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Garden Tractors / Mowers / Scooters Vintage lawn and garden tractors, mowers, snowmobiles and other old machinery that is driven by small air cooled engines.

Garden Tractors / Mowers / Scooters

Briggs and Stratton Model N That Smokes. What's the fix...


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  #1  
Old 01-12-2018, 03:54:21 PM
R Davis R Davis is offline
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Default Briggs and Stratton Model N That Smokes. What's the fix...

I've tried getting a Model N that doesn't smoke the easy way, by buying one, but I own 3 now and still don't have one.

The strongest of the 3 is on my David Bradley 57561. It smokes a 'little' all the time, but really pours out the blue smoke for a minute or better when it's first started. So oil's getting in the cylinder after it's shut off... I guess. I can't figure how the oil would get in there.

My question is, will honing the cylinder and going to an oversized piston fix it? And what else might be wrong, worn valve guides maybe? I've read about what can happen with a sticking or faulty crankcase vent, so replaced mine with a NOS that's never been used. It helped, but the motor's still an oil burner...

Any suggestions? I won't be tearing the motor apart 'til warm weather gets here, but it'd be nice to find out if it can even be fixed before I start buying pistons, cylinder hones and headache medicine... lol
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Old 01-12-2018, 06:41:44 PM
Thaumaturge Thaumaturge is offline
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

Clean (rinse out) crankcase breather....
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Old 01-12-2018, 07:36:18 PM
G.M.Johnson G.M.Johnson is offline
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

If the motors have been run hard and never really had a proper oil change it can be just worn out rings and/or scored or worn cylinder. There is also a possibility that the oil return holes in the piston oil ring groove are blocked or plugged! I've seen a few that where that way and the oil has no way to drain back. Just another possibility.
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Old 01-12-2018, 09:01:44 PM
R Davis R Davis is offline
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

Thaumaturge... replaced the original crankcase breather with a 'new old stock' breather still in the box. It helped, but not enough.

G.M.Johnson... what you're saying makes sense. I've got a 2nd Model N sitting on the shelf that smokes worse than the one I'm using, but will fire right up. That'll probably be the motor I tackle first, thanks for the help...
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Old 01-12-2018, 10:23:33 PM
akuna akuna is offline
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

Dumb question. Could oil be leaking past the valve guides when setting?
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Old 01-13-2018, 07:17:18 AM
R Davis R Davis is offline
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

Akuna... I was wondering the same thing, but with the way the motor's built, any oil leaking past the valve guides would end up back in the crankcase.
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Old 01-13-2018, 01:19:45 PM
akuna akuna is offline
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

Well if you look before you start it, is there oil in the breather tube or carb?
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Old 01-13-2018, 02:00:49 PM
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

Too much oil in oil bath air cleaner?
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Old 01-13-2018, 06:08:49 PM
R Davis R Davis is offline
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

Back inside after pulling the air cleaner and breather tube. Inside the tube and carb didn't have any oil, they were pretty much dry. The oil bath air cleaner was a tad low on oil, just below the full line.

When I'm plowing snow with the tractor, once I've pushed the snow up to the pile, I'll keep right on pushing with the tractor and drop the handle bar. Sometimes the handlebar will go all the way to the ground while the plow raises up to pile the snow higher. That would probably tip the air cleaner enough to spill a little oil, but doesn't seem like it'd be a problem.

It seems like what G.M. Johnson says about the oil return holes in the piston oil ring groove being blocked or plugged is probably what's going on. I'm still open for more suggestions though, we've got more snow forecast for later in the week and I can't tear the motor down just yet. I was going to wait 'til spring, but probably shouldn't let this thread die without getting an answer to what's wrong...
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Old 01-13-2018, 06:11:56 PM
akuna akuna is offline
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

I thought you had 2 others that you could look at?
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Old 01-13-2018, 07:58:18 PM
R Davis R Davis is offline
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

I do. I'll take the motor I'm describing off the tractor and put one of the other motors on. They act a little different, the one that's going on smokes all the time...lol

I'll take the motor apart that's on the tractor, look at the piston and rings, then get on here and let you know what I find.

It'll be a few days, but I'll report back.

And thanks for the help...
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Old 01-14-2018, 12:32:58 AM
gdstew gdstew is offline
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

The only real way to know is to take it apart and look at it. I'm guessing it has a worn out bore, worn out rings, and probably an undersize crankshaft. Could be opening up a real can of worms!
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Old 01-14-2018, 07:32:58 AM
R Davis R Davis is offline
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Default Re: Briggs and Stratton Model N that smokes. What's the fix...

gdstew... I hope you're wrong about the can of worms. In hindsight, I probably should have waited 'til I was ready to work on the motor before asking a question here.

Now that've started this thread, I really ought to take it far enough to answer the question I asked. These discussions do come up in Google searches, so I feel like I owe it to the owner of this site to finish what I started.

One way or the other, I'll eventually have a decent model N powering the tractor, so tearing this motor down was going to happen sooner or later...

I appreciate your post though, you might be right...lol
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Old 01-14-2018, 11:55:58 AM
gdstew gdstew is offline
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Default Re: Briggs and Stratton Model N That Smokes. What's the fix...

I hope I'm wrong too. If you expect the worst, then you won't be disappointed when you get it apart. However, you might be pleasantly surprised instead.

I've met lots of disappointed people while in business.
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Old 01-14-2018, 02:56:39 PM
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Default Re: Briggs and Stratton Model N That Smokes. What's the fix...

