6-31760, they both run good except the one has an intermitant smoke unit that will go on and then about ten seconds latter go off, and stay off until I hit smoke button again.  I run them in a lash up and the front engine will stay on but the rear does the intermittant thing.  But even if the one engine is ran by itself the same intermittance happens.  I don't run to much smoke but when I do would like to have them both running fine.  Any advice?  Thanks in advance.