Briggs 31R headache...

[StarTech]

Well this darn engine is back again. It is the second that misfiring I had in a month. This had a blown head gasket at first. I replaced it and mowed an acre before returning the mower without any further problems. Well I was fool by this one. It was used by the less than 30 minutes and now it is backfiring [I mean out of the carb]. I haven't found a thing out of the things normally causes this. Valves are in time, Flywheel key is intact, ignition coil was replaced with a known good one. No carb leakage, Intake manifold sealed.

In 18 years I never had one to do this without an oblivious reason. Look like I am going have to call this one and return the customer his money. So embarrassing that I can't find the source of the problem. There got to be something that failed but what ? Apparently it is one of those once in a lifetime problems.

 

[ILENGINE]

Have you checked valve lift to make sure you don't have a worn lifter. Also something blocking the exhaust causing back pressure.

 

[slomo]

Back slamming out of the carb is improper ignition timing, super lean air/fuel mix, valves out of spec or weak spark.

 

[StarTech]

Valve lift identical on both rockers.

And I will just remove the muffler after resealing the rocker cover. I have thought about that being a very rare case. Never seen one but anything is possible.

BTW the spark plug is carbon fouling so I don't it is a lean mixture and besides it is impossible to adjust the mixture other than enlarge the main jet. Plus if I did oversize it there is no replacement for that Ruixing carb. Just another carb.

And as the spark both plug and coil have been replaced. And the flywheel magnet is strong.

As I said this is for the books.

 

[ILENGINE]

Valve lift identical on both rockers.

And I will just remove the muffler after resealing the rocker cover. I have thought about that being a very rare case. Never seen one but anything is possible.

BTW the spark plug is carbon fouling so I don't it is a lean mixture and besides it is impossible to adjust the mixture other than enlarge the main jet. Plus if I did oversize it there is no replacement for that Ruixing carb. Just another carb.

And as the spark both plug and coil have been replaced. And the flywheel magnet is strong.

As I said this is for the books.

In my 28 years I have seen 3-4 mufflers were the internal baffle broke loose and blocked the outlet

 

[Hammermechanicman]

Pull the head and check the intake is seating and sealing properly. Had one where the valve looked good but gave random backfire in the carb. New valve and a lapping job fixed it.

 

[slomo]

And as the spark both plug and coil have been replaced.

Are they resistor plugs? What is the resistance measurement through the plugs? Run some non resistor plugs. I try to cross over non resistors for all my OPE.

How large of a gap will the coil fire?

Fouling plugs can be weak spark.

Valve lift identical on both rockers.

How about cam timing at the cam gear?

Far as the muffler goes does it puff out air when running? Does it sound like a jet engine where you can hear the restriction in the muffler? Like an old Cadillac 472 V-8 though single exhaust? SHHHHHHHHhhhhh.

 

[StarTech]

Done did both a leak down test and pulled to head due a bad head gasket.

 

[StarTech]

Are they resistor plugs? What is the resistance measurement through the plugs? Run some non resistor plugs. I try to cross over non resistors for all my OPE.

How large of a gap will the coil fire?

Fouling plugs can be weak spark.


How about cam timing at the cam gear?

Far as the muffler goes does it puff out air when running? Does it sound like a jet engine where you can hear the restriction in the muffler? Like an old Cadillac 472 V-8 though single exhaust? SHHHHHHHHhhhhh.

Of course it is a known good 2460 NGK. As far the spark the coil and plug are from my personal mower. And the flywheel holds a 10 inch screwdriver out horizontally.

I'm pulling the muffler tomorrow, after the RTV sets up.

 

[slomo]

Ok so most everything mechanical sounds like it's working as normal.

Dump the gas and try some fresh E-0 unleaded juice.

Wonder if someone, has been in the carb with a welding torch tip cleaner enlarging the main jet/s?

Choke staying wide open once warm?