Motorcycle Repair: Timing a Bike 1981 Yamaha Virago 750, yamaha virago 750, tdc mark

Question
QUESTION: can you help,im trying to time my 1981 yamaha 750 virago

ANSWER: Stephanie,I have a question why does the bike need the timing set? Were the cylinder heads off or the chains or the cams replaced or was the chain tensioners replaced,just wondering..

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QUESTION: sorry...one of the heads were changed on the bike.now when you try to start the bike it back fires, so it sounds like its off time..

Answer
Look at the cam on the head that wasnt worked on. Pull the timing plug off thats on the left side of the engine also pull the center cap off so you can put a socket on the crank bolt. Turn the crank so you can see the timing mark for TDC for the cylinder head thats still original. The intake lobes on the cam should be closing the intake valves and the timing marks for that cam should be in the window. Look at the marks on the cam there may be a TDC mark and lines that may align with the top of the head. If so do the same for the head in question there should be its own timing marks on the flywheel. If the marks are all lined up it may be a question of valve clearance. Check the clearance intake should have 3-5 thousanths and the exhaust should be 5-7 thousanths. Try this to see if it will help.