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Murray Riding Mower Compression Issue


ttow's Avatar
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05-04-07, 08:25 PM   #1  
Murray Riding Mower Compression Issue

The issue is this:

The mower (Murray 40631A with B&S 28V700 series) will turn over one revolution than stop. It turns smoothly by hand (twisting the top fly wheel screen) and I can feel/hear good compression. Iíve replaced the starter with a new one and am using a brand new, verified, 12v deep cycle battery jumped directly to the starter positive terminal and chassis ground. Iíve verified the chassis ground location. But itís still like the starter is too weak or the engine is providing too much resistance. Iím confident the starter is correct, good and is getting appropriate power.

I removed the spark plug and engine cranks normal speed and even started within seconds after replacing plug. But after killing it, it wouldnít restart. So I assume that verifies itís a compression issue (since I released it with plug removal).

After googling the symptoms, the only answer that sounded reasonable is the exhaust valve is not releasing compression and needs valve adjustment or new compression release mechanism??? (Briggs & Stratton IPL for this engine Ė 28V700 - doesnít show anything called that).

Can anybody give me some guidance on this, as Iíve never messed with valves or the CRM before. Iíve done timing belts, water pumps, engine/trans replacements, etc so feel confident I can but I know enough to know I need to bounce this off a few experts first.

Thank you for your time,
Travis

 
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ttow's Avatar
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05-04-07, 09:10 PM   #2  
Follow up

To follow up my own post, after more googling and a reply from allexperts, it looks like I have to do a valve adjustment.

I've done timing belts, water pumps, engine/trans replacements, etc so am confident I can do it but have never done a valve job before.

Is there a "small engine valve job" guide somewhere? I have the measurements but not even sure if it's for the gap between the fat part of the valve and valve seat or at the bottom of the shaft.

I appreciate any guidance anyone can give me on my first valve job.

Thanks again,
Travis

 
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05-04-07, 10:15 PM   #3  
You don't need a valve job, just a valve adjustment. Remove the valve cover and measure the distance between the rocker arm and the valve stem. Loosen the torx set screw in the center of the adjuster nuts, and make this adjustment while the engine is a tad past TDC on the power stroke. It will probably take a few tries because the setting will change when you tighten it all up. I generaly like both valves to be .005".


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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05-04-07, 10:32 PM   #4  
Thank you for the assistance.

I know what a valve cover and rocker arms are on a car, but in the B&S IPL for 28V700 series:

http://www.briggsandstratton.com/ipl/pdfs/100/MS5150.pdf

neither are listed or shown (or anything that resembles those on a car). I feel dumb asking for clarification on a valve cover but I don't see it in the IPL.

I'm sure I have to get to the small end of the valve, I assume there is just a nut I loosen to make adjustments? And that adjustment would be vs the tappet valve?

Sorry for asking what is probably obvious to most but I do sincerely appreciate your time and expertise.

In a weird way, I'm glad I had this problem as it led me to this great site.

Thanks again,
Travis

 
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05-04-07, 10:43 PM   #5  
My apologies, I didn't check your model#. You don't have overhead valves, and your valves are not adjustable. Your engine also does not have a compression release mechanism.

Is your oil level overfull? Does the oil smell like gas? Did this problem slowly develop, or appear all of a sudden?


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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05-04-07, 10:52 PM   #6  
I just bought it used.

The previous owner said it ran all last year and couldn't get it started this spring.

And yes, I just checked and the oil is approx two inches above the full mark and does smell like gas.

 
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05-04-07, 11:01 PM   #7  
There's your problem. Your carburetor is leaking gas into the cylinder, which is seeping past the rings and getting into the oil, making the oil overfull and thin. You'll want to remove the carb bowl, clean it out, and replace the inlet needle. Make sure the float moves freely, and reassemble. Drain and replace the oil with fresh SAE30. This should solve the problem.

The reason the compression is too high is because the gassy oil is in the cylinder, occupying space that is needed for the compressed air. Since the gas and oil are there, the air must compress into a much smaller space, which increases the compression. That's why it started after you removed the plug, spun it over, and reinstalled the plug. It blew out the gas/oil that was in the cylinder. Probably not much at all, but it only takes a tiny bit.


"Who is John Galt?" - Ayn Rand (Atlas Shrugged)

God bless!

 
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05-05-07, 12:11 AM   #8  
Already draining the oil and will do as you advised with the carb.

Thank you very much.

 
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05-05-07, 06:39 AM   #9  
Azis
FYI: When looking for parts info or help on briggs engines, you need to also list the Type along with the model #. This should be the middle number on the shroud. I was unable to access a drawing until you posted the link, and therefore had no input :P
Sounds like you should have it now
gl

 
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05-05-07, 03:58 PM   #10  
Sorry about that and noted.

Thanks again for the help.

 
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