390 new points now backfiring

[GerryP]

Popping out the exhaust is usually a sign that you are over advanced.  Pull the vacuum advance and see if that helps.

[wayne]

Unhook the vacuum  advance  i have had them go bad more rpm the more it missed .

[390owner]

Thanks for all the replies. It ended up being bad gas. I pulled the top of the carb and the liquid in the carb hardly smells like gas. So I am going to pour some heat in the tank and hope I can run the half of tank out.

[oldiron.fe]

                                                                                                                                                           do not try to (run it out) you can damage stuff like break a ring / gum the carb /or-??   just get clear plastic hose work in fill cap -can put short piece coppertube to weight to bottom of tank siphon out--  or take line off before fuel pump use rag and air line over line apply a little air to help out---or drain by
open  -    fitting before fuel pump  take rag and air line to fill cap apply a little air to push fuel out--after all old stuff out add a little fresh fuel to rinse -- fuel gage/pickup usually get to drain /view/ might find-stuff-clean. I usually pull gage-now you see what you have-clean make it right-then one area you dont have to chase your tail-if you push fuel out by pump - jack rear up use stands to get tank line at lowest point to get all stuff out -do it right---store with good fuel / oil mix-- i have mix i used over 50 years in any engine or compression NEVER a problem !!

[427John]

Yes get all of the old gas out and start with fresh gas,I have an old electric pump that I use to pump the gas out of a rig thats been sitting long enough for the gas to go bad,which with todays gas can be a year or less.A jumper box works great to power it up and a hose on the discharge to stick in a gas can.I made the mistake once of trying to run the old gas out of a car that had been sitting for a while and the varnished gas ended up coating the valve stems and guides and causing some valves to stick.I initially got the car running by running a hose from the engine driven pump into a can of fresh gas after cleaning out the carb and it ran great for a couple of hours.After checking it over for leaks and checking the timing and carb adjustment I switched the fuel supply back to the tank and took it out for a test drive.After taking it on short drives over a couple of days it started running worse and worse,after checking it out I found multiple valves sticking,and after swapping the heads out and inspecting the old ones the stemms were heavily coated with hard varnish and you could see where it was being scraped off by the guides.