Idle Jumps after a few minutes

[efranzen]

I'm having issue with my 1978 XS400 with Mikuni CV carburetors. I have balanced them with dual gauges and set the Idle to around 1200. I have adjusted the fuel mixture screws by turning in until the engine stumbles and then back out 1 turn. The bike will idle for a few minutes and then the idle jumps to around 3000 and just hangs there. I can turn the bike off and then restart it. It will idle fine again and then take off again. I thought it was the sliders, but I do not see them move once the idle jumps.
Carbs are clean and float level is correct. New carb to head mounts. Vents are connected to the gauges and the bike dies if I unplug them.
I'm using pods with 137.5 mains and 47.5 pilot jets.

My next test is going to check the float level with clear tubing to see if it's changing. Anything else I should check for?

 

[JPaganel]

Have you replaced the throttle shaft seals?

They harden with age and start giving you air leaks, which is what it sounds like you are having. This is especially noticeable with warmup.

The seals are still available from Yamaha: 256-14997-00-00 or 2A2-14997-00-00, whichever you like better, they are the same part. Not actually listed for XS400, but I have fit a few of them. You need four, two per carb.

You have to remove the butterflies. Butterfly screws are peened over, and you better be damned sure to have the correct JIS screwdriver for them, or you will strip them out. You also may want to dremel the screw ends before trying to take them out.

 

[efranzen]

Thank you for the info, we did notice a slight RPM change when spraying the shafts. I can see that messing with trying to set the mixture screws, but not why the idle would run away.
I went down the ignition advance hanging today and it was well lubed and move freely and there are tension on the springs.

 

[JPaganel]

I can see that messing with trying to set the mixture screws, but not why the idle would run away.

That's pretty simple. It's the same as setting the mixture screws too lean - you also get hanging idle. So, when the hardened rubber warms up and creates a leak it's like you suddenly screwed in the mixture screws.

 

[CaptChrome]

The idle issue sounds like a lean condition. Based on what you have done already, I would do what @JPaganel is advising.