I have a 70 harley, when I first start it it runs great, after it warms up I can not adjust the idle, I drive it a while it will start to pop through the carb. The tank is warm to the touch, I have a new vented gas cap on it the tank builds up so much pressure the cap sounds like a tire leak, I take the cap off put it back on it resumes hissing

Most motorcycle problems involve timing or carburetion. Have you checked each cylinder's timing with a strobe light? Never run the timing light off the motorcycle's battery; instead, use a separate battery. Popping through the carburetor sounds like the timing is off. If it's not that, I would remove, clean, and rebuild each carburetor. With duel carbs, you need to adjust them together, so that the exhaust pressure is equal. Only use the same plugs that originally came in the Harley. Cycles are hard on plugs.

Thanks for the info.

On cycles of that vintage, the left cylinder's timing was adjusted by rotating the distributor's base plate. The right cylinder's timing was adjusted by changing the point gap. It's best to install a new set of points and condenser--filing usually doesn't work well at all. Clean your feeler gauge with brake cleaner. Ignition systems on those old cycles were weak to begin with--you don't want to attenuate the spark by contaminating the points.

Before doing any of this, remove and fully charge the cycle's battery with no more than a 2 amp charger. You can use a ampere damper on a 6 amp charger. Weird stuff happens on cycles, when the battery is not fully charged and battery terminals and connections are not clean.