Better to expect the worse, and be disappointed, then expect a easy fix and be disappointed.
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Old 01-14-2018, 03:28:39 PM
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Default Re: Briggs and Stratton Model N That Smokes. What's the fix...

What oil are you running? Is it the same oil in all three? Could be a simple change in oil weight or type might help. Two possibilities here. Too thick an oil can cause worn rings to plane over the oil film on the inlet stroke resulting in lots of burning oil, especially on a cold engine. Too thin an oil can get sucked up the inlet guide or simply go by worn rings, especially on a warm engine. If it were mine I would try a straight non-detergent 20 or 30 weight as a trial to see if there is any change. Perhaps even consult Briggs (or find an owners manual or service guide for that engine - they do exist) and see what they recommend for oil.

Could be as simple as worn or stuck rings or could be as bad as needing a rebore which, frankly, is not such a big deal. Biggest problem wit reboring is finding a suitable, hopefully NOS oversize piston. Bigger problem is dealing with a worn crank, bigger yet is finding that the cam and lifters are badly worn and misshapen.
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Old 01-15-2018, 12:59:58 PM
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Default Re: Briggs and Stratton Model N That Smokes. What's the fix...

beezerbill... I'd already swapped motors on the tractor before seeing your post and why I didn't respond sooner to your oil question. Synthetic 10W30 is what's in it..

Took the shortest route possible to get a look at the piston and bore on the Model N, anyone not familiar, the oil pan is separate from the rest of the block and held in place with just 2 bolts. Aside from removing the head and air cleaner, that was it.

The piston's scored pretty bad, especially above the last piston ring, but also pretty consistent below the bottom piston ring all the way to the end of the piston, just not as deep. Micrometers showed 1.987 to 1.988 above the top ring and 1.993 to 1.994 just below the bottom ring. The bottom piston oil ring holes weren't plugged, but all 3 rings are scored consistent with the bottom of the piston.

I don't have a way to measure the bore, but it's smooth where the piston rings travel and scored pretty bad below.

Also, when I pulled the head, there were small particles of grit laying in the area where the spark plug screws in. There's a small build-up of carbon around the exhaust valve also, maybe some of it broke loose.

No play between the crank and connecting rod and the camshaft doesn't show any unusual wear.

I didn't know what to expect, so didn't get disappointed...

Opinions appreciated as to what I need to do next...
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Old 01-15-2018, 02:00:15 PM
gdstew gdstew is offline
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Default Re: Briggs and Stratton Model N That Smokes. What's the fix...

Bad air cleaner allowed dirt to enter engine on top side of piston. Dirt went past rings and scratched bottom side of piston. Dirt mixed with oil made a fine lapping compound and "polished" everything it touched.


Have a shop measure the bore and the crankshaft/conn rod journal. Check with the guys on the other "model N" thread for parts availability and prices and go from there. Don't forget to look at the valves too!
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Old 01-15-2018, 04:26:45 PM
Andrew Mackey Andrew Mackey is offline
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Default Re: Briggs and Stratton Model N That Smokes. What's the fix...

The oil ring wears the fastest as it has a much less cylinder contact area, and it has a scrapping actiion on the bore. You have issues. One, the cylinder is now polished in the ring travel area. This combined with worn oil rings allows oil to pass the oil ring. You also have about .005" of bore taper. This will make the rings move in and out of the grooves in the piston, which will make for excessive side play in the rings. This means oil passes the side of the oil ring. Two, you are using a multigrade synthetic oil. Synthetics are more 'slippery' than a standard oil, this they get past worn surfaces easier. This means more oil consumption. Also the 10W is relatively thin for an engine built to use either a straight 20 or 30 oil (summer).

You could try honing the cylinder, and if it cleans up, put in new rings and see what happens. i think that will not work well in your case. The biggest issue would be the bore taper. If you hone and remove it, you will be better than halfway to the next oversize, in which case you will now need new standard pistons, as yours are also worn undersized. Now that the bore is larger, you now end up with a piston clearance problem. Back to ring flex as the piston flops in an over large bore. personally, i would have the piston and cylinder taken to .010 over, if that cleans up the cylinder, and go from there. If .010 doesn't do it, you may have to go .020 over, to get a true, straight bore.

If you rebuild the engines, I would run for 5 hours at load for break in, with a straight 30 oil. After that, you can switch to a synthetic and see what happens. I would not recommend synthetics for break in, as they have superior lube characteristics. This will inhibit ring seal on these older cast iron engines. As i stated earlier, once the engine is broken in, you can switch and see if it smokes.
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Old 01-15-2018, 04:51:47 PM
gdstew gdstew is offline
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Default Re: Briggs and Stratton Model N That Smokes. What's the fix...

"I don't have a way to measure the bore, "

The measurements listed were for the piston, not the bore. It may be already +.010" or more, who knows!!! Get the engine measured up first, then go from there. No use cleaning up a cylinder that may be no good to start with.

---------- Post added at 03:51:47 PM ---------- Previous post was at 03:39:43 PM ----------

Bore gauge--for those of you with basic micrometers or dial indicators, you can make a simple tool for measuring bores. Get a 1/4" or 5/16" turnbuckle and cut off each end. Make the ends round and smooth and lock nut one end in place. Grind a couple flat spots on the "bolt" part of the other end to aid in turning it in or out. Put a wing nut on that end.

Put tool in bore, adjust bolt to widest spot, snug wing nut. Remove and measure.
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