Best Time for Motorcycle Adventure
What the best time of the year to get adventurous on a motorcycle and be able to see a lot of places via pavement and dirt roads.
Alaska is doable on 2 wheels May through September. The best times, in my experience, is June through mid-August. As I get older, I am more inclined to ride when the weather is the driest and warmest.
If you haven't already, check out Advrider.com and follow some of the Alaska ride reports. Also, get a copy of the Milepost travel guide to the Yukon and Alaska (money well spent).
I concur with Libertine. If I was planning a trip I'd probably shoot for sometime between mid May and mid June. That would give you the best chance at staying dry and seeing long cloud free vistas. Also it seems like the tourists don't show up in big numbers until July when they are sick of the heat in the L48 so traffic shouldn't be too bad. If you wait until August you will have a much higher probability of rain and lots of hunting traffic in the interior but it is beautiful that time of year.
I suggest that you take tire choice seriously for dirt and gravel roads especially if you proceed when wet. Sometimes the conditions are fine for hundreds of miles then turn scary quick so don't get complacent.
A lot of that depends on whether you are camping or not. I made one late August - early Sept trip to Montana. Left Fbx in 39 degree rain and was very chilled by the time I reached Tok. An hour break for lunch in Fast Eddies in Tok warmed me up enough. Once into Canada the weather kept improving until I hit 100 degrees in MT. Trips in June - early July through the Yukon tend to be very buggy if you are camping.
Labor Day weekend of 2013 a buddy and I attempted a run to Whitehorse and over to Skagway which we knew was very iffy. We were hit by torrential rains coming into Haines Jct and laid up for a few hours to dry out. Finally caught some nicer weather heading toward Whitehorse but decided to camp to dry out by a fire and drink a few beers. The next day we rolled into Whitehorse under warm blue skies. The road to Skagway was snowy & icy so we turned around in Whitehorse.
Too late to keep it short but after the 5-6 trips I've made to the Yukon and beyond, I'd aim for mid-July to mid-August for the usually better weather and lessening of bugs. Traffic will be high so be careful. Keep an eye out for Liard Hot Springs in northern BC. If you are considering a side trip up the Dempster Highway toward Inuvik, NWT, count on heavy bugs the farther north you feel like exploring. Tons of places to pull off and camp in Canada unless you are looking for "campgrounds". Bring an extra 2 gallons of gas to ensure you do not rum out. I can get ~180 miles/tank and have rolled into stations on fumes.
Enjoy the trip, it's great ride. Set aside enough time to have time to lay up for adverse weather. Sometimes shi* gets crazy on the way here or back. Make sure the tires are new or nearly so, some of the gravel roads can tear them up.