Exhaust popping through intake briggs 10hp

[Barney56]

So.... I feel like a dope but I noticed something when I put the head back on and checked the rocker arms again. Sometimes I will get the full lift out of the exhaust valve, and other times, (most of the time now) it only opens a very little bit. I can't say I've seen that before but I'm guessing a bad lifter? Maybe broken or bent on one side of the contact face of it? I think if it was a camshaft it would be constant so my thinking is that since the exhaust valve isn't always opening properly, it's allowing the extra exhaust gasses to stay in the chamber and be compressed a bit more until the intake valve opens and then it blows out that way. Ugh.. I guess I should have payed more attention initially...

That was going to be my suggestion, pull the valve cover and watch the rocker arms. I have that same engine on my Chipper. Same exact thing happening. I ordered a new cam and lifters for it. Just need some time to change it.

 

[georgPru2]

I have a 10hp OHV briggs engine on a yard blower. It starts up fine and idles but it pops like crazy through the intake as soon as it goes up in rpm. If I take the air filter out it sounds like a 2 stroke engine without a muffler. The engine sounds like its struggling and it never really gains much rpm as the popping gets worse.

Has new carb and plug, both valves are set to spec for clearance. They seem to open and close as they should, and the same amount as each other. Seems to hold compression well when pulling over slowly. Any ideas? I'll try to post a link to a YouTube video of it.

Have a leakdown test done. Compression may be seeping back thru int vlv when it fires, igniting fuel/air in intake manifold.

 

[tkos115]

Well I took the engine apart as a leak down test showed the valves as being good. The exhaust valve cam lobe is pretty much worn down to nothing. I ordered a new cam and the gaskets needed to do the repair. No idea why it was opening a lot more on occasion but I suppose it was just me seeing what I wanted to see hah.

 

[bertsmobile1]

I'm just thinking out loud here.
Backfiring through the carb, means the fuel is ignited while the intake valve is still open.. Or has a gap big enough for exhaust to get through, Correct?

So there's only two things that could make that happen. Firing at the wrong time or valve problem.

A warn cam wouldn't allow this to happen, would it? Since the lobe is shortened.

IT depends if what Ikos115 is actually seeing
To come back through the carb obviously the inlet valve must be open and there must be more pressure inside the cylinder than outside .
Some times it can be caused by the exhaust not opening completely so the engine fires but the exhaust can not get out so when the inlet opens the exhaust which will still be around 50 times the volume of the cylinder at ambient pressure will blow back through the carb .
Normally it will just whoosh out but some times it can cause an secondary ignition when mixed with fresh fuel & air.
If this is rushing out obviously little to no fuel will go back in so the engine comes to a stop.
Then you can have over heating so a glowing ember of carbon anywhere inside the cylinder can ignite the incomming charge
Loose carbon can get stuck under the valves stopping them from closing at random times so compression comes & goes.
But as he has found the exhaust cam worn down it is just excess pressure in the cylinder when the inlet starts to open escaping through the carb.

 

[VRR.DYNDNS>BIZ]

Either flywheel key partially sheared effecting timing OR intake valve to tight OR valve guide has migrated and valve does not close all the way. Since you said it seems to have good compression when you pull it over, either valve guide interference with piston or you actually have compression in which case if it still has real compression on a slow pull and not a sudden interference with the piston, then must be timing firing before the intake valve closes. In all cases, it is firing with the intake valve open - guide migration, timing ( key), valve lash tight, or as one suggested but less likely, the valve seat is loose in the head and bypasses when firing. One last possibility - burnt valve or seat but again, compression loss. There is also the possibility of a mechanical decompression cam is used instead of a extra lobe cam and it is bent and always stuck up holding the intake valve open a bit, but again compression issues.

 

[Barney56]

Have a leakdown test done. Compression may be seeping back thru int vlv when it fires, igniting fuel/air in intake manifold.

Pull off the rocker arm cover, pull the starter rope and watch the rocker arms. If the exhaust rocker isn’t moving as much as the intake, the engine needs a camshaft.

 

[tom03]

I was thinking intake valve spring flowting at higher rpm

 

[ARN Greencare]

Dude, your cam is worn out. It's obvious from the video you posted of the valves working.

 

[Scrubcadet10]