Furnace will light - if I hit it!
 

Hi
Ruud furnace.
Achiever 90.
About 13 years old I think.
I can hear the electric spark trying to light the furnace, but
it doesn't light.
If I hit the furnace right above that area when the spark is
trying ... furnace lights!
Trouble is, I don't want to stand by the furnace day and night.
Any ideas?
Thanks!

Dirty burners. Need cleaned.