According to this page http://www.rcplanet.com/Glow_Plugs_s/870.htm the MC-8 would be a terrible choice for any four stroke engine.

Mc Coy Glow Plugs Chart

MC-4C : 4-cycle plug and used in all Saito engines.

MC-55: The MC-55 is considered a hot plug and is designed to run on fuel with a low nitro content. Applications for this plug would be planes, tether cars, etc.

MC-59: Like the MC-55, the MC-59 is considered a hot plug. The difference between the 55 and 59 is that the MC-59 is designed to run on fuel with a higher nitro content (low to medium nitro content) than the MC-55. Applications: Boats, planes, helicopters, and cars.

MC-8: The MC-8 is a medium to cold plug, designed to run on fuels with a medium to high nitro content. Applications: Boats, cars, and planes.

MC-9: The McCoy MC-9 is considered a cold plug and is designed to run on fuels with a medium to high nitro content. Applications: Cars, boats, ducted fan and Giant Scale planes.

MC-14: The MC-14 is a hot plug, designed for helicopters and 4-stroke engines. This plug has a 24K gold stem.

=======================

First I would get a 'four stroke' glow plug. I like the OS-F but others probably also work well.

Next I would check the valve lash and adjust if needed.

Gray Beard mentioned making sure the fuel pump is working correctly. I do not know anything about pumps but it makes sense that it could wear out or get debris in it. You know it is at least three years old. I have no idea how long the pump diaphragm and other parts should last.