Briggs 500 Series - Interesting troubleshooting case for the pros. help needed.

[motormonkey]

Hello!

First of all I want to say this place is great. been reading for a long time but just signed up.

Ok so... Here's a little story on the background of the issue...

I was asked to fix a lawnmower with a B&S 500 Series (Model # 10T502) on it.
When trying to start it by pulling the recoil it wouldn't even fire and there was a knocking sound in the crankcase.
I removed the fuel tank and drained the oil. I removed the blade and took the motor off the frame and set it up on a motor stand to work on it.
upon removing the head I found a stuck intake valve, lots of carbon on the piston head and on the valves and head but the cylinder walls were in awesome shape actually.

I then split the crankcase because there was no way to un-stick the valve and I needed to work the valves over, clean them up etc.

After removing the camshaft, and removing the valve lifters I could access the bottom of the stuck valve with a very large screwdriver and pry it straight up after removing the valve spring retainer.
I pried the valve up and finally was able to remove it and clean and sand the valves, valve seats and I cleaned the valve stems very lightly with very fine grit sandpaper.
Unfortunately I didn't have a valve lapping tool or compound to to it with so I sanded the valves and valve seats with extra fine sandpaper(500 - 1000 grit) until they seated beautifully. cleaned down the valve guides etc. the valves were working beautifully with just a SLIGHT play in the valves ( and rightly so due to the age, wear and lack of oil changes in the mower).

after that I reassembled the valves, put the springs and retainers back on, the valve lifters in and the Camshaft back in and set my timing marks dead on with cam and crank. (doubled checked my timing by the way the valves moved at TDC on exhaust stroke.)

Then I put a new head gasket on, a little Red Grease (High-Temp) to help protect the head gasket from the heat and reassembled the mower.
I did the old "thumb-Check" for compression covering the spark plug hole with my thumb and it had normal compression. (obviously my thumb can't read psi but you get a feel for it)
put the plug back in, Filled the Oil to correct level, fresh fuel etc,.

primed the carb with 3 squirts from it's PulsaJet carb and it started right up on the 2nd Pull.
It purred like a kitten, I mowed with it for a little bit... then I let go of the bail handle to kill it then I immediately started it back up with 1 pull.

then... I shut it off and came back to it about 15 - 20 minutes later I came back... and.... it wouldn't start.... it had that knock sound of the Camshaft turning but not able to lift 1 or more valves...
I proceeded to remove the valve spring access cover. (took the plug out) and had someone pull the mower rope.. and sure enough.. the valves were not moving.... ( I could move the exhaust valve with a screwdriver no problem... but the intake valve is apparently stuck again.

SO.. I am guessing.. the Camshaft is bent from before.
the cam would about have to be bent for it to be able to turn over if the valves aren't moving.. other wise it would be seized if the cam was straight wouldn't it?

Can someone please give me some insight into why this would stick like this again, so quick, after purring like a kitten. I know I have to tear it down again but some advice.. or moral support would be wonderful.

Many Thanks in Advance.

-

 

[reynoldston]

When you install a valve you want about .003 between the valve stem and valve guide. You need some oil clearance.

 

[motormonkey]

When you install a valve you want about .003 between the valve stem and valve guide. You need some oil clearance.

Hello Reynoldston,

Then it sounds like in that regard we are OK because there is some oil clearance.

 

[motoman]

motor, Not a pro, just a tinkerer...Try some checks. Did you take the lifters out and look? Lobe lift is the difference between a mike meas on the "base circle" and the lift . Compare the cams for wear and check the lift spec. My Intek had ground down its ex valve so that there was only 1/8 " lift and it still ran. (That cylinder finally went away). Stuck valves seem a bit of a mystery. Plane guys and some tractor guys see carbon (coke) buid-up as the culprit. This often happens on a very hot engine being shut down without reaching a reasonable temp. But in those cases bent pushrods are seen , or bent push rods drop out. I do not rebuild ac engines, but .003 guide oil clearance seems excessive. This may allow "cocking" of the stem in guide. You have not mentioned a guide seal. As you know these are used to control oil into the combustion chamber. I am surprised my Intake has a seal on the (? I think) ex valve as this is where the heat is generated. So with a seal the Briggs guys don't seem to want .003" oil film. I would carefully check all valve gear for cracks and wear and replace the springs. Hope some of this blather can help solve the problem or motivate the great forum knowers to respond, BTW removing and reinstalling valve guides did not work for me and BS does not sell guides (for my Intek , anyway), only assembled heads. My guess is that these engines fail mostly from valve mechanism problems if they are not fried from lack of oil/overwork. (As in using a 3 bagger on a hill with damp grass in 85F weather).:laughing:

