Briggs backfire through intake

[RMR]

Hello.
I've got a 19.5hp Briggs 31P677 0130 E1 in the shop that has a random misfire/backfire through the carb. I have rebuilt the OEM carb and adjusted the valves with no improvment. I replaced the carb with an aftermarket one to see if things changed, which it did not. Also checked the valves again.
I am attaching a YT like to a video of it running, I'm thinking burnt or carbon stuck valve at this point. Any one have other opinions??
https://youtu.be/aXGMuP6nNyo

Thanks
Chad

[KE4AVB]

Have you replace the intake manifold gaskets?

Also when adjust did you check the amount valve lift?

Both a heavy worn exhaust lobe and intake air leak can cause this. I had one 31 series that would sneeze out the intake randomly. And I already had three 31 series last year with badly worn exhaust lobes causing back fire out the carburetor.

[RMR]

I did not change the intake to head gasket. However I did leak check it with some carb cleaner and showed no signs of leakage.
And I did not check valve lift. Which hind sight I should have thought of that when I went back to do the valves the second time.
I did get the go ahead to dig deeper into the problem, so that will be the first thing I check as well as a leak down test.
On a side note, I have had very mixed results when doing a leak down on small air cooled engines. Sometimes it will convince me of a problem or no problem, and sometimes it just confuses matters more.

[bluemower]

more thoughts:

1. over time, ethanol fuel will gum up valve stems. The valve may not be closing all the way.
2. Carbon deposits in the combustion chamber may be glowing red and causing the incoming fuel mixture to ignite before the ignition event.
3. The valve may not be closing squarely on the valve seat if the valve guide has excessive wear.

[Arkie]

I would replace the spark plug with a new one of the correct type and gapped properly.

If not go I would remove the valve cover and check the valve lift of both intake and exhaust.
I would manually push the valve stems for feel and then adjust the exhaust valve for max lash instead of minimum lash.
If still same symptoms I would adjust the intake for max lash and test.
Be aware that going to max lash on the intake might result in the engine not getting a compression realease when cranking.

Doing each valve lash one at a time might give a hint as to which one if it's a valve.

Is it worse hot or cold????

[RMR]

Update. Now its back to hunting as well as still having the backfire. In no particular order I did the following (aside from the last step. It was hunting before that as well)

Checked valve lift open and closed and both valves match.

Did a leak down (like I mentioned before, it left me even more confused). I had over 50% leakage, but no indication from where. I use a stethoscope with a hollow tube on the end to pin point leaks. VERY little air from the exhaust- nothing near what you would hear with 50% leakage

Pulled both Intake and Exhaust manifolds and did a light test with zero light leakage.

Camera scoped the cylinder with no obvious indication of valve damage.

Changed and gapped spark plug again.

In desperation I even did the unthinkable and drowned it with seafoam snake oil right down the yap.

Replaced both intake gaskets.

That did actually smooth it out if I controlled the volume of liquid. So I'm chasing a lean condition?? However, the plug I pulled out did not look lean, it actually looked great.

This late season equipment has been reeeeeallly testing my patients. I have a couple others fighting me as well that I may need help on. But that is for another day.

Thanks for the input so far ya'll.

[Arkie]

50% leak down leakage needs investigated further.
leak down when checking bad is not lying to you even though it seems to confuse you and us.

When doing the leak down and seeing 50% or more try moving the flywheel little bit more CW as viewed when looking down at the flywheel. (normal rotation when engine is running) pay careful attention to leak down to the first 1/4 turn of the crank. If leak down gets worse as crank is rotated 1/4 turn CW try CCW movement and compare. (see if moving the crank back and forth reduces the leak down leakage somewhat)

How does low idle act with little bit of choke and no choke??

How does the hunting and surging act when running if you add little bit of choke when it's hunting?

You say very little leakage from the exhaust valve. Little bit is like being just a little bit pregnant. Should not be any leakage at the exhaust during a leak down test.
Try pressing on the valve stem manually and see if you get a a erratic leak no leak as the valve's stem is wiggled open and closed while injecting leak down air.

I've seen them clone carbs have hunting issue's and spitting but would usually clean up somewhat as choke was applied by restricting the air intake at the carb throat with fingers or with the choke butterfly thus giving a hint of a engine running lean due to intake air leak or a lean carb itself. I've seen them Nikki's CHINA clone carbs can indicate all kinds of strange lean symptoms.

Post up some pictures of your original carb with the bowl removed and the clone carb with it's bowl removed showing the emulsion tube area and the o-ring area that seals the emulsion tube at the bottom of the emulsion tube at the bottom of the bowl.

[RMR]

OK, last post before tearing it down because I'm to the point that the head needs to come off.
I did another leak down and at the very top of the stroke (past the compression relief, on it's way back down) at TDC, I get leakage from the intake side- Not good!
But if I rotate it slightly CW, the leakage leaves the intake and goes straight to the exhaust side.

What is that all about??

[Arkie]

Before you pull the head take a close look at the exhaust rocker/pushrod action and even set the exhaust lash at max specs and test again.

If you see no leakage from either valve or the leakage switches valves as the crank is slightly rotated and the ACR is not involved would make me want to think about camshaft. (instead of pulling the head) You say moving the crankshaft slightly will get a seal on the intake and exhaust but not both at the same time.

Camshaft lift lobes not acting correctly?????????????

At least the Leak down is showing some results of what the engine is doing when running.

I'm not that familiar with those 21, 31, 33 series of Briggs cams other than the fact some can do some strange things and I have a 31 that ruined the engine when it failed and a low hours 33 series engine that the ACR is not working at all and a 31 donor engine for the 33 maybe.
I see several china clone cams for sale and have heard of some sad stories about those. Seems some of those Single cylinder Intek cams last forever and some shell out at low hours. Mine failed at low hours. (the ACR)

I'm not saying your cam is bad, but suspect of something amiss, I'm just saying that you need to inquire more before pulling the head.

Just a heads up if you ever go looking for a replacement Briggs 33 series camshaft. The oil pump type engine and the non oil pump engine list the same part number. You have to make sure get the correct replacement if it has the oil pump.

[Arkie]

RMR:

What is the latest on that dawg?
Did you get it going?