I have a honda 250 recon & a 350 rancher both are having trouble staying running after being started. The recon 250 will not stay running after starting. It will run only with the throttle held . When I let off throttle it will die. The 350 rancher will start fully choked, but will die after about a minute of high idle. I have checked the sparkplug. It is black & wet. It seems to be running rich. What can I do ?