Misfiring engine

[Ian Williams]

If the engine has sat for three years you may find that, depending upon storage conditions, the valves are no longer sealing properly due to corrosion on the seats. Check compressions first to eliminate this from the equation, if they are low or inconsistent regrind the valves. I would actually always recommend you do this when recommissioning an engine as any drop in compression will noticeably affect performance. As others have said it sounds much like an ignition problem consistent with fouled plugs or weak spark, however a weak mixture can also cause misfiring. What do the plugs look like when you remove them, are the black and sooty, black and oily, a greyish brown, or whitish grey? Are they all the same or or is one or more cylinders inconsistent with the others? You should get a good indication from the plug colour/colours where the problem lies.