Outside air temperature affecting idle return. Vac leak?

Went for a ride yesterday evening. I thought I had fixed the slow idle return by removing a kinked push cable. Not so much!

As the sun went down, the longer I was riding the slower the idle was returning. Went from 2 seconds when I started the ride, to 4-5 seconds when we stopped to get coffee, three hours later. When I got off the bike it clicked, the colder it is, the longer it takes for the rpms to get back to idle. Which leaves me to think that I have a vacuum leak. Now if the outside temperature doesn't affect my situation I'm at a loss here, but since cold air takes up less space, it would only make sense that the vacuum leak, if there is one, sucks up more air, thus changing the mixture and causing the slow idle return.

I'll try the WD40 trick. If there is a leak, is there a way to stop it? I guess it depends where the leak is. I suspect the carb holders. They are in good looking shape, but, they are hard as rock. Shouldn't they be a littls soft? 31 year old rubber has a tendency to get hard!

Anyway, happy riding ppl, and thanks for the input!

Oh, and I forgot to mention, I removed 2 of the little rubber plugs on the carb holders, and there was fuel in both of them. Is that normal? Maybe if I try to put tight tie-wraps around them it would help?

Thought it would be worth mentionning.
Cheers

a trick i used to soften up the holders was to use wintergreen oil. take a pot that your wife doesnt care about anymore, put just enuff water in it to cover the holders(make sure u use something to keep the holder off the bottom of the pan,i use an old grill rack and bent tabs to keep it about an inch up). cover the boots with just enuff water, add about 4 oz. of wintergreen oil(can get at walgreens) and slow boil them on low for about two hours. i did this on my boots two years ago(hard as rocks) theyve stiffened up just alittle but are still pliable to slide the carbs in and out easily and tighten up. my boots were so hard before i couldnt get them to seal. never a problem since boiling them. i havent found anything else that works for more than a few hours other than this. boiling them in water will soften them for a short time, but then they go back to rocks. something in the wintergreen actually draws out the old gas out of the rubber somehow. i dont know how it works, just know it been working great for me for over two years. i suggest you do this outside or on the grill, i did it in the kitchen thinking the smell would be gone by the time the wife came home, i was wrong.

WD-40 will be the best bet at tracking down where the leak(s) are located, regardless if there are any cracks visible or not.

If you have the original intake manifolds and airbox boots on your bike, then it's almost guaranteed that they've weathered and gone bad over the years = yes, they should be somewhat soft and pliable.

Z1 enterprises has the intake manifolds and airbox boots for a much better price than the dealers will. And IIRC, they also have the rubber plugs for the manifold brass fittings, which I'd also recommend replacing.