Briggs 8HP only 20lbs of compression

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  #1  
Old 06-20-08, 11:14 AM
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Briggs 8HP only 20lbs of compression

I have an old briggs that has only 20lbs of compression. Valves seem to be seating OK, replaced head and gasket off a different 8HP. Nothing. Can you "see" if the valves are closing enough? Also piston looks OK but is it possible the compression ring is shot. Ran fine last fall. Any ideas? Thanks
 
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  #2  
Old 06-20-08, 01:16 PM
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Compression readings can be misleading on small engines, so
do not fixate on that.

Post the engine's model numbers, which will be helpful in helping you.

Model, type, and code numbers, stamped in the shroud.

Thanks,

Fish
 
  #3  
Old 06-20-08, 11:31 PM
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Fish, Heres the numerals

Model 190707 Type 0852-01 Code 75010712

Hope this helps. PS I plugged with a rag and blew air into the carb throat with the head off and there are small fuel bubbles from under the intake valve. There is also some leakage around the piston. Normal? 110psi air compressor.
 
  #4  
Old 06-20-08, 11:54 PM
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If you blew the air from the carb throat, it is going to leak past the valve. It's air pressure from the other direction that matters. What's wrong with the engine, won't start? As Fish said, compression readings on small engines aren't the best thing to go by. If it will blow your finger off the plug hole when cranking, then it's probably fine.
 

Last edited by cheese; 06-22-08 at 12:30 AM.
  #5  
Old 06-21-08, 06:19 AM
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With the numbers you provided, then your engine originally
had a points type ignition. I would suggest putting it back
together, and starting fresh with a troubleshooting procedure.

Put a dribble of fuel in the carb/or shot of carb spray, then try to start. If it starts, then dies, then most of the problem
is likely in the carb. Checking for spark is a must to do as
well, weak/intermittant spark is not good.

Fish

Do both of these things, and let us know.
 
  #6  
Old 06-21-08, 06:21 AM
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Cheese- Yeah it won't start. Spins over and sounds to me like it has enough comp but won't fire. Tried carb cleaner right in the cylinder, carb etc. Won't backfire or pop or anything, Tried plug of course. Good spark with a glow tube type tester and spark strength tester, Changed out the coil, Checked flywheel key. Looks like it maybe a tiny bit twisted but not like others I have seen.. This thing is throwing me for a loop.
 
  #7  
Old 06-21-08, 06:25 AM
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Is there a way to time these engines other than the flywheel key. If it is a point system can they get out of time. Not to familiar with points. I try to switch everything to magnetron.
 
  #8  
Old 06-21-08, 06:43 AM
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If you have some compression, a new magnetron coil, new
plug, and nothing, you need to recheck your flywheel key, it
sounds like it is sheared again. You have to torque down the
nut/clutch real tight..

Fish
 
  #9  
Old 06-21-08, 06:51 AM
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I didn't put a new coil on it just a used one from another boat anchor 8HP. I did not know it had point ignition. I thought the wires that were under the flywheel were from the alternator. Thanks for the help.
 
  #10  
Old 06-21-08, 09:24 PM
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Guys I put my thumb over the plug hole and it wouldn't blow it away. You thinking rings? Let me know if there are anymore options. Thanks
 
  #11  
Old 06-22-08, 12:34 AM
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You have checked for spark right at the plug, correct?

Even if the compression is low, you should still be getting a sign of combustion.
 
  #12  
Old 06-22-08, 09:01 AM
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Yep checked spark at the plug. It's a good blue spark. Tried a different plug just in case it wasn't firing under compression. It doesnt suck my thumb in either for the intake stroke. Valves open and close. I can spin the exhaust valve on it's seat at TDC but the intake I cannot. But if it was a valve I should be backfiring through intake or exhaust whichever isn't sealing. Correct
 
  #13  
Old 06-22-08, 10:18 AM
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If the valve can spin, that means it is not closing.


Fish
 
  #14  
Old 06-25-08, 11:21 PM
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Fish whats the procedure for getting that valve to seal? Can I lap it of does it need to be ground?
 
  #15  
Old 06-26-08, 05:55 AM
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You could likely get by with adjusting/grinding the valve
stem to spec., but a good valve job would be better. There
is no clearance on the valve that spins.

When these blocks overheat, they tend to warp back, making the valves lose their clearance. Unfortunately, the valve seats also are cocked, and to do a proper valve job,
the seats and valves should be cut and lapped, which requires
expensive tools.

Fish
 
  #16  
Old 06-26-08, 07:31 AM
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Fish-We are pretty sure this is where all the compression is going right. One more thing what is the valve stem clearance on these engines. Thanks for all of you help fish
 
  #17  
Old 06-28-08, 11:30 PM
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Cool

Thanks a million fish!!! Got it running today. Replaced valve from another engine. Lapped it in. Everything you said makes sense. The seat does seem messed up a little. When lapping it would grind hard, light, hard etc. I didn't do a valve job cause this is an old mower for a friend. Of course, they don't want to spend anything on it. Thought it was odd that the valve stem lost the .010 to the lifter. I didn't think that would ever happen. But in all reality that is the only think that would hold the valve open besides a weak spring. Thanks again
 
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