OT: small engine conundrum


Got a B&S 12.5 HP engine on the lawn tractor. Model 28V707 if you
worry about such
things.
Started to go a little strange recently.
It would blow off white exhaust, loose power and die. Oil fouled plug
and even oil in the carburator chamber.
Figured rings were the problem. But when I took it apart the rings
seemed fine. Thinking about it again maybe the oil ring gap was too
large and I intend to recheck it.
After I put it back together, I get what now seems like a timing
problem.
Turning the motor over gives me detonation once every so often but
the exhaust comes out the air filter!
OK. Timing problem. Right? But...
I've got the crankshaft lined up "properly" with the cam shaft. (Set
the
pips, drop the gear onto the crankshaft key.) There's only ONE way for
this to fit AFAIK.
Thought it might be the tappets. If the tappets were different lengths
it would
affect the timing. So I reversed them. Nothing!
Checked other obvious things like valve return... Everything seems
OK.
My next idea is to visually check the piston cycles compared to
the valves and see if the timing looks about right.
I don't mind taking this sucker apart, but I'm runing our of ideas
about what might be wrong.
Tanks for any input.
DOC
What beer?
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Reply to
doc
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By "chamber", do you mean the throat, or the float bowl? If throat, then your problem could be (is likely) a leaking PCV valve. It could be just a tiny piece of grit in the flap. Take it off & take it apart.
Bob
Reply to
Bob Engelhardt
I've used the repair manual for Briggs for years, it's this one:
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Oil on the plugs and out the exhaust means it's coming from somewhere, probably a worn cylinder. You can't detect this just by looking at it, a set of telescoping gauges or running a ring up and down the cylinder and measuring the gap to give you an idea of the wear is the way. You might need to pull the head and check the valves and seats. First, check to see if the oil level is high and whether it's got gas in it. If you've got gas in it, you'll have to find out where and how. Then you'll have to undo what you just did. And the piston usually goes back only one way round, if it's not marked and you didn't mark "front" on the top, that'll complicate things. Backfire means that the valve timing is off or you've got a busted valve.
The later Briggs engines have an electronically-switched magneto system for spark, mostly it's trouble-free and all you have to do is to make sure is that it's got the right gap from the core to the magnet on the flywheel. Sometimes they go thermal or get intermittent, about the only way to find out and eliminate it as a possibility is to swap it. No real way to change timing built-in, if the flywheel key is partly sheared or loose and wallowed-out, that'll change timing. The book has details on all the other systems.
Stan
Reply to
stans4
Well! That was interesting.
I got frustrated. Had a couple strong beers. Then went for a nap.
When I revived, :-) I started to think about the only other place directly involved in timing. The flywheel!
So I pulled it and found the key nicely sheared in two with the flywheel about 180 deg. out of phase. One of those WOW moments!
Fires up nicely now. I may still have an oil problem, but I can handle that without too much strain on the brain.
Thanks for the suggestions guys.
Oh, and the name of the "magic beer" is available free upon request. :-)
DOC
...
Reply to
doc
I was going to tell you to check the shear pin/flywheel key - you have now done that and found the problem - but did you use the soft key that will shear on overload? that's important, it protects the crank if something stops the motor suddenly - as I recall the keys are zinc - I don't have any spares any more (at least not that I know of) so I can't check for you. If you use a steel key, you are putting the engine at risk
Reply to
Bill Noble
I had an oil burn problem with an old 3.5 B&S vertical, traced it down to an earwig blocking the drain hole from the tappet chamber which caused the oil to be sucked into the carb. Don't know if this has any bearing on your problem but it is one of the stranger things I have found when playing with my junk. Gerry :-)} London, Canada
Reply to
Gerald Miller
Well, you found one problem, but I bet it's not the reason for the oil in the carb chamber. I have the same motor on an old riding mower and the same problem, but not a sheared key. Pulled the head and watched as I rotated the flywheel. I could see one of the valves cock to one side and let all the compressed fuel and oil from bad rings go right out the intake valve and into the carb. Absolutely no compression. The valve guides are shot on mine and since the whole machine has been repaired for years, it is now junk.
The flywheel key usually shears when the blade hits a root or some such object and stops the engine, but the flywheel continues to turn. Normally the crank breaks. You are lucky.
Paul
Reply to
co_farmer
ROFLMAO!....if it works for others, I may start to drink.
Gunner
One could not be a successful Leftwinger without realizing that, in contrast to the popular conception supported by newspapers and mothers of Leftwingers, a goodly number of Leftwingers are not only narrow-minded and dull, but also just stupid. Gunner Asch
Reply to
Gunner Asch
OK - between rings/valve guides or the PCV valve that I posted about earlier: if the problem has been building awhile (getting worse over period of years) it's very likely rings and/or valve guides. If the smoking/stalling/oil-in-carb was sudden, then it's the PCV valve.
