Engine auto revs up when warm

Hello everyone, first time poster.

Bike in question (first pics after i got it out of the shed)




I have a 1982 yamaha xs400 special that i have got running after my dad had left it in a shed for 4 years, had to clean the carbs. I have now got it in such condition that it starts up on first kick and will idle nicely (going by sound here, ~1000-1200 rpm) once it has gotten slightly warm.

But when it has been running for like 5 minutes it will rev itself up to 3500-4000 rpm with the slightest throttle i give, and it will stay there. Only way to bring it down is by giving it load or by quickly flipping it off and on. I have tried to search for the solution and the most common answer seems to be vacuum leak, i have sprayed manifolds and the petcock hose with motorstarter without finding a leak.

So could it be mixture related? I have very little understanding about how mixture effects the motor, other than that I want to err to the richer side.
When I cleaned the carbs (mikuni bs34) i set the pilot screws to 3 complete turns, but i am not really sure what i am adjusting, is it fuel or air? turning screw in means giving less, regardless of medium? The screws sit in turrets above the butterfly valve.

When i take it out for a spin it will give good response and acceleration up until 5000 rpm when it really flattens out.


I am a complete noob when it comes to engines and its only my newly discovered love for motorcycles that has ignited the will to fix this.

EDIT

One of the cards doesnt have the main jet screw, exactly what is its function? Assuming that the needle jet stays in its place.

That main jet screw may be an issue. Someone sent me this link a while back, some good info on carbs. http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

I would have a quick look at the throttle cable too, make sure its all nice and free and well routed. Sometimes it can be simple things. If the boots are original they are probs pretty ratty, how do they look? I changed my boots and it stopped the high idle. Another thing to check is if you have a idle screw tip broken off in the carb body. It can be removed if you have I have done it (I made a thread on it).

Thanks.

The manifolds look dry and cracked but i cant find leak... But i guess im buying new ones.

The throttle cable was my first thought, but its notthe peoblem, the butterfly valve remains in its positions while the engine auto revs up. Im thinking that i have a rich mixture plus vacuum leak

Clean the carbs again. And verify that the pilot circuit is clear by blowing cleaner through them. Then clean them again. We've been telling this to people since 2010, and even had it printed onto T-shirts. For some reason newbies don't want to believe it. No kidding...