Thread: 350 backfires through carb

Tweet

Hope somebody can help!Just rebuilt a 1972 chevy 350 with a new
crankshaft,edelbrock torquer plus cam,torker manifold,holley carb,
summit 1.6 ratio rocker arms,HEI distributor.This motor is in a 1979
nova and ran well before.Now when trying to start it backfires
through the carb something terrible.Have tried readjusting the
valves,and thetiming looks good for starting(checked with timing light).
Any ideas?I'm dying over here.

First, go back to basics and recheck for proper firing order, Again, and again until you're sick of doing it. Second, check for enough fuel going into the beast. A lean condition will cause this always.
Those are the two biggest proponents of carb backfiring.

you could be wipping off a cam lobe

Firing order first, intake valve riding open second, vacuum leak third thing to check. My money is on crossed wires.
That's assuming the cam is timed right.

I've checked the firing order countless times and adjusted the valves according to
edelbrocks instructions.I've rebuilt several motors(fords,chevys,and pontiacs)and
they've always started right up.I will check for vacuum leaks and check fuel
pressure as soon as I can.Thanks for the tips.

re check 5 and 7 wire

It sounds like a timing issue. I know that you’ve probably looked at this until you’re blue in the face. The sketch below represents what you should be looking at from the top. This is real basic – but unless you’re building a race motor, the following will get you where you need to be.

Start all over with number one at TDC. Make sure that when the piston is at TDC (as indicated by the timing marks on the harmonic damper) that both valves are closed on number one. If not, your cam is out of time and there may be some big problems. You didn’t mention any bad “crunching noises” so we trust the cam is in time with the crank.

Remove the distributor cap and the spark plug wires. The distributor rotor should be in the approximate position shown. If not, you’re off and need to take the distributor out and re-insert so the rotor matches the sketch. Snug the distributor hold-down bolt but not so tight that the base of the distributor will not turn by hand. Reinstall the wires. I always follow the firing order, i.e., 1-8-4-3-6-5-7-2. This might sound basic – but do it.

Now install your timing light on number one and start the engine – it will start. The little things are really the big things

Good Luck,
Glenn

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.

how old are the sparkplug wires? rotor and cap? do you have points? does it idle at 2500rpm? more info please?

I think he's 180 out also, great info from tech and all.
It doesn't matter where you put the #1 wire as long as the dist cap matches, wires may be short in some cases.
Also make sure the firing order is in the correct direction with the rotation of the dist.
Good luck.

are you using a COMP.CAM? the reason is ask is because i got a bad cam from them for my 455 t/a when they first came out was the first and last time i used there products.long story

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...

This may sound stupid, but is the fuel line connected to the right port on the carb? I learned this the hard way. When I changed a carb once the mechanic put the fuel line on a vaccuum port because that is where the fuel line was originally. So then, everytime I would try to start it, it would catch on fire.

If you are all done laughing at me, it was my first "hot rod" and I dutifully learn from my mistakes.