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just4fn
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2025/07/14 23:32:03 (permalink)
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What should the compression be?

I have a toro 7.25 HP mower.  I think is has a decompression cam.  What should the measured compression be?  I'm getting 40lbs.  I think that is ok with a decompression valve but not sure. 
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    SRTsFZ6
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    Re: What should the compression be? 2025/07/15 06:28:44 (permalink)
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    40 is super low...
     
    Can you post an engine model #????
     
    .

    Scott
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    just4fn
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    Re: What should the compression be? 2025/07/16 07:28:31 (permalink)
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    model # 128M02-0017-F1
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    SRTsFZ6
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    Re: What should the compression be? 2025/07/16 15:25:58 (permalink)
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    https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/briggs-stratton/briggs-stratton-engine/100000-19z999-series/128m00-to-128m99/128m02-0017-f1
     
    That model # is showing a "flat head" engine-NO overhead rockers. 
    I don't believe they had a flat head engine that large..
     
    IF it is a FLAT HEAD engine, check  / tighten the THREE HEAD bolts CLOSEST
    to the exhaust valve.  They loosen up (very, very common) and will cause
    no start issues...   
     
    About 15' lbs on the head bolts.  
     
    If you have an overhead valve engine, advise.  I own a 7 HP Briggs OHV
    engine and can get you a pressure #...   Advise..
     
    .

    Scott
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    just4fn
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    Re: What should the compression be? 2025/07/16 19:16:11 (permalink)
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    Thanks for the info.  I will look at it this weekend and report back. 
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    just4fn
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    Re: What should the compression be? 2025/07/20 19:44:26 (permalink)
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    It is a flat head. The  head bolts are tight.  It is acting different from what i know about engines.  I have replaced the coil, spark plug, checked the ground wire even though I have spark.  It starts when it's cold but is really hard to start after warmed up. The only smoking gun is the 40lbs compression. Why it would start when cold is beyond me. It won't even chug with starting fluid. That led me to check spark and it has plenty of spark.  I also check the keyway and it's fine. My next step is to take off the head and check valve clearance, lap the valves and install a new headgasket. I don't own a leak down detector. Are you able to find what the compression should be? Can the cam get stuck and be the cause of low compression or is it the opposite and there is too much compression to turn it over if it's broken?
    post edited by just4fn - 2025/07/20 20:00:31
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    just4fn
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    Re: What should the compression be? 2025/07/20 19:53:20 (permalink)
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    Looking at the camshaft on ebay, it looks like it is a compression release cam.  It looks different from what I'm used to seeing but it does have some extra parts on it. The cam is a 590463.  Here is a link to some pictures on Ebay:  https://www.ebay.com/itm/...7Ctkp%3ABk9SR9DKmKeFZg
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    SRTsFZ6
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    Re: What should the compression be? 2025/07/21 07:15:56 (permalink)
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    I don't currently have a flat head engine to check but can give you a # on my OHV 7.25 Briggs.
     
    Usually, the minimum is 100PSI.
     
    I agree, that cam does have a compression release. The valve lash is checked
    under the cover under the intake manifold.   To adjust wider, the valve must be removed and the tip slightly ground down to open the gap.
     
    Now, with that said, when the engine gets hot, parts expand.  The valve clearances also tighten up
    causing a possible , slightly open valve lowering compression even more.
     
    I'll post a # later today.  I do have an old 2 hp flat head Briggs I'll check too...
     
    .
     
     

    Scott
    Quote  #8
    Mikel1
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    Re: What should the compression be? 2025/07/21 08:38:30 (permalink)
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    I have checked a 5hp flathead at 60psi that was running. I would check the valves because flat heads valves pound on the seat.
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    SRTsFZ6
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    Re: What should the compression be? 2025/07/21 10:15:30 (permalink)
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    My flat head  2 HP Briggs is 65PSI ,
    My 7HP, OHV is 85PSI, both cold..
     
    .

    Scott
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    just4fn
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    Re: What should the compression be? 2025/07/21 19:24:07 (permalink)
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    Thanks all!  That's the next step after I get the headgasket adjusting the valves.  I'll post what I find.
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    just4fn
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    Re: What should the compression be? 2025/07/24 18:11:32 (permalink)
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    I removed the head and found the headgasket blown. Looks like exhaust getting into the combustion chamber. I should be getting a new gasket on Monday.  I'll report back. 
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    SRTsFZ6
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    Re: What should the compression be? 2025/07/24 18:17:02 (permalink)
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    Glad you found the  / one issue...
     
    I would still check valve clearances while the head is off and adjust.
     
    Clearances are also viewable here:
    https://outdoorpowerinfo.com/engine_specs/briggs_head_torques.asp 
     
    .
     
      

    Scott
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    just4fn
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    Re: What should the compression be? 2025/07/24 22:19:53 (permalink)
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    will do..tomorrow.  I tried to upload a picture but it wouldn't go.  It just spun. 
    post edited by just4fn - 2025/07/24 22:21:24
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    just4fn
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    Re: What should the compression be? 2025/07/26 08:43:55 (permalink)
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    The exhaust valve was at .003.  I removed it and adjusted it to .008.  What a PITA getting the valve retainer back on  without the correct tools! The intake is correct at .006.  I hesitate to lap the intake valve since it was so hard getting the exhaust valve back together. 
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    SRTsFZ6
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    Re: What should the compression be? 2025/07/26 10:17:24 (permalink)
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    Well, that tight exhaust gap would explain a good part of it.
     
    The engine gets hot, tolerances TIGHTEN UP, as does the exhaust clearance-
    Making the valve stay OPEN (just slightly but enough to drop compression)...
     
    Plz advise once all together .
     
     
    .

    Scott
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    just4fn
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    Re: What should the compression be? 2025/07/26 10:52:12 (permalink)
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    Should have the headgasket on Monday.  We will see how I feel after work. 
    Quote  #17
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