5.7 back fires through carb but timing is good.

[rydog]

I take it that you have the intake off at this moment?

I don't but I can have it off pretty quick If I need

 

[legopnuematic]

It’s got to come off if you are going to pull the heads anyways. If I were you I’d-

-Pull the intake
-Remove (loosen) all of the rockers so the valves are all closed
-put your leak down tester on one plug hole at a time
-fill the intake ports with water, kerosene, diesel. Some liquid that won’t evaporate too quickly, maybe like marvel mystery oil. Some thing fairly thin. Probably don’t need too much, just enough to cover the valve and see/hear bubbles
-pressurize cylinder with leak down tester one at a time. Observe for bubbles from intake ports.
-also observe if when you add fluid to intake runner if the level remains steady. It the level drops quickly then there is a major valve sealing issue. (If the chosen liquid evaporates too quickly it’ll be hard to tell if it’s leaking into the cylinders or not.

That should show visual proof of poor intake valve sealing.

That’s my thinking at least.

 

[75 Chevy SWB 4x4]

Before you pull the heads, just go through and readjust the valves. If you have an intake valve that's too tight, it can stay open and cause a backfire through the carb. After you get to zero lash, I usually just go 1/2 turn more on the rockers. Service manual used to call for 1 full turn, but with any aftermarket cam, lifters, and springs, the motor wouldn't run normal.

It'll be a lot faster to pull the valve covers, and readjust the valves, and then see if it will run normal without back firing, dieseling, etc.

 

[rydog]

Before you pull the heads, just go through and readjust the valves. If you have an intake valve that's too tight, it can stay open and cause a backfire through the carb. After you get to zero lash, I usually just go 1/2 turn more on the rockers. Service manual used to call for 1 full turn, but with any aftermarket cam, lifters, and springs, the motor wouldn't run normal.

It'll be a lot faster to pull the valve covers, and readjust the valves, and then see if it will run normal without back firing, dieseling, etc.

When I was doing the leak down test I had the valve covers off and when I seen I was tdc I felt that the pushrods had no pressure on them and that there wasn't a bunch of play between the push rod and rocker arm. They all seemed fine isn't that a decent check?

 

[rydog]

It’s got to come off if you are going to pull the heads anyways. If I were you I’d-

-Pull the intake
-Remove (loosen) all of the rockers so the valves are all closed
-put your leak down tester on one plug hole at a time
-fill the intake ports with water, kerosene, diesel. Some liquid that won’t evaporate too quickly, maybe like marvel mystery oil. Some thing fairly thin. Probably don’t need too much, just enough to cover the valve and see/hear bubbles
-pressurize cylinder with leak down tester one at a time. Observe for bubbles from intake ports.
-also observe if when you add fluid to intake runner if the level remains steady. It the level drops quickly then there is a major valve sealing issue. (If the chosen liquid evaporates too quickly it’ll be hard to tell if it’s leaking into the cylinders or not.

That should show visual proof of poor intake valve sealing.

That’s my thinking at least.

Seems logical to me. I might go pick me up some marvel mystery oil after work and try it.

Seems like a good way to avoid buying one of those smoke machines.

 

[fast 99]

40% is high but that alone isn't enough to condemn the engine. Old cold [when tested] engines can read high and be good runners.

Was it leaking by any of the valves or into crankcase?

 

[75 Chevy SWB 4x4]

When I was doing the leak down test I had the valve covers off and when I seen I was tdc I felt that the pushrods had no pressure on them and that there wasn't a bunch of play between the push rod and rocker arm. They all seemed fine isn't that a decent check?

No, that's not the correct way to check valve clearance on hydraulic lifters.

If you are able to have the engine running, I remove one valve cover at a time and start the engine and let it idle. Then I back off one rocker nut at a time until it starts ticking. Then I slowly run it back in until the ticking just stops. Then I look at my 3/8" ratchet for which direction the handle is facing, and I give it one half turn in more. Then I move on to the next valve doing the same exact process until you have all 8 valves done on that head. Put the valve cover back on and tightened down properly and do the other valves on the other head the same way.

 

[rydog]

No, that's not the correct way to check valve clearance on hydraulic lifters.

If you are able to have the engine running, I remove one valve cover at a time and start the engine and let it idle. Then I back off one rocker nut at a time until it starts ticking. Then I slowly run it back in until the ticking just stops. Then I look at my 3/8" ratchet for which direction the handle is facing, and I give it one half turn in more. Then I move on to the next valve doing the same exact process until you have all 8 valves done on that head. Put the valve cover back on and tightened down properly and do the other valves on the other head the same way.

The old thing has solid push rods not hydraulic lifters

Ohh seems like I'm wrong but not running anyway.

Getting tempting to just spend 2500 bucks on a crate motor I've never been into internals too much

 

[rydog]

40% is high but that alone isn't enough to condemn the engine. Old cold [when tested] engines can read high and be good runners.

Was it leaking by any of the valves or into crankcase?

To me it seemed like air was going out the intake/carb.

The pull the intake and use marvel mystery oil to look for bubbles seems like a good idea.

Getting bored of testing though and kinda just want to start replacing and see what happens

 

[DoubleDingo]

.....Getting bored of testing though and kinda just want to start replacing and see what happens

Don't. If you do you'll never know what is causing it. You have gotten solid advice, run through the tests, ask the questions, and then we'll all find out together. If you shoot the parts cannon at it you'll just waste money and ultimately get even more frustrated and park it or sell.

 

[77Dmax]

I would check for a flat cam. Turn it over with the valve covers off and have a look.