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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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  #11  
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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  #12  
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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  #13  
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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  #14  
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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  #15  
Old 01-14-2018, 02:56:39 PM
akuna akuna is offline
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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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  #16  
Old 01-14-2018, 03:28:39 PM
beezerbill beezerbill is offline
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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
R Davis R Davis is offline
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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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