Rough Idle and or Idle Speed Fluctuation

[youngSTer]

I have been putting up with rough idle/speed variation for as long as I have had my St.
I was browsing through my workshop manual on Saturday and found a section entitled FAULT FINDING Dooh!!!
and guess what???
If you have rough/variation in idle speed and your bike has not been used for 3-4 months the fuel has most likely
coagulated in the breather hoses (yes plural) that connect between the top Fuel Tank and the lower one.
Suggestion is that you blow the hoses out to clear anything that may be blocked.
So I did as suggested and blow me down there was some kind of stuff ( yes I said stuff) in 2 of the hoses,
NOT Mud Wasp Nests.
Result being that my ST now idles at a consistent 1200 rpm with no fluctuation. PRAISE THE LORD!!!!
Added bonus is that I have just come back from a 160k ride and in 5th gear she pulls as if I was still in 4th.
Yes I did check numerous times as I thought that I had forgotten to change up too.
I hopefully have put an attachment showing the fuel tank and relevant hoses.

[Marcus]

R.T.F.M.

[Brian]

Valuable info .... Thanks for sharing... And enjoy the extra horses

[StinkyPete]

Good find.   Another potential problem that my local dealer put me onto is a blocked fuel tank breather.   It will cause poor running and the engine to regularly cut out.   It will run OK for a little while and then stop again.   On every service on STs now, they squirt a little CRC into the breather pipe and gently blow some air through, watching for a dribble of CRC out the bottom.

[youngSTer]

Oh and I forgot a mechanic mate of mine said that if you can get some 100 octane + fuel that will break down any remaining deposits of bad fuel and also clean up sticking injectors which gum up while trying to use the bad fuel.

[Esstee 13]

Are we looking at numbers 9 and 10 on the diagram?
 

[youngSTer]

That is right. 9 & 10 and also number 40 even though it appears not to be under the tank. To gain access
You have to raise the tank and support with the stand provided. Also remove the seat. Do one hose at a time and blow back into the top tank with the filler cap open. Then blow the other way down into the main tank, you can't hurt anything by blowing the tank except to remove any blockages.

[Esstee 13]

Thanks Young One. SO you dont remove the hoses at all? By blowing it back in to the tank, where does it go?
 

[youngSTer]

You can remove hoses 9 & 10. These are easy to remove at where they go by jiggling each one in turn where they come out under the frame near the seat/main tank. No. 40 is impossible to get at without removing the main tank (major job). When I blew that one out there was a loud pop as something hit the inside of the main tank. That is why my mechanic friend suggested using some 100 octane fuel to dissolve anything that came out. By opening the filler cap I found that whatever was in the top tank blew out the filler cap.

Hope this helps.

[youngSTer]

Sorry I forgot to mention that I removed the hoses under the top tank one at a time and blew each one back both ways. By doing one at a time you can't mistake where they came from.

[Esstee 13]

Thanks Young One. I will give it a go next weekend.