Possible serious problem on a 350 chevy?

Question

I put in gas in my 74 chevelle with a mildly modified 350. I put in somwhere around 8 gallons of gas with part if a bottle of octane boost. I don't know if the fuel had anything to do with this but soon after I got gas it started to make a loud noise and it wanted to die at stop signs. I later fond out that compression is coming back through the carb.. when I took out the number 4 spark plug the compression stops coming through the carb. The number 4 cylinder has close to 150 psi of compression and so does the number 3 cylinder, those are the only 2 cylinders that i have had checked. So whats the deal is it an intake valve not sealing or what?

I checked the timing its about the same as it has been I think. Also valves were recently adjusted.

Answer 1

if the compression is exhausting out of the intake valve and manifold up through the carb then the problem is with an exhaust valve that is not opening. That usually means a worn lobe on the camshaft on the chevy 350. as compression is checked with the valves closed a worn cam lobe won't show up on a compression check. It could also be caused by a bent pushrod for that cylinder although that's pretty rare.

the other possibility is that the rocker arm for the exhaust valve on that #4 cylinder has had the lock nut come loose and back off so that the rocker arm is not pushing down on the valve spring to open the valve. either way the best thing to do is to pull the valve cover off on that bank and look at the rocker arm for the #4 exhaust valve. It should be easy to see--its the valve that lines up with the exhaust header for that cylinder. if the rocker is tight, start the engine and see if the rocker is moving up and down. if its not moving, or moving very little either the cam is worn or the pushrod is bent. be careful not to run the engine for very long and keep back when running the engine with the valve cover off as oil will splatter. its best to check this with the engine is cold as doing so on a warm engine will throw hot oil alll over the place

my guess is that the cam is worn
bad news, but I hope that helps

Answer 2

I own a shop, and build many 350 small block engines. If you got compression coming back through the carburetor, you have either a lobe worn on the cam, a bent push rod, a loose/worn out rocker, or a burnt valve. Run a vacuum test to see what it is doing. I'll bet the needle is all over the place, and if you read the chart for the vacuum gage you will see what I mean. I have seen the lifter get a pin hole worn in it from a worn out cam & lifter set, thus causing this problem. Either way a vacuum test should help you locate the problem. A timing problem such as the rotor button, or cap causing a crossfire will do this too. A worn out distributor can do this. See if you can move the shaft from side to side in the distributor. Put a timing light on the engine to see if the timing is way off, or moving around. It could be the timing chain & gear set going out. 150 pounds of compression is good, and I don't think you would have that much with a burnt valve. This may sound crazy, but I have saw an engine do this with a bad spark plug as in the ceramic broken causing fire to jump. Fire jumping to the center post through the rotor button will do this. Look for a carbon trail that looks like a pencil has been drawing inside the distributor cap. I really don't think the octane booster is doing this, unless you got a bad bottle of it for some reason. Make a line that will come off the fuel pump into a jug of gas. This will tell you if you have bad fuel if it stops doing it. The vacuum gage is your best bet.
Honda Guy is very correct in what he is saying. I agree 100% with him. I would rule out the firing thing with the cylinder, before I pulled the valve cover. With it running you will be able to see a valve that is not opening up, as it will look different. You can make a cardboard shield to keep the oil from going everywhere, and don't rev the engine up, or you will have hot oil all over you and the car. If nothing is broke, bent, or worn out, then you must for some reason have a plug firing at the wrong time forcing the exhaust out the intake.
Glad to help out, Good Luck!!!

Answer 3

Chevy 350 Compression

Answer 4

You have either got a valve adjusted too tight or you have just ruined a camshaft. Backfiring through the carb. when the motor gets revved is one of the smyptoms of that. Compression tests will not find this as it is the valve train having a problem, not a burnt valve. I would suggest removing the valve covers and watch the motor as someone else turns it over. Chances are the rocker arm that barely moves has no cam lobe left on it. I am wondering why the valves were adjusted, once they are set that is it unless you were having a problem. More checking nec. here. This is a big and ugly job if you need a new cam. Good luck.

Answer 5

Valve adjustments should be done on a warm engine while it's running. If adjusted cold, they might be too tight when it warms up, causing the problems you describe. If you race it or hot rod it, it could have bent a valve, but the compression seems OK. The octane booster could have knocked a chunk of carbon deposit loose and it stuck to a valve, a valve guide could be excessively worn, or the timing gear lost a tooth or 2. The plastic gears are bad about that. You might check for a broken valve spring, too. That's a good possibility, also.

Answer 6

If you have 150 psi the values are sealed on the number 4 and number 3, you may have the timing out or the engine may have jumped timing and caused the problem. It probably has nothing to do with the octane boost.

Good Luck,

Answer 7

if any of the valves were not closing all the way and sealing you deff would not have that much compression in that cyl. sounds like the exaust valve is not opening on that cylinder for some reason, then when the intake valve opens the exaust comes out that way.

Answer 8

i own a repair shop and Chevy,s was real popular for bad timing chains and gears ,and you may also have a bad valve on it but the compression was good on it 150 pounds is good,you usually wont get that much compression if you have a bad valve on it,so id be looking at the timing chain and gears right now on it,good luck i hope this helps.

Answer 9

Possibly a crossed plug wire? Fuel had nothing to do with it, and quit with the "octane boost". Stuff is crap. "Points" are (.1) not (1.0) or (2.0) or whatever the bottle promises. Turns 93 octane into 93.1. Not much of a performance boost. Buy higher octane fuel if you need it.

Answer 10

Valves may be burned, make a compression test on all cylinders, they should be similar.