Small Engines (Lawn Mowers, etc.): only idles at 3/4 choke, blown head gasket, briggs straton engine

Question
hi i have an 11 horsepwer briggs straton engine in my portable generator its about 12 years old. latley i have not been able to idle it unless its at 3/4 choke and the vibration always runs it back to "RUN" which cuts it off. i have changes the -fuel filter-sparkplug- fuel line! cleaned the carb. changed fuel - runs great if i hold it at 3/4choke.. Paul any idea what is wrong? im lost.. thx joe

Answer
hi joe

1)have you checked for a blocked muffler ,?

2)is compression ok ?

3)check for a exhaust or inlet valve leak ?

4)have you checked for  for a  manifold joint air leak ?

5)is spark at right time

dry compression test,  record readings then ( pour into cylinder sae 30 oil tablspoon full) @ tdc compression stroke ,leave for 1 minute ,the wet compression test - record readings
if wet test compression result alot higher than  dry compression  test result - worn rings indicated but if wet test result no higher  than dry compression  test result  - burnt valves or blown head gasket or tight valves indicated .  

if all these are ok , then i would suspect something blocked in the carby , by closing choke to 3/4  you are making the air pressure less and letting engine vaccuum travel througout carb  venturi while atmospheric air pressure forces volume of fuel from main jet and emulsion tube for startimg and engine runs ok but open choke it starts to die     due to no fuel from venturi and or progression ports / idle ports

if  the 5 points above check out and are fine then  recheck all carb openings / etc / jets / nozzles / vents


cheers  paul

ps let me know what you find  , if i can help  just holler