1977 XS360 - Carburetor Adjustment Question Regarding High Idle

[got20z]

I have a '77 XS360 with stock airbox and all stock carburetors/jetting, but it has the MAC 2 into 1 exhaust.

I currently have the idle screw ran basically all the way out to where it doesn't want to idle when it's cold but when it is warm after riding it the idle likes to hang. It is to the point now of not doing it every time I pull the clutch in or stop but it does it ALMOST every time. When it does NOT do it, sometimes the RPM is still slow to drop but it will after 5 or so seconds.

I have read a lot on here and other places online for solutions and not really finding anything good. I'm no carb pro but I can usually figure things out but this has me stumped. I originally thought that the idle screw was running itself back in from vibration when the idle would hang consistent but now that it is not every time I have my doubts there.

To the experts, I was wondering if you think adjusting the needle clip down maybe 1 notch to richen it a little would benefit me any or if I should try something else?

I synced them about a week ago and the bike runs great besides this problem. It runs smooth and pulls hard through all gears. No sputtering or backfiring.

I had similar issues on an XS750 last year but it was running hardcore lean because of the exhaust AND pod filters so I had to do a lot of jetting to it to get it running more rich.

I have routed my throttle cable about 3 different ways with really no change. The cable has slack and I don't think that is my problem.

Any help appreciated.

 

[NewHavenMike]

My first guess was that your throttle cable is sticking. Have you tried closing the throttle at the carb by hand to see if it drops?

You could have an airleak at the carb boots, You do have the vacuum nipples block off right? Those could not be sealing properly.

I would go back to stock settings on the idle mix screws which is around 2.5 turns out.

Have you looked at the plugs to see what color they are? It could be your mechanical timing advance sticking on full advance.

High idles are usually caused by air leaks(causes lean conditions), timing, the throttle cable.

 

[xschris]

Make sure the diaphragms are in good shape and check to make sure the idle mix screws have not broken off in the carbs.

 

[got20z]

The carb boots are new.. Just put them on a few months ago.

The vacuum caps are blocked as they should be.

On the mix screws, mine still have the plastic caps on them. I did not take those off and with them on it only allows the screws to turn around 1 full turn if I recall correctly (I am at work and can't directly go look at it). Maybe I should take the plastic caps off of them and adjust them again?

The plugs look good. I put them in about a month ago. They have about 100 miles of riding on them and still look great.


Thanks for your posts.. I'll keep digging on it in a day or two.

 

[NewHavenMike]

Having the plastic caps on is fine. Even better once you get the screws dialed in where you want and the cap will provide some security. You get about 1/4 to 1/2 a turn with the caps.

You should be making all adjustments with the bike fully warmed up too, yes I know its a painful process lol...

The bike should stall when you screw an idle mix screw in all the way. That will give you a really good idea that you have the right size pilot jet.

Have you checked the timing lately? That needs to be on point before you do anything, along with the valve lash. You cannot continue until you time the bike with a timing light. and get the points gapped properly. You can also verify that the timing advance returns home when you let off the gas..

The mechanical advance unit on my bike was sticking a bit and this caused a higher idle and had me going crazy because it didnt stick every time. You can verify easy movement by turning the timing cam by hand and the advance weights will swing out behind the points plate. I ended up stripping down the advance unit and cleaning it and its been working just fine.