18HP Briggs Opposed Twin - 1990


Old 04-14-14, 12:34 AM
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18HP Briggs Opposed Twin - 1990

I have a 18HP Briggs Model 422447 Code 1216 01 out of a 1990 Grasshopper 718. The mower has 1600+ hours. I bought it two years ago from the previous home owner. 4 acres was a bit much for a push mower!

The mower has run great for the past two season. It doesn't smoke at all. It uses a lot of oil. I'd check the oil every time I'd mow and it always needed about 1/2 quart. The air intake was always oily.

I decided today that I would start to tear into it. I probably should have done a compression check, but I never had any run issues. There was so much grime on the engine, I'm surprised I haven't overheated and thrown a rod. The spark plugs are somewhat ashy, but not oil soaked. This particular engine has two breathers, one of which has been discontinued which could pose a major problem. However the one breather that can be replaced was missing all internal components. No diaphragm on the bottom and no foam insert on the top. The oil smelled like gasoline and was somewhat thin.

I'm planning to buy a bunch of parts, short of new piston rings. I probably will pull the crank cover and heads to replace the gaskets and have a good look.

Could the one failed breather have caused this problem in the first place? Not having the diaphragm and allowing atmosphere into the crankcase?

I'm just baffled by the strong engine with 1600+ hours that consumes oil but doesn't smoke.
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Old 04-14-14, 07:26 AM
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As far as going through oil an engine will either leak it or burn it, so if it isn't on the ground , it's being burned. If it's the latter, the question is how it's getting into the combustion chambers.

If could be a crankcase breather letting the pressure build and blow oil into the upper engine, either past the rings or into the intake through a breather tube.

It could be a leaky float valve on the carburetor that leaks into the crankcase, thins the oil which makes it more likely to blow past the rings. That usually has other symptoms with it.

Or the rings you have may be simply worn with use and need replacing. That would allow excessive blowby to pressurize the crankcase and worn oil rings to allow the oil to pass on the way up.

AS far as the color of the oil burning, it may just be getting started where the ratio of oil to gas is still small and not as noticeable.

Since you plan on a tear down, my take on it is, replace the rings and all gaskets and plastigauge the crank.
Old 04-14-14, 11:21 AM
Join Date: Dec 2006
Location: Canada
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Most likely if the oil smelled like gas then you probably have a leaking inlet needle on the carb. Cleaning the carb would be the best recommendation but I would also add a manual fuel shut off valve like this, between your gas tank and fuel filter:

3.25 in.W x 1.25 in. H Fuel Shut-Off Valve-5091 at The Home Depot

This might resolve all your problems, since most of the gas that would make it to your crankcase would probably happen when the mower is not in use. As marbobj said, it could be this thinned oil that is blowing past your rings and also evaporating through your air filter/breather, etc. This valve should put an end to that.
Old 04-14-14, 04:23 PM
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The carb on this engine is supposed to have a fuel cutoff solenoid. The solenoid is still present, however the wire has been severed. I've capped the end of the live wire thinking I would replace the solenoid at some point, but at a cost of $75+, I'll opt for a manual cutoff.

I read somewhere that these engines could hydrolock from fuel being dumped through the carb when shut off. I've never experienced this issue, but it could still be dumping fuel in, just not enough to hydro lock.

Should I heat the case before taking out the head and case bolts? I read somewhere that thread locker was used from the factory and it would be beneficial to melt it before trying to break the bolts loose. So far everything has been easy out due to having been soaked in oil for several years.

Thatnks for the insight. Planning to tear it down tomorrow. Will post updates.
Old 04-14-14, 05:11 PM
Join Date: Dec 2006
Location: Canada
Posts: 528
The fuel solenoid on the carb will not stop the carb from leaking fuel. It just stops it going into the high speed jet and then backfiring.

If it was leaking fuel into the combustion chamber it can occasionally cause hydro locking, but what it usually does it crank and crank and crank and crank and then papump, papump, then a few more cranks until it catches when it starts. What it is doing here is blowing out that liquid fuel in the combustion chamber and the intake line, out through the exhaust. Eventually it may start but it can take time. How is your starting. Does she catch right away or crank a lot.

In any event those manual shut off valve should be standard equipment, but they are not. Every time the leak happens you should change the oil. That thin oil isn't helping it any. The shut off valve is good insurance and there is a snowballs chance it may fix the oil burning as well.
Old 04-15-14, 04:29 AM
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I never had any trouble getting it started. The first start after the winter to get it from the shed to the garage, it cranked for maybe 10s before it caught. On a weekly basis, I think it would take maybe 5s to start. The fuel pump is a vaccuum type, so these times would be normal I would expect.

I'm going to go on the assumption that due to 1600+ hours, the rings are just worn out creating a lot of blow by. This increases the amount of fuel that gets into the crankcase as well as the excess pressure, which lead to the oily breathers and fuel in the oil.

Tear down tonight! Should be fun.
Old 04-15-14, 06:34 PM
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If the oil smells like gas and is thin, then gas is getting in it. This is a leaking needle problem in the carb most likely, or less likely a fuel pump problem.

If it is losing oil and the inside of the intake is oily, coupled with the fact that the guts are gone from the breather, I would say it's very safe to assume that you found much or all of your oil consumption problem. The rings probably have some wear that is contributing to the blowby, but if it's not smoking, I would put most of the blame on the gutted out breather assembly.

If you are tearing this apart with the intentions of making it as good as you can before putting it back in service, pay attention to the rod/crank clearances, especially on the top cylinder. This is usually the first place to exhibit wear and the most common place for this engine to fail. The top rod gets the least oil and is usually the one that wears out first, creating a knock, then blowing up the engine.

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