2 for the Price of One (High Idle Speed and Faulty Thermostat)

[fast al gordo]

Hi Guys

First things first, I've just cleaned the carbs and replaced all the orings and seals using one of the kits from litetek. Set the pilot screws to 2.5 half turns out.

1) With out any input on the throttle stop screw and no choke, the bike will idle at 2k rpm, I'm guessing I need to adjust the pilot screws. But is it more or less turns out.

Also top tip, it's a lot easier to put the airbox onto the carbs before you put the carbs onto the engine, wasted 20-30mins before giving that one a shot.

2) When would you expect to see the rad fan kick in? I was planning on balancing the carbs tonight, so started warming her, the temp gauge kept rising till it was at the end of the black range.

Thanks

[Squishy]

1) What happens if you set the idle stop screw lower? The engine can't maintain 2000rpm with the throttle valves fully closed unless it's drawing false air.

2) You can test the fan by simply bypassing the thermostat temp switch (they die sometimes) to see if the fan still works.
Fan should go on after maybe 5min idle from warm engine? Can't say.

[fast al gordo]

1) The idle stop screw is all the way out. What is strange is it was idling around 1k yesterday with the airbox off, and that was with a few turns in on the idle screw.

2) How do you go about bypassing the thermostat, bike electrics is not my strong suit.

[greg737]

The GSF400 Service Manual says that the Radiator Fan Control thermo-switch is supposed to go "on" when the coolant temperature rises to approximately 95 degrees C (203 degrees F).  When the coolant temperature cools through approximately 88 degrees C (190.4 degrees F) the thermo-switch is supposed to go "off".

When referring to the components of the cooling system it's best not to use the word "thermostat" unless you really mean the actual thermostat.  The "thermostat" is the spring-tensioned, wax-core-activated coolant valve that is mounted within the bike's thermostat housing.

Are you working with a PDF copy of the Service Manual?  If not, you should find one.  It's available out there on the Interwebs if you look around for it. 

I've personally found and downloaded two english language versions of the GSF400 Service Manual: One it the original "M, N, P" version of the Service Manual (the letters correspond to Suzuki's VIN number 10th digit year-model code: M=1991, N=1992, P=1993), and the other is the later edition "M, N, P, +V" version (which includes an addition/supplement for the Variable Valve engine)

[fast al gordo]

Hi Greg

I've got the pdf, and tbh I only looked at after I wrote the post.

I guess the first thing to do is to test the two thermo switches and the rad fan as per the manual.

I suppose I was just being lazy and hoping someone could answer my question.

[greg737]

I understand, it's quite a tear-down (tank, coolant drain, hoses, wiring, etc.) to get the thermostat housing off the bike.

But as Squishy mentioned you could easily operation-test the fan motor by simply grounding the wire that normally connects through the thermo-switch in the thermostat housing (the thermo-switch causes the Radiator Fan to run by completing the circuit through the fan motor by providing a path-to-ground for the 12 volt power, which is always available, hard wired, into the Fan).

The Radiator fan has two wires going to it: one that supplies 12 volt power (this incoming 12 volt power line is Orange colored) and a ground-path wire (Black colored) that runs out to the thermo-switch mounted on the thermostat housing.

Identify the Fan wire connector that normally goes to the thermo-switch, disconnect it and insert a piece of wire from that Fan connector to the bike's battery negative.  Then key-switch the bike to "on".  The fan should run.

[fast al gordo]

Thanks for that Greg.

The tank is off at the moment, but following your advice would be easier than draining the system and pulling it all apart.

[greg737]

Wait... a '91 GSF400 VC model has a temp gauge?  I didn't know that.

I thought it just had an "idiot light" like my '93 B4.

[fast al gordo]

Indeed it does, I wasn't sure what the water thermo switch was for at first, had a wee look at the wiring diagram and it shows it feeds to a light. I'm guessing that in the 91 VC's case it feeds to the temp gauge!

But that doesn't really sort out the issue with the high idle speed though!

[greg737]

Since you already have the fuel tank off the bike you should take a close look at the thermostat housing. 

You'll notice that the thermostat housing has two sensors screwed into it, one on the top and one on the bottom.  The sensor on the top is connected to the gauge (on your VC model, the idiot light on my '93), and the sensor on the bottom is connected to the Radiator Fan wiring.

Just trying to clarify the situation a bit by pointing out that the temperature gauge wiring is not related to the radiator fan control wiring.

[Squishy]

1) The idle stop screw is all the way out. What is strange is it was idling around 1k yesterday with the airbox off, and that was with a few turns in on the idle screw.

That's weird the bike should die with throttlevalves closed, unless it's getting air from somewhere else.
Does RPM go down when you spray brakecleaner or starterfluid around the intake area?

I'd suggest setting the pilot screws to 1.75x out with stock exhaust or 2.0x out with open exhaust.

[fast al gordo]

MMM, I'll have a look at it again tomorrow.

[greg737]

Also top tip, it's a lot easier to put the airbox onto the carbs before you put the carbs onto the engine, wasted 20-30mins before giving that one a shot.

So I'm guessing you've recently removed and replaced the carbs?  If you've accidentally left one or more of the carb-to-engine connectors loose you'll have idle problems.  This would set up a situation where air is being sucked in (in spite of fully closed throttle plates).  Even one cylinder sucking extra air will cause the idle to go high.

[fast al gordo]

Hi Greg



That could be the case, I know I got the two outside clamps nice and tight. The inner clamps, not so sure about.

I was planning on replacing the clamps with jubilee clips at some point, now might be a good time!!

[ventYl]

wrong ballance of carbs can cause very high idle too. usually in combination with extremely slow decrease of RPM after closing the throttle