Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

Looking for some insight and feedback from the small engine experts on this forum.  I have a 2002  6HP Craftsman 4-in-1 Yard Vac (non-propelled version) that will suddenly lose about 50% of it's power after it's warmed up (about 7-10 minutes from the initial start).  The engine is a 6HP Vertical Briggs & Stratton 123K02-0240-E1  (self throttle and manual choke).   The engine starts on the first pull and runs at full power for the first 7-10 minutes, then starts to sputter for a second or two (almost like it's running out of fuel) and then runs at about 50%-60% power.  If I stop the engine, it will immediatly restart on the first pull, but again only at 50% power.  The engine never dies or cuts-out, