Thread: 350 backfires through carb

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Hi peoples,I'm back.I've checked for vacuum leaks and have plenty of gas,
even replaced the fuel pump.I took the valve covers off and while cranking
looked for any sticking valves,but there were none.When trying to start
this thing it will backfire and start the inside of the carb on fire.Has anyone ever heard of a camshaft being ground wrong?I'm at my wits end here
and have never seen anything like this.Any ideas?

I believe your 180* out.

Remove #1 spark plug and rotate the crank clockwise until you feel pressure building against your thumb while holding your thumb, or your buddy's thumb, over the spark plug hole. Watch the harmonic damper ring and bring the TDC mark on the inertia ring up to match the timing tab on the front cover. Now you are at TDC firing #1.

Remove the distributor cap and verify the rotor is pointing at about 5:30 O'Clock as you stand in front of the motor. (more or less pointing at #1 cylinder). If not, remove the distributor and re-stab it, positioning the vacuum advance can so that you will have room at the manifold runners to turn the distributor housing clockwise and/or counter-clockwise to change initial timing later. Plug #1 plug wire in at the 5:30 position, matching up with the rotor. Going clockwise around the cap, plug in the remainder of wires in firing order....1-8-4-3-6-5-7-2.

On the front of the crank pulley, make a mark or attach a little piece of masking tape at 12:00 O'Clock, 3:00 O'Clock, 6:00 O'Clock and 9:00 O'Clock.

Back off both rocker nuts so that you have a little play. Grasping the pushrod with your thumb and forefinger, jiggle it up and down while you hold the rocker arm tip down against the valve stem tip. Adjust the nut until all jiggle room is taken up and you have zero lash. Give the nut an additional 1/2 to 3/4 of a turn. Do both the intake and exhaust on #1 while you are there.

Rotate the crank clockwise 1/4 turn (90 degrees) so that your 9:00 O'Clock piece of tape is now at 12:00 O'Clock. Back off the intake and exhaust nuts on #8 cylinder and adjust both valves the same as you did on #1.

Rotate the crank another 1/4 turn so that the 6:00 O'Clock piece of tape is now at 12:00 O'Clock and adjust both valves on #4 cylinder.

Keep rotating the crank 1/4 turn at a time and continue through the firing order until you have all valves adjusted. 1-8-4-3-6-5-7-2. Driver's side front to rear, 1-3-5-7. Passenger side front to rear 2-4-6-8.

It takes a lot of experience to get the valves adjusted correctly if you are using the "spin the pushrod" method of removing lash at the rockers. I would guess that 99% of the fellows doing it this way will get the valves too tight. This "jiggle up and down" method was passed on to me by Denny W and is bulletproof.

Originally Posted by sd455

Hi peoples,I'm back.I've checked for vacuum leaks and have plenty of gas,
even replaced the fuel pump.I took the valve covers off and while cranking
looked for any sticking valves,but there were none.When trying to start
this thing it will backfire and start the inside of the carb on fire.Has anyone ever heard of a camshaft being ground wrong?I'm at my wits end here
and have never seen anything like this.Any ideas?

I dont know a whole lot about engines, BUT I had this same exact thing happen to my chevy 355 engine. I found out it was the + battery wire to the distributor.. It had a break in the wire and only a few strands holding it together... maybe something to check out...