 

[reynoldston]

motor, Not a pro, just a tinkerer...Try some checks. Did you take the lifters out and look? Lobe lift is the difference between a mike meas on the "base circle" and the lift . Compare the cams for wear and check the lift spec. My Intek had ground down its ex valve so that there was only 1/8 " lift and it still ran. (That cylinder finally went away). Stuck valves seem a bit of a mystery. Plane guys and some tractor guys see carbon (coke) buid-up as the culprit. This often happens on a very hot engine being shut down without reaching a reasonable temp. But in those cases bent pushrods are seen , or bent push rods drop out. I do not rebuild ac engines, but .003 guide oil clearance seems excessive. This may allow "cocking" of the stem in guide. You have not mentioned a guide seal. As you know these are used to control oil into the combustion chamber. I am surprised my Intake has a seal on the (? I think) ex valve as this is where the heat is generated. So with a seal the Briggs guys don't seem to want .003" oil film. I would carefully check all valve gear for cracks and wear and replace the springs. Hope some of this blather can help solve the problem or motivate the great forum knowers to respond, BTW removing and reinstalling valve guides did not work for me and BS does not sell guides (for my Intek , anyway), only assembled heads. My guess is that these engines fail mostly from valve mechanism problems if they are not fried from lack of oil/overwork. (As in using a 3 bagger on a hill with damp grass in 85F weather).:laughing:

I wouldn't think .003 is excessive at all. But I am thinking a car engine and don't know what the Briggs engine calls for. I would say go to the service manual and see what it dose call for because I was just guessing on that number. Oil dose take up space and you do need this space to lubricate the valve stems and without proper lube they will stick when they get hot.

 

[Rivets]

I don't feel that .003 is too great a clearance, but I would like to kno how you got that. You should have measured the valve stem with a mic at multiple places in the travel area. There should be no more than .001 difference in any of the measurements. You the measure the valve guide at the top, center and bottom with a small hole guage and mic. Again you should have no more than .001 difference between any measurements. The difference between the valve stem measurement and the valve guide measurement is your clearance.

Second, you never said what was the cause of the valve sticking in the first place and what corrective action you took. If the valve was bent or the guide tapered, you are spending a lot of time and effort for nothing.

 

[reynoldston]

I don't feel that .003 is too great a clearance, but I would like to kno how you got that. You should have measured the valve stem with a mic at multiple places in the travel area. There should be no more than .001 difference in any of the measurements. You the measure the valve guide at the top, center and bottom with a small hole guage and mic. Again you should have no more than .001 difference between any measurements. The difference between the valve stem measurement and the valve guide measurement is your clearance.

Second, you never said what was the cause of the valve sticking in the first place and what corrective action you took. If the valve was bent or the guide tapered, you are spending a lot of time and effort for nothing.

To be true full I am thinking back when I was working on truck engines about 25 years ago. They had replaceable valve guides and when you replaced one it had to be reamed. Its all part of a valve job on larger engines. Its just something I never got into on mower engines seeing I have never seen a replaceable valve guide in a small engine. Now I am not saying there isn't such a thing? If the valve guide is bad it is either new head or block time. Maybe oversize valve stems?? I think you have a very good point about the bent valve. I have never measured the valve stem clearance when I do a valve job on a small engine other then just by feel. I will assure you that .001 isn't something you will feel by hand.

 

[Rivets]

Briggs does have replacement valve guides. They are not that hard to replace if you have the correct tools. It used to be a common job to do so, but the labor costs today (about $50) lead most people to just decide not to go that route. We've gotten to be a throw away society wnen it comes to doing repairs on small engines. I used to do 25 rebuilds a year, but have only done 3 this year if my memory is still working. Reynoldston, you are correct, they must be reamed to size once they are installed, and most of the time I would install a new valve.

 

[motoman]

There used to be a rocking check on stem to guide for some car engines. Raise valve off seat so much and then take side to side dial reading, but I guess that is something that would have to be worked out with geometry if not given by the mfgr. It was once fashionable to knurl valve guides to reduce the ID , then ream. The lands created by the knurling also held oil. I could not find any replacement guides for the Intek anywhere, including looking at motorcycle sites, only fully assembled heads. I suppose a lathe mod would work. Still believe Briggs knows (on Intek anyway) that guide problems mean softened head alloy so replace.

 

[reynoldston]

knurl valve guides to reduce the ID

Now that you said something about it I remember that also. Its been a few years back for me and my memory isn't as good anymore. I still got the tools to grind valves but don't remember the last time I used them. On the small engines I just clean them up with grinding compound any more, I think they make valves and seats of better material now then in the older days. I don't care what is said older isn't better newer is.