Small Engines (Lawn Mowers, etc.): lawnmower problem, engine model type, valve spring compressor

Question
QUESTION: I have a b&s 16 horse twin cylinder riding mower. When it is running it blows gas up through the carb into the air filter. What causes this? thanx

ANSWER: How old is the engine?  Have the valves ever been serviced?  How much gas is blowing into the air filter?

What is the engine model, type and code number?

Let me know.
Eric

---------- FOLLOW-UP ----------

QUESTION: I'm not sure the age of the motor. I just purchased it. The guy I bought it off of didn't say anything about any motor work being done to it so I'm doubting it has had the valves serviced. It is blowing just a mist of gas into the filter but it does it the whole time it is running. It runs and cuts fine but I don't want to keep mowing with it if there is something wrong with it.

Engine model       Type        Code Number
402707          1240 01     9312095B

Hope this helps. Thanks for your help also.

Answer
Based on the code number the engine is a 1993 model.  Since the enigne is 17 years old it is bound to have some blow-by from worn rings and possibly from poor valve sealing.  

A valve job is not really that hard to do but you need a few special tools.  A valve lapper, feeler gauge, valve spring compressor, and a torque wrench.  You will also need to replace the head gaskets.  If you have any friends that work on cars they may have a couple of tools you could borrow if you do not have them yourself.  If you had to buy all the tools and parts you'd probably spend about a $100...been a while since I bought any tools.

Here is a good link for servicing valves:

http://engines.myfaqcenter.com/Answer.aspx?p_faqid=3441&body_0$tbKeyword=valve#l

As for grinding the valve stem for valve/tappet clearance, a flat file or dremel tool can be used.  You don't have to grind very much at all.  Usually a few strokes with a hand file is enough to get the proper clearance.  

If you decide to tackle the job yourself, the valves should come out real easy.  If they don't, check for burrs or rough spots on the valve where the retainer sits.  These burrs are generally very small and be cleaned/smoothed with a flat file; again, it doesn't take much.

Is the air filter getting soaked with gas or is there just a fine, slimy/slik film in the bottom and/or top of the air filter housing?  Honestly, if it is just a film and the filter is not soaked I would probably just run the engine the way it is.  My old 95 VFR750 motorcycle has a light amount of fuel/oil in the air filter housing every time I service the air filter...and yes, the valves have been serviced.  Doesn't hurt the performance and I just wipe it clean when I service the air filter.

Let me know if this helps.
Eric