how do you set the timing on a big bear 400 engine i keep getting blowback through the carb?

Question

i lined the f timing mark with the line on the timing sprocket and it keeps back firing like its 360 off

Answer 1

Always give year, make, engine size and model # when asking questions.
-The lobes on the cam should be pointing down. You can see by looking in the in & ex inspection covers. No tension on the rocker arms. I think the dowel pin for the sprocket is on the top.
-Bring the piston to TDC and line up the f mark on the flywheel.
-Remove the tensioner.
-Install the chain and sprocket. If you have the cam set up like I said, the timing mark can't be in the wrong position.
-Push your finger through the tensioner hole to remove the chain slack and check the timing again.
-Tighten the cam sprocket.
-Install the tensioner.
-Slowly turn the crank. If you feel any resistance, the piston is probably hitting a valve. Stop, reverse direction and figure out what you did wrong.
-If the motor turns freely, double check the timing again.
-When you get everything right, adjust the valves.
-It's possible you set the timing wrong and bent a valve and that's why it's backfiring.

To check for bent valves, bring the piston to TDC of the compression stroke.
-Turn the crankshaft in the direction that the engine turns when it's running.
-Watch the in valve go down (as in "it's opening") then come back up and line up the f mark on the flywheel.
-The in & ex valve rocker arms should have a little free play. The cam lobes shouldn't be touching the rocker arms.
-Remove the spark plug and look inside (or use a probe to feel) to make sure the piston is at TDC
-With an air gun, blow into the cyl through the plug hole.
-Listen at the ex pipe and air box. You shouldn't hear any air escaping. If you do, sorry...bent valve. The cyl head has to come off.

If you're lucky, it might just be a fouled plug, or you set the valve clearance to tight.

Answer 2

not familiar with that specific engine but all engines 4 strokes are based on the same principle. Locate cyclinder no.1 at tdc on the compression stroke. at TDC both valves should be closed. Now with regards to the firing order you want the plug to fire a few degrees passed TDC. If your getting blow back on yoru carbs that your very off. either that or your intake valve isn't seated properly. check out the image below. From what I hear it sounds like your intake isn't closing fast enough on yrou compression stroke thus teh blow back.