520 Won't Start

[84Workhorse 3]

I'm looking for help in troubleshooting a starting problem with my 520. I used it yesterday to mow my yard for about 1.5 hours and when finished I let it cool down before shutting it off. It sat for about 2 hours and I put fuel in it and drove it on the ramps to clean the deck and after rolling it off the ramps I hit the starter and the engine made a noise and tried starting. I tried several times after that and no pop at all so I did a compression test and the front cylinder was fine and the rear was poor so I immediately thought of the valve seat being loose wedging the valve open. This morning I took the head off and much to my delight I did not see the seat as I thought but the head bolts were not very tight so I understand the poor compression (see pictures). The piston goes back and forth so that seems fine so I'm not sure where to focus my attention next. I am new to the 520 world and I tried searching the forums to find a thread that might walk me through this to no avail. I'm going to hold off putting the head back on just in case someone says to check something else related. This machine is new to me and has been smooth running, no smoke, good power, and always started easily with very little choke so I'm very surprised at this. Any help would be appreciated.

[oliver2-44 9,726]

I'n not an Onan guru, but here some basic engine things to work through.

For the engine to run you need compression, fuel, spark, and air.

Certainly looks like that head gasket was leaking, and the loose bolts could be part of the culprit.  I would clean and then sand that head flat.  Use a piece of glass and start with 220 wet/dry sandpaper.  I use a black marker to "paint" the area to be sanded, then I can see how its progressing.  While sanding use a figure "8" motion to do it evenly. When about 80% of the black area is sanded off, I switch to 320 or 400 grit wet or dry and finish it.  You can use water or mineral spirits of the wet/dry paper.  Get the head bolt torque spec's from a manual, and torque Both heads, even though you didn't remove the one.  After you run the engine an hour, retorque the head bolts white it is hot.

 

Decarbon the top of the piston, head seating arean and valves using a plastic scraper.  Don't use any hard metal as you don't want to gouge these surfaces.

 

Since you have the head off, if you have the tools I would take mic reading of the cylinder bore at top and bottom, at 90 and 180 degrees.  This will give you a baseline for the future.

 

You also can check for spark before you reassemble things.  Hold (ground) the body of the spark plug with a rubber insulated pliers agains the engine, then crank it and watch that the plug has a white/blue spark.  Check this on both cylinders.

 

Remove the fuel hose at the carburator and crank it to make sure your getting fuel flow.

 

Check that the vent hole in the gas cap is open. 

 

when you have the head back on, be sure to clean insid ethe engine tins to ensure good cooling air flow to the cylinder.

 

Let us know what you get with these checks and we can guide you from there.     

 

 

[84Workhorse 3]

Oliver2-44; I did all the things you list with the exception of measuring the bore of the cylinder since I didn't have the correct tool to do that. I have air, fuel, and fire but it will not start. The fact that it ran so well prior to this has me stumped. I read different threads on the different forums to see if something could be a possibility and of course there are plenty of rabbit holes to run down and spend some time and money but I'm wondering what the next logical step is so I'm going through the troubleshooting possibilities from the manuals. Just for reference this is a '97 model with 727 hours on it. Again, any guidance is much appreciated.

[Bill D 1,916]

You may want to pull the flywheel and see if the key has sheered off

 If it has it will change your timing as it holds the trigger ring in the correct position.  You will have spark but at the wrong point in the cycle.  If someone changed the ignition module but didn't torque the flywheel properly it would just be a matter of time before that happened when starting the engine.  Bill

[lynnmor 7,303]

You can check the timing by looking down the hole in the tin where the large rubber goes up to air filter.  A long line cast into the flywheel should line up with the timing mark on the engine when it fires.  See the Onan manual for more information.

 

But first try pouring a teaspoon of gas down the carburetor throat and quickly cranking the engine.  If it briefly sputters to life, you have a fuel delivery issue.

 

Those white valves tell me that you have had a lean condition for a while.

Edited May 4, 2020 by lynnmor

[84Workhorse 3]

Bill, The noise I heard when attempting to start it after rolling it off the ramps could have been the key giving way. I will do a quick timing check as lynnmor suggested and based on what I discover I may be taking the flywheel off.

Lynnmor, there seems to be plenty of gas getting to the carburetor because I pulled the line at the carb and put it in a soup can and cranked the engine and within a couple of seconds I had a 1/4 can of gas. After reconnecting the hose and cranking it again it backfired out of the carb and muffler a couple of times. I'm wondering if that lean condition can be attributed to the loose head bolts. I barely needed a wrench to loosen them.

Thanks for the tips and suggestions.

[Bill D 1,916]

If it backfired out of the carb that would make me think timing issue.  I had a Briggs on a Snapper walk behind that sheared the key.  It was only half sheared, about 1/8" off from where it should have been, but it would rip the starter cord out of your hand when you tried to start it. 

 

Your Onan has a big, heavy flywheel and a thin key.  Easy to shear if the flywheel isn't torqued down.  The flywheel cover is a pain to remove.  When reassembling make sure the ignition module is hooked up correctly or you will fry it.  Also, the condenser connects to the positive terminal unlike engines with points.  Bill

Edited May 5, 2020 by Bill D

[lynnmor 7,303]

Just because gas is getting to the carb doesn’t mean it is going thru it.  Do the teaspoonful  trick and see what happens.

[953 nut 55,163]

9 hours ago, 84Workhorse said:

I'm wondering if that lean condition can be attributed to the loose head bolts. I barely needed a wrench to loosen them

              Sound to me like a previous owner was playing around and didn't re-torque the head bolts once it was hot.

[Bill D 1,916]

If the heads weren't torqued properly, the flywheel wasn't either.

[84Workhorse 3]

Problem found...sheared flywheel key. I did the timing check from the manual per lynnmor's suggestion and it wasn't checking out so off came the flywheel. The pictures show what it looked like when it came loose along with some pretty healthy metal shavings. Bill you were spot on, the flywheel bolt was barely tightened. Where do you get the torque data from because the one I'm looking at on page 8-1 Onan service manual doesn't list one for the main bolt but it does for the cap screws. I'm wondering if that is the correct data to be using. It's time to take the torque specs for all the different locations and spend some time checking things over after placing the order for the new key. No luck locally. Thanks for the guidance.

Edited May 5, 2020 by 84Workhorse
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[lynnmor 7,303]

Page 4-1 has the torque specifications.  Flywheel is 50 to 55 lb. ft.

 

That key doesn't look correct and is there still material in the crankshaft keyway?

 

Pull out that light gray plastic piece and check the keyway in it for damage.

Edited May 5, 2020 by lynnmor

[84Workhorse 3]

Here's more of a reassembled profile view of the key with some size reference. The length of the key makes it about flush on both ends of the flywheel groove. The keyway on the crankshaft is clean and the one in the plastic piece looks perfect. My bad on the page no. for the torque data. I saw the 50-55 lbs but wasn't sure if it was referring to the 4 screws that hold the chaff screen since it is plural or if it meant the main flywheel bolt. Also of interest in the instructions on that page, it says all threads must be clean and lubricated with new engine oil before torquing. That's an important piece of information because the torque would be significantly different without it.

[Bill D 1,916]

Glad I could help.  55 lbs is the torque spec for the center bolt that holds the flywheel.  60 lbs wouldn't hurt if you ask me.  Jack's small engine has the key, but shipping tends to be slow.  Clean everything good while engine is apart.  Good luck with it.  Bill