Motor wont idle, revs when hot

[steptoe]

replica seats are available on line

[Michaelk]

Sorry, should have introduced myself before, but had a mission to solve. I live in Longford Tasmania.

I did a full resto on a CB900 a couple of years back, and was looking for another project.
I picked up the XS (1978) on gumtree for what I thought was a good price. The carbies were of it and the brakes didn't work. The fuel tank has had numerous attempts to fix leaks with metal putty but the bike was supplied with spare good tank. Otherwise it is a solid original old bike with really good chrome. Not sure if its a keeper. I'll try and get it running right , then decide. If I keep, it will just get paint and tyres.   OH and I will have to do something with the seat which has had someone attempt to fiberglass repair bad rust in the seat pan. Not sure how to tackle that as I imagine they are hard to come by It was last registered in 2002.

                                                                                                                                                                   ............….Michael

[steptoe]

it's always the simple things in life,,
it's like having the choke on first thing, takes a couple of minutes to warm everything up before revs move up

have you said where you are and I,ve missed it?

[Michaelk]

Oh dear,,, this is embarrassing,, it appears that I just had the idle screwed  up too high.
  still doesn't make sense to me why only when hot, or maybe I have just tried too many things and you forget where you were.

Engine still not running perfect. I am going to buy the Gunson color tune thingy, renew plug leads and caps and try to give it a
tune up.  See where we end up.

[steptoe]

how much slack have you got on the inner accelorator cable, you should have 2 or 3 mm of free travel on  the throttle before the carbs start to react
easiest way to adjust is the small long nut and lock nut at the hand piece

ooooo and welcome aboard our info library

[Christian Raith]

[Jonesy :-)]

colour tuning is using a clear spark plug that allows you to see the colour of the flame when you are adjusting the idle mixture screw. Search "Gunson colourtune"
since the carbys were off it when you got it maybe someone has messed with them before. too weak a spring or no spring on the diaphragm/slides would allow the revs to climb. just thinking out loud.

[Michaelk]

Colour tuned don't know what that is
I have synced the carby's with vacuum gauges.

I now know the importance of doing this. My cb900 would not rev over 6000rpm. It had me stumped for a long time until I bought gauges and tuned them. They were not far out but the bike revs to redline now.

[excess.11]

You didn t say..have you colour tuned and carbtuned the carbys ?

[Michaelk]

Thanks for quick reply's
Carbs were cleaned well and blown out with air. all jets etc either proven clear or new.
Running stock air cleaner.
diaphragms inspected and all good.
 I will try the WD 40 trick.
The rubber connecting boots did not have a gasket , just raised rubber bit to seal . so if the are leaking I will seal with sealant.
 Just seams strange it only happens when completely warmed up to operating temp.

[XSIIE]

God day Michaelk,
Welcome to the Frey. 
It sounds very much a Carbie problem.  My "E" model did exactly what you described a few years ago. 
You have put new manifolds on? Have you started the bike and sprayed WD 40 on the manifolds with the engine running to check for leaks? As soon as you spray the revs will increase if there is a leak.
When you cleaned the carbs did you meticulously clean EVERY tiny orifice with oxy top cleaners.
These carbs are finicky to say the least.
What air filter (s) are you running? Pods? Stock?
Do any of the diaphragms have pin holes in them?
Balanced?
Just a few things to check/ confirm.
Eddy

[excess.11]

Maybe just needs a colour tune and carbtune synch.

[Michaelk]

hi, I have a new project. 1978 xs in stock trim. when I got it the carbies were off it.
I have stripped, cleaned and fitted new kits to the carbs. after refitting with new carb to head rubber boots, the bike started and ran first go. HOWEVER when the bike gets to operating temp(hot), if I blip the throttle the revs don't return to idle. stop and restart the bike revs to approx. 4000rpm. when the bike is not to full temp it idles nice at about 1000rpm.

I have tried with vacuum lines blocked - no change . the throttle appears to be closing off properly
Maybe need to make sure rubber boots seal properly?

I would appreciate any thoughts or ideas to remedy this problem.
Thanks