Bob
Reply to
Bob Engelhardt
On Tue, 22 Jun 2010 07:04:03 -0700, Gunner Asch wrote the following:
No, don't start drinkin', Gunner. Just keep counting potential cullees and drift off with a smile on your face. ;)
Speaking of drinking, a drunk in a red Jeep almost hit me on Monday. I was coming up the 199 highway into GP when he came flying up and hit his brakes about 8' behind me, with me at 55mph. He kept creeping up, wandering over the double yellow line, and wandering over onto the right shoulder for 2 miles. Finally, the road widened and I immediately got out of his way. He sped on, passing me in the double double yellow area for about 1/4 mile. I was praying for a cop to see him when, to my complete delight, a white Mustang came onto the highway right behind me. As it passed me, I saw the thin white bar on top and knew it was a radar cop. As he sped toward the Jeep, I saw a white puff in front of him and the red Cherokee went spinning over to the right side of the road, landing at a 45 degree tilt on the far side of the gully. Ahead of him was the ancient Datsun pickup he rear ended. The guy got out of his totalled Datsun pickup and was lamenting over the new 30-degree bed- to-cab angle. Luckily, only the drunk was (potentially) hurt. Well, he's off the road and Oregonians are safer tonight.
-- Peace of mind is that mental condition in which you have accepted the worst. -- Lin Yutang
Reply to
Larry Jaques
Blame it all on the chipmunk!
In the spring, when there was enough grass to be mowed, I did my usual check out the lawn tractor, change oil...
When I was fiddling with the tractor, out pops a chipmunk. Looked to be about as surprised as I was. Ah well, sorry to wreck your home buddy, but there's grass to be cut!
I got a stick and poked around under the shroud to get rid of some of the nesting material. Thought nothing more about it and started to cut grass.
Month or so later the problem that I described started to happen. After a little while running, the engine would cut out in a puff of white exhaust.
I thought it was a rings problem so I stripped down the engine. Along the way I noticed that about three of the bolts that hold down the cylinder head were a little loose. Didn't pay much attention, and when I got to the rings they looked fine.
When I put everything back together I somehow didn't get the key for the flywheel to seat properly. It sheared off and caused obvious timing problems when the flywheel was out of phase.
Now that I fixed that problem everything seems fine.
So here's the diagnosis:
The chipmunk jammed up the engine cooling fins with bedding material. I didn't remove all of it so the engine overheated. Over a period of time this caused some of the bolts in the cylinder head to become loose.
This caused the loss of power. I'm not too sure about why there seemed to be oil in the mix.
So the next time I find a chipmunk nest in the lawn tractor, I'll know to clean it out thoroughly!
DOC
Something about beer...
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Reply to
doc
I just knew I should have posted and said something about that key, but you said you installed it with no problems or something like that. Father-in-law had major problems with those keys, just kept sheering them off.
Maybe compressed air in the spring.
I had lots of fun yesterday fixing my generator that hasn't run in years, think the same in law put diesel in it. The power went off around 1 am. and wasn't coming back on and the wife is funny about food getting a little bit warm. I've been over stressing the tendons lately and was trying to figure out how to get this thing up on the operating table. Was thinking ramps and that would be too steep, then crane,yes, so I drag the generator inside the line and just about to swing that puppy out and realize no electricity! So, I dead lift the heavy bastard up cause I wasn't about to work laying down next to it. Gas tank and lines where bone dry, probably from baking in the vegas sun forever. Checked the spark plug, oil, governor linkage mickey mouse rigging job I did long ago, and really wanted to air out all that baffling with no air. Put together and gas her up thinking how much fun it is gonna be to redo the carb with no air, first pull purrs so softly I wasn't sure it was running at first.
I just knew I'd put that job off until it was needed. Anyhow, good thing I did it cause it took 16 hours to get power. Wow, if it takes them that long in the summer... I need a bigger homemade generator.
SW
Reply to
Sunworshipper
As another poster said to make sure you use the soft key. Also torque the flywheel nut to full spec. I kept shearing my B&S keyway on a 5.75 HP Verticle mower engine, until I got the torque wrench out, that problem went away, for my engine it was 115 ft.lbs. ignator
Reply to
ignator
I don't think I did a soft key. It was one that fit from my stash.
The blades are run by the crankshaft using pulleys so I figure that there probably would be some slippage/clutching there.
I've done this sort of thing on a spinner robot I built a while back. If the blade hit something really solid and stopped, it wasn't likely that a lot of that force would get transferred back to the motor or the robot itself.
115 ft. lbs seems like pretty tight on the taper shaft. I'm wondering if you are actually supposed to get any slippage there.
DOC
Something about robots...
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Reply to
doc
The problem is that on sudden stoppage, the flywheel is, well, a flywheel. It wants to keep spinning, and the crankshaft doesn't. Back in my motorcycle mechanic days, I fixed a LOT of these.
If the key doesn't shear and puts a nasty gash in the crank, all is not lost. I have had good succcess cleaning up the burred shaft and then lapping the taper with Clover compound. Then just leave the compound on the shaft and torque it in place. The grinding compound digs into the shaft and flywheel and does an amazingly good job of holding it in place without a key.
Of course, you could also bust the end off the crank. Id I were you, I'd oput in a proper shear key.
Reply to
rangerssuck
Years ago I was getting all sorts of errors out of a Fanuc E-310 robot. Mostly ot limit errors that went away. Eventually I pulled the covers off and found a bunch of mouse turds and sound deadener along with chewed wiring for the axis limit switches.
This had been a clean room robot and the exhaust port that we didn't have connected to a vacuum source was the point of entry.
Wes -- "Additionally as a security officer, I carry a gun to protect government officials but my life isn't worth protecting at home in their eyes." Dick Anthony Heller
Reply to
Wes
As RS said, you really should replace it. You have that key in the fly wheel and often a blade adaptor that is designed to fail if you hit something you should not. Better a 3-15 dollar part than scrapping your mower engine.
Unless you have a perfectly manicured lawn, you will hit something unseen eventually.
Wes
Reply to
Wes

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