Unfortunately I could not get the WD40 today, will try your suggestion on Friday

[KEITH KRAUSE]

Sounds more like a mechanical problem than an intake leak. Does it idle high after you throttle it when just started and not warmed up yet?
Might be a sticking throttle plate or problem with your bench synch. Could also be a diaphragm/piston assembly not installed correctly.

[Guus de Jong]

I tried the sprying with WD40 but the speed did not go up nor down.
The construction of the carb is a little different than on the picture in workshop manual. The piston spring (part 2) is keeping the the needle into position , there is no circlip. Is it possible to see the carb piston going up together with the speed with the air filter connection being removed?

[Guus de Jong]

I removed the air inlet pipe and with a little mirror I could look into the inlet pipe. When the speed goes up the carb piston goes up like it should on both carbs. I found out that there is no O-ring between the carb and the inlet pipe. Although I think there should be a ring it does not cause the problem. I checked the inlet for leaks with the WD40 spray.

[BRycraft]

C- Clip missing

Is the c-clip missing? what happens is when you open the throttle there is a pressure change in the venturi, this forces the diaphram upward over coming the spring tension on the slide this in turn raises the jet needle, the fuel mixture is then measured by the jet needle and needle jet opening, the greater the opening the more fuel is mixed with the increased air thru the venturi. If that jet needle is not lifting up as the diaphram is raising then you will have serious problems. That carb would never operate correctly if that jet needle is not raising up. The spring inside the cover is in place to hold the slide down during low speed operation such as idle, when you open the throttle the vacuum created is what over comes that spring, if that slide is lifting up and the jet needle is stuck in the lowered position then that carb will constantly be in idle mode. Be certain that circlip is in fact in the throat of the slide assembly. Unless it is some other type of carb that I am not familiar with that is a major problem...
Part of that slide assembly has the ability to raise and lower that jet needle as you wish to adjust the fuel mixture as say the air filters are altered.

With the airbox off and engine running it is easy to see the slides lifting and lowering as you open and close the throttle...

Bill

[BRycraft]

O-Ring

Didn't catch the 2nd page before I posted the above reply...
If there is a missing o-ring then that will in fact cause problems that you may not even be aware of. The spray test is not always conclusive, first of all remember that at higher speeds there is a greater suction in and around the carb inlets and a more likelyhood that it will suck extra air in leaning out the carbs on a subtle leak. I pulled mine and replaced them because I had the carbs off anyway and since they are dirt cheap its a good investment. I tried the spray test and didn't have any change whatsoever. I still replaced them because I had them off and I didn't want to go thru that again should I still have a problem after it all went back together. What caught my eye was #4 manifold, when I pulled it I noticed a covering of what must have been dirt totallly covering a portion of the oring on the back of the intake boot. I had to take a thin screwdriver and literally scrape the dirt out that gotten behind the oring itself that was inbedded in the groove the oring sat in and it made itself like a bridge of dirt over the oring and was smooth like sanded bondo, probably from years of a leak drawing in dirt and getting impacted behind the boot. When I reinstalled the boots I used bolts in replacement of the screws that were original on my bike. I had to take a long chisel and actually use it to break the screws free as there was no way there were coming out, thats why I got bolts with lock washers and used some medium strength lock tight when I reinstalled them.
I didn't have any leaks that I could detect with spraying, but those o-rings were totally flat and hardened and the motor has only 11K on it. It now idles so much smoother then before, its literally like night from day.
Get those orings replaced, they were put there for a reason, if it didn't make any difference Suzuki wouldn't have put them there. At the very least should you have any more problems you will be able to eliminate that as a possible area of concern.

Bill

[Guus de Jong]

Bill,
Thank you for your detailed explanation! Now I understand the working of the needle-spring-diaphragm. I checked the working again and it seems to go up (with the needle) if speed goes up.

The missing circlip was not the one holding the needle in position. The carb I have on this bike is a little different than the one in the workshop manual. The needle is kept into position by the spring.

I will get the O-rings tomorrow from the shop. Maybe indeed I have drawn the wrong conclusion using de WD40. At the workshop manual I have, the Manifold is not mentioned at all. Reading it again very carefully I found the frase ........14. Check that all the mating surfaces on the carburettor body are flat by using a straight-edge laid across the mating surface. Ensure that all the O-rings and sealing gaskets are renewed when reassembling .....Thats why I did not miss the O-ring in the first place.

[Guus de Jong]

Unfortunately adding the O-ring did not give any improvement.
The stationary speed went up a bit and could be lowered by turning the stationary ajuster. When I force the speed of the engine to go down it will stay down but once I open the gas it stays high.