BACK FIRE

[drp198 0]

i have a 16 hp twin briggs on my work horse and today i switched the gas filter and i changed the oil but i put 10w-30 synthetic blend in it instead of regular oil. i was driving it around on high throttle like i usually do and it sounded good then i went to park it. i put the throttle down and turned it off and it back fired! its never back fired before,  is it because the oil i put in it?

[oldlineman 1,450]

Not sure about the oil would not think that would cause it, but you should let an engine idle for a little bit before turning off.

[lynnmor 6,812]

I think that is caused by the ethanol gasoline that causes a lean condition..  I let mine idle for about a minute before turning it off.  The newer lean carbs add to the problem, I've replaced them with older models and the problem, for the most part, went away.  It is possible that you need to clean the carb, but first try some ethanol free gas if available.  Higher octane might help as well because it won't detonate as quickly.

[shallowwatersailor 3,213]

On my two 5xi with Kohler Command twins, I let them idle down (I believe that is in the owners manual as well) and then when I turn the key push the throttle to full on. That prevents the backfire. Backfires will eventually ruin the muffler first.

[pfrederi 17,169]

My 1951 Case VAC does the same thing if I shut her down after right after I finish mowing.  Let her idle for a couple of minutes or drive slowly back to the barn no problem.  

[bmsgaffer 2,043]

2 hours ago, shallowwatersailor said:

On my two 5xi with Kohler Command twins, I let them idle down (I believe that is in the owners manual as well) and then when I turn the key push the throttle to full on. That prevents the backfire. Backfires will eventually ruin the muffler first.

 

The Commands all have the fuel shutoff solenoid, don't they? When you turn the engine off it actually starves it of fuel, so there will be no fuel allowed through to backfire. 

 

I have heard that backfires start to occur as carbon builds up inside the engine allowing enough heat and compression to build up that even when you dont add spark it is enough to make the gas explode (backfire). You may want to pull the head and check for carbon (or at least run seafoam through it for a couple tanks). 

[cschannuth 3,795]

 Any engine, old or new, can backfire if not properly idled down for a minute or so after hard use. I get on my wife and kids for that all the time. 

[rjg854 10,762]

I get a backfire once in a while just idling down, am I idling down to fast?