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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.

This is a link to it running, the throttle is close to wide open and the loud poping sound is coming from the carburetor intake.

I would check the flywheel key... have seen them sheared just enough to run bad...

Cam is bad
Valve is bent
Valve seat is loose
Time to pull the head or do a leakdown test if you feel inclined

I was wonderings about it being an ignition issue as well. However it does seem like a valve issue from the way it is popping back like that. I will probably end up pulling the head to see what's going on. For what it's worth too I probably remove the rope start assembly and hub so I can see if the flywheel key sheered first though just to make sure.

The only things I've physically checked so far on the valves/head, is valve clearance and that they both appear to be opening and closing the same amount.

Too far advanced and the engine will not start, backfire violently through the carb, run fast if it does start but with a drastic loss of power.
Too far retaarded and the engine starts easily, backfires through the muffler and looses power the faster the engine goes but usually will never get to full speed.

Could a bad igntion coil cause this? I seem to be getting a pretty weak spark from it, sometimes sporadically it seems. It might just be I cant get a good ground on the plug too. Personally I think that's too easy. It sounds like a loud exhaust coming right through the carb when the throttle is set to 1/4 or more throttle

NO
If the coil does not produce a spark then all that happens is the unburned fuel goes into the muffler when it may be ignited by the following hot exhaust or a red hot part inside the muffler.
If the timing key in the flywheel is sheared and the spark is arriving while the inlet valve is still open then you will get a backfire through the carb and then generally the mower will stop as this cause it to starve itself .
qA mower engine is not a magic prop from Penn & Tellers's Fool Us
It obeys the rules of physics as such the magneto is a device that converts MECHANICAL energy ( from the rotating flywheel ) into ELECTRICAL energy using the magnetic fields as the conversion agent .
The MECHANICAL energy is of course proportional to the SPEED that the flywheel is moving
The faster it moves the more energy that is available to be converted the bigger the spark
Now these systems are optimumised to speeds of around 3500 rpm where the engine should be running and because of this cranking speeds are just barely within the operating range of the magneto system so the spark you get at cranking speeds is poor .
But don't take my word for this.
Redo the experiment you should have done in high school by pulling the blower housing off and rigging up an electric drill with a socket on the flywheel nut
take the plug out and clamp it tight to the engine, block off the fuel supply ( don't wand a fire or explosion ) then spin the flywheel at different speeds. best done in the dark
Make sure feet are well away from any thing underneath ( removing belts is a good safety idea )
If you want to have some fun, you can change the gap and see the wider the gap the faster the engine has to turn to produce a reliable & regular spark and then if you have them a couple of different plugs and see when they start to fire and at what speeds the spark becomes consistent
Stand on something that is wooden and do not touch the mower anywhere els or you might very well also confirm a spark will follow the path of least resistance ( ouch )

Gotcha, so in the mean time I did yank the head off it and pull the valves out. As far as i can tell the guides are tight and the valves are sealing as they should. The seats are tight and clean and the valve faces look good. I might lap them anyway while its apart. The flywheel key is good. I did check that before pulling the head off. Pushrods are also straight and I measured the lift and they are both approximately the same maybe minus a couple thousandths to get technical.

tkos115 said:

Gotcha, so in the mean time I did yank the head off it and pull the valves out. As far as i can tell the guides are tight and the valves are sealing as they should. The seats are tight and clean and the valve faces look good. I might lap them anyway while its apart. The flywheel key is good. I did check that before pulling the head off. Pushrods are also straight and I measured the lift and they are both approximately the same maybe minus a couple thousandths to get technical.

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Did you remove the inlet valve all together then try to pry the seat out ?
It must be in there tight

bertsmobile1 said:

If the timing key in the flywheel is sheared and the spark is arriving while the inlet valve is still open then you will get a backfire through the carb and then generally the mower will stop as this cause it to starve itself.

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More likely it will fire during the compression stage and cause a violent backlash. I don't think an engine firing during the intake stage at cranking speed could possibly produce enough force to get through another cycle and actually start.

bertsmobile1 said:

The MECHANICAL energy is of course proportional to the SPEED that the flywheel is moving

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Mechanical energy is proportional to the square of speed. Voltage is proportional to the speed. Double the speed and you double the voltage, which roughly quadruples the energy /per spark/, but you also double the number of sparks. 8x the power. Except in the so-called "Southern Hemisphere" which I doubt even exists.

bertsmobile1 said:

Stand on something that is wooden and do not touch the mower anywhere else or you might very well also confirm a spark will follow the path of least resistance ( ouch )

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Couldn't tell you how many times I grabbed a coil wire or plug wire to check for voltage. This has NOTHING to do with memory loss... just saying I don't remember.

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...

you should get slight open as the piston nears TDC on the comoression stroke, that's the automatic compression relief. makes starting easier. on some models its on the intake.

Yeah I can still see it do that, but it can take several revolutions sometimes before it will open more.

Sounds like a Lean Condition to me. Remove and deep clean the carb and add NEW fuel.

Did this happen at once?

tkos115 said:

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.

Click to expand...

I replaced the carb on a rear engine riding mower and it did the same thing. Backfired through the air intake. Figured running lean so I bored the jet slightly larger. No change. Checked valve clearances, flywheel key, everything. Finally took the old carb and used the bowl cover and gasket from the new one and it ran perfectly.

This is the video of the valves, I'm gonna take the engine apart tonight and see what the cam looks like.

bertsmobile1 said:

Too far advanced and the engine will not start, backfire violently through the carb, run fast if it does start but with a drastic loss of power.
Too far retaarded and the engine starts easily, backfires through the muffler and looses power the faster the engine goes but usually will never get to full speed.

Click to expand...

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.

PTmowerMech said:

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.

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A worn lobe wouldn't open it as long....

tkos115 said:

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...

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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.

tkos115 said:

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.

Click to expand...

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

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.

PTmowerMech said:

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.

Click to expand...

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.

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.

georgPru2 said:

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

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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.

I was thinking intake valve spring flowting at higher rpm

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