Exhaust popping through intake briggs 10hp

[tkos115]

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.

 

[tkos115]

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.

October 24, 2021

youtube.com

 

[Scrubcadet10]

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

 

[bertsmobile1]

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

 

[tkos115]

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.

 

[bertsmobile1]

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.

 

[tkos115]

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

 

[bertsmobile1]

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 )

 

[tkos115]

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.

 

[bertsmobile1]

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.

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