Small Engines (Lawn Mowers, etc.): B&S 17hp twin running erradically follow up, engine acts, little engines

Question
QUESTION: B&S 17hp twin engine runs badly.  At normal full throttle, the rpms drops low then bounces back up and runs good for a five seconds, then drops back down for a few seconds, and comes back up and does this consistently while running.  When you put it under a load, it runs smoother and does not do it as much and almost goes away.  As soon as you unload it, it starts all over again.  With just the engine running not doing anything, if you run it at low throttle speed, the rpms drop so much that the engine will die when the rpms drop.  This happened suddenly from one day to the next.  A while later, it stopped doing it completely using it a couple times to plow snow.  Now it is back to doing it consistently and seems to be getting worse.  Engine acts like it is on the verge of backfiring while running and every time I shut off the engine, it backfires one time really loud every single time.  I maintain it very well.  Have changed all filters, spark plugs, and use full synthetic oil, etc.  Has run perfectly for years.  Riding mower is about 10 years old.  I even own a service manual for it, but not sure where to begin with the diagnosis.  A couple of years ago, the air cooler passages got plugged with leaves and I may have overheated it, but it ran great after cleaning out the leaves, so I don't think I did any damage.  I have a strong automotive knowledge, but don't know these little engines nearly as much.  Engine info is as follows:
MODEL# 42A707
TYPE# 2653E1
SERIAL# 9801095B

I would very much appreciate your advice.  If you have any automotive questions I would be glad to help.  I am ASE certified.

Thanks

ANSWER: Did cleaning the main jet inprove engine performance?

Eric

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

QUESTION: Thanks for the quick response.  I tried to get back to you earlier, but it would not allow me to re-submit another question.  When I got your answer back, I pulled the plug out of the carb and attempted to remove the main jet.  When trying to remove it, it was taking a lot of force with the alan wrench and I was afraid to strip it.  So I left it in the carb and took a can of carb clean with a tube and sprayed it directly in the jet several times.  Looked in there with a flashlight and all looked clean and good.  Put it back together and tried it out and it ran perfectly for about five minutes.  After that, it suddenly started running badly again- the same way it was before.  I shut if off and checked for any loose bolts on the carb or the intake and checked for any vacuum leaks.  Pulled spark plugs and they both looked good- burning maybe just a little on the lean side.  I have a compression tester, but have not yet checked the compression.  I don't think it is an issue because when it runs normal, it runs really good.  I am still going to check it- just have not done so.  So after doing these things and letting it sit for about ten minutes, I took it out and tried it again for another 15 minutes and it ran perfectly.  I didn't change anything however, and I think it will start acting up again most likely.  I am planning on getting a carb rebuild kit for it next week and will go through the carb and see if that takes care of it.  What seems really odd to me is that the problem is intermittant.  Could it be that once the engine warms up it has a deeper issue like a blown head gasket or something that would make it run like that?  Let me know what you think.  Thanks a ton.
Chase

Answer
Maybe a leaky head gasket.  Did you check the head bolt torque?

http://www4.briggsandstratton.com/miscpdfs/RNT/Engine%20Specifications%20Chart_m

Compression testing is done with a cold engine, per the manual, but you can also check the compression when the engine stops running.  This might help isolate a leaky or blown head gasket.

On a really odd not, I once had a Tecumseh engine, really old one, where a couple of head bolt holes were so wore that they would start leaking when the engine warmed up.  Took me forever to track down the problem.  Ended up oversizing the head bolt holes and installed bushings to match the orginal hole size.  Ran great.  However, I have never seen this happen on any other engine brand.

Could also be valve gap but I'm betting there is something in the carb...maybe a little bit of water.  Today's carbs are tuned so precisely that it only takes a drop of water to mess them up.  Most of the time I rarely find anything inside the carb but a good cleaning solves most of the problems.   In fact, carb cleaning is the most common repair procedure and I have stopped even trying to find dirt/debirs or water in the fuel system.  Just drain, flush and clean the carb/fuel system and they run great again.

Eric