Resurrecting an old B&S Flat-head vert shaft motor

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sleddriver

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B&S Sprint vert shaft 3.75Hp gas engine on a 20" MTD Lawnflite chassis. Age ~ 10yrs.

My neighbor was going to junk this because the pull cord broke. I fixed it, pulled the head (the bolts were loose), cleaned a bunch of ugly black junk out, put in a new plug, changed the oil and replaced the diaphram on the carb.

It runs ok, but is very difficult to start w/o ether. There was visible scoring on the cylinder bore and it burns oil. Anyone ever rebuilt one of these before?

I have noticed the foam air filter easily clogs in dusty conditions (it's on the same side as the discharge chute (small wonder)), which makes it run rich and guzzle gas.

I have a small yard though, which takes me 15 to cut then I'm done. So I'm hesitant to spend $$$$ on a fancy mower.

What say you?
 
I would run it until it craters. The bore of most of these engines is flashed with chrome and usually cannot be bored oversize.
 
I wouldn't bother with trying to rebuild it. It might end up being more aggravation than you planned on and as FX4 said it might not be feasible anyway. If the deck is really nice, Briggs sells complete shortblock assemblies with gaskets, but it will probably cost as much as a Wal-Mart mower.

What type of starting system does it have? Choke or primer?

I have an AYP cheap mower that's at least 20 years old. It has a 3.5HP Briggs flat head and has a choke, no primer. Usually it takes about 4-5 pulls to get it running, and from there about 30 seconds at half choke to get warmed up, but once it's warm it will mow all day. Mine actually doesn't use all that much gas or oil, and it doesn't smoke. When I got it back in 2009 it had the usual tar oil and brown cake air filter. I did a basic tuneup, but did not replace the diaphragm and I've never taken off the head too look inside. It has seen a whole lot of use since and has never had a problem aside from the slow starting.

Also, I like this stuff...
http://www.blastercorporation.com/Lawn_Mower_Tune-Up.html
It's at least good for a smoke show.
 
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It uses a bulb primer & a plastic carb. It runs very rich and the plug is oily & black. I read somewhere that a rich condition is usually caused by a bad gasket between the carb and the tank. Can't remember if I replaced that when I did the diaphram. Could also be dirt in the carb.

I'm going to pull the carb & tank and thoroughly clean. And use something heavier than 30wt in the crankcase. I added about 15ml (1/2oz) of RL Si1 to a gallon of mower gas and the mower is running worse! Interesting...

I keep my machinery in good shape and it goes the distance. The problem with this one is it's been abused by it former owner so I'm dealing with someone elses neglect.

Thanks for the reply.
 
If you're not sure on the carb, definitely take it apart again. I've seen new diaphragms stick on these engines before. Make sure the spring is good and the little screen is clear. Usually a new diaphragm will come with those and a gasket.

Also, you can sometimes unstick a diaphragm by putting your hand over the carb right as the engine starts (air filter off).
 
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Put ridiculously thick oil in it, too. Straight 50 or something would be great. It'll help seal up that cylender, likely making it easier to start. You might not be getting adequate compression due to excessive scoring causing leakdown. The oily blackness on the plug could actually be oil rather than a rich mix.
 
Just throw in some Restore!!
smile.gif
 
Originally Posted By: greenaccord02
Put ridiculously thick oil in it, too. Straight 50 or something would be great. It'll help seal up that cylender, likely making it easier to start. You might not be getting adequate compression due to excessive scoring causing leakdown. The oily blackness on the plug could actually be oil rather than a rich mix.
Yep..good plan. I'll take a photo of the cylinder scoring. It's an aluminum engine, no iron sleeve. I might also pull the valves and regrind the seats for grins.
 
Originally Posted By: sleddriver
..Yep..good plan. I'll take a photo of the cylinder scoring. It's an aluminum engine, no iron sleeve. I might also pull the valves and regrind the seats for grins.


If you want to do it for fun, why not.

I'd throw a donor engine on it, or a new cheap OHV Chinese import before I sweated over rebuilding a cheapy old pulsa-jet Briggs.
 
would be a good candidate for trying out a lot of oil additives that stay help stop oil burning.

i think it depends more on if you prefer a reliable mower that will start reliably all the time and let you cut your grass in 15 minutes with zero hassle, like when you get home from work and have 15 minutes of daylight. Then it might be worth spending a few hundred on a quality mower.
 
Engine Model: 096902-3113-01; Code: 92031225

Finally got it all back together and the cold hard start persists. When I pump the prime bulb, it never fills. I looked down the carb throat and didn't see any gas shooting into the intake manifold. I did notice gas leaking below the bulb, (which is only a year old).

