Exhaust popping through intake briggs 10hp

[Richard Milhous]

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.

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.

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

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.

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 )

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.

 

[tkos115]

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...

 

[Scrubcadet10]

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.

 

[tkos115]

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

 

[Gord Baker]

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

 

[mystic240]

Did this happen at once?

 

[Jml]

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.

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.

 

[tkos115]

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

October 27, 2021

youtube.com

 

[PTmowerMech]

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.

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.

 

[Scrubcadet10]

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.

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