First, we'll calculate the amount of carbon that could be emitted by a volcano. We'll use the Mount Saint Helen eruption of May 18, 1980 as our model, but we'll assert that 100% of the material released is coal and all of its carbon enters the atmosphere in the form of CO2.

The volume of material released was 3.7 billion cubic yards (Reference) = 2.6 billion cubic meters.

Coal can be 1.5 times as heavy as water, so a cubic meter could weigh 1500 kg, and as much as 75% of it could be carbon, so each cubic meter could contain 1120 kg of carbon.

This fictitious eruption, then, would release 2.9 * 10^12 kg or 2900 million metric tons of carbon.

The atmosphere contains 827,000 million metric tons of carbon.

This amount is 285 times as much carbon as the fictitious volcano could emit. So, such a volcano would inject an amount of carbon into the atmosphere equal to 0.35% of what's already there. In comparison, the concentration of CO2 in the atmosphere is rising about 0.5% per year. For volcanoes to cause such an increase, there'd have to be a major volcano every eight months, and the material released would not be ash or lava, but 100% coal.

For comparison, we can see the effects of volcanoes in 1982 and 1991.

http://www.noaanews.noaa.gov/stories2006/s2654.htm

http://www.esrl.noaa.gov/gmd/aggi/

What we see is that there was no upward spike in CO2 concentration in those years. Whatever effect they had was imperceptible.