So I fired it up with ether. Idled very well, puffed a bit of smoke. Mowed great. Then it began smoking even more. I stopped and noticed quite a bit of gas leaking on top of the fuel tank, running down the tank onto the deck. (This area was dirty before; thought it was the carb tank gasket so I replaced both gasket & diaphragm).

Removed the engine cover, air filter, carb & fuel tank and noticed light smoke in tank! Maybe oil smoke. Hmmm. . . I'm beginning to wonder if the cheap plastic carb is Toes Up. It's not meant to be repaired. The primer system doesn't work. Like a check valve is stuck open.

On the exhaust side, I removed the exhaust manifold and noticed oil had been leaking out of the bottom of the crankcase breather, which coveres the valve springs. I did replace the gasket, and was going to replace the breather, but the service center was out of stock.

So I'm thinking that if the check valve in the breather is Toes UP, then it's over pressurizing the gas tank, causing the leak out of the bottom of the primer bulb, particularly if some check valve is stuck open.

I cleaned the carb out best I could. It's cheap plastic, not designed to be taken apart much.

Perhaps increased crankcase pressure is forcing more oil past the rings, causing the smoke show?

Fishing for suggestions, & ideas!
 
I sprayed more carb cleaner into the crankcase breather/flap valve, shook it a bit, and began to hear a rattle inside. More flushing out till the liquid ran clear. Set it aside for now.

Learned that the prime pump ball, seat & spring ARE removable from plastic carbs. The ball has surface corrosion (from ethanol?), but the seat was installed backward by someone else, which caused the primer to never work right.

Also discovered that there should be a gasket between the air filter and the carb. Never was. I made one out of thin closed cell foam as the dealer was out of these.

The gas leak below the primer bulb onto the top of the tank was fixed by realizing that the long 5mm bolt that holds the air cleaner on, goes through the carb through the gasket/diaphragm and into the fuel tank. I'd been makind due with something else because this was missing when I got the thing. The dealer had one and with the gasket, it sealed up tight and leaked no more.

I put it all back together, filled with gas, and after a few presses on the primer bulb, I could feel it full of gas! Success! She started right up after 5 squeezes and idled smoothly.

With the motor running, I observed no leaks below the primer bulb or on top of the tank. Also noticed no oil leaking from the bottom of the breather.

After about 10 min of running, she began puffing blue smoke though. Not near as bad as the other night, but it continued till I shut her down.

With everything more sealed up, I wonder if the crankcase pressure has now increased to cause it to smoke more than before. The cylinder bore does show a few scratches I'm able to hang a finger nail on. But I don't have a hone and didn't want to tear it completely down.

Perhaps I'll have to go to 40wt oil to minimize the smoking. That's all for now.
 
aee486d2.jpg

This is the only image I could upload. PB seems to have a unspecified problem with the CH photo. Not enough information to proceed.

Anyhow, this was mostly black. I went after it with carb cleaner, syn steel wool and a brass brush. The CH is just as clean now. So are the valves. I chucked them in the drill press, spun at > 2000rpm and polished/cleaned with the 3M syn steel wool (great stuff). Then lapped them in.
 
Thanks!! I sent you a PM then realized you'd responded before....sorry, my mistake.

Perhaps I should look on craigslist for one of those golden lawnboys and work on that? Thirty years ago, I finally convinced my dad to buy one and never complained again about having to cut the yard. Great mower.
 
No problem, I've never rebuilt the bottom end on one of these. If you can feel score marks in the cylinder though, it's going to burn some oil. There's probably not much you can do with this engine aside from mitigating it with the heavier oil. Give the 40 weight a try and see what it does for the smoking.
 
Thanks. You mentioned shortblock...never heard of it...so it's just part of a motor? Perhaps that's the answer for a DIY'er. Please elaborate.
 
Briggs & Stratton sells complete bottom ends for most, maybe all of their engines. Basically it's everything below the cylinder head completely assembled and ready to go. The kit comes with all of the necessary gaskets so you don't need to buy anything extra to install it. You just remove the carb, gas tank, recoil and housing, cylinder head, etc. from the old engine and swap them over to the new block with the supplied gaskets. Install the engine back on the mower, set the governor, and you're back to mowing with a brand new bottom end.

If you get the complete model number off the engine, a dealer can look up the shortblock part number for you or you can probably find it by Googling.

Here's a list of Briggs shortblocks at Small Engine Suppliers...
http://www.smallenginesuppliers.com/shop/briggs-horizontal-engine-shortblocks.html

They are not cheap, but you are getting a new assembled engine and it will probably cost a little less than a complete Chonda engine.
 
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I just noticed the link I gave only lists horizontal engines. If you can find the model number on the engine though we might be able to figure out the right part number for it. I know they make vertical shortblocks too since I've installed a couple. They are usually purchased when someone runs a mower without oil, lol.
 
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