I have a 20 HP honda on a porter cable generator. Carb was nasty. Cleaned it up, new kit etc. got it running this morning. After about five minutes I got flames out the muffler, when I shut it off there was a huge back fire.

Any suggestions?

when exhaust valve is not seating and the intake valve comes up and if that exhaust valve is stuck or something causing it to not seat properly that creates enough gap that fuel gases can go into that exhaust area this in turn can cause a spark to ignite fuel vapors in other words it is going to let out backfire if you shut the engine down cause all them trapped gases are forced thru the muffler exhaust area to where it just goes boom in terms is the backfire we hear when things like this happens


clean them valves

with valve lap compound which auto zone advance auto has it or even napa can get it


if the valves are not seating properly then change them out with new ones

i dont think you have a fuel issue i think its a valve issue

the symptoms you describe points to valve seating issue

P.S. Agree with LMTech, but have seen this symptom caused by a valve hanging up from stale-gas varnish. If carb. was ugly, valves may be too. AND make sure you have fresh fuel in tank!
Paul

For a quick fix that often cleans sticking valves, get her warmed up, open throttle, dribble a little Automatic tranny fluid into air intake, she smokes like hell, repeat a couple more times, then dribble enough atf so that she stalls, leave for hours, preferably overnight.....The solvent in atf will dissolve laquer on valve stems....It has worked for me many times.....Good luck!.....ET.....