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~30 psi compression on 300ex


Jacob Blocker
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I have not been able to start my 300ex that I just got off a guy. I was able to getting it running once, then never again. After a week of trying to make it run I got a compression tester and it is saying about 30 psi. Is this definitely the rings just worn out or can it be something elses? I can't even make it pop when I spray stater fluid in it.

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Dream Car Giveaway

Either  rings or  valves..  Check the  valves to be  sure  neither of them  are being held  open   by maladjusted  valve  lash . If they  are set correctly  then  it is  most  likely  a ring problem.

There  is  a way to  check to  see if  blow by  is due  to a bent or  burnt  valve or    rings. It  requires  a  compressor  though ..  You  bring the  piston to  exactly top dead  center  on compression.  Connect  and  air  hose to  the  spark  plug  hole and  apply pressure. If  you are  at  top  dead center, the piston  will  not  move  down.... any  off either side  of  TDC  and  you  have to  try  again  because the  piston  will  move  down  and a  valve  will open.  Once  you  have the  piston  TDC and pressure  in the  cylinder you will  be  able to  tell if pressure is  getting  by  the valves  and  escaping out the  carburettor  or  is coming out of the  crankcase  breather  because of  bad  rings.

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27 minutes ago, davefrombc said:

Either  rings or  valves..  Check the  valves to be  sure  neither of them  are being held  open   by maladjusted  valve  lash . If they  are set correctly  then  it is  most  likely  a ring problem.

There  is  a way to  check to  see if  blow by  is due  to a bent or  burnt  valve or    rings. It  requires  a  compressor  though ..  You  bring the  piston to  exactly top dead  center  on compression.  Connect  and  air  hose to  the  spark  plug  hole and  apply pressure. If  you are  at  top  dead center, the piston  will  not  move  down.... any  off either side  of  TDC  and  you  have to  try  again  because the  piston  will  move  down  and a  valve  will open.  Once  you  have the  piston  TDC and pressure  in the  cylinder you will  be  able to  tell if pressure is  getting  by  the valves  and  escaping out the  carburettor  or  is coming out of the  crankcase  breather  because of  bad  rings.

Alright I'll try that, also when when ever I try to start it up there is smoke coming out of the carb and engine breather, I don't know if it comes out the exhaust tho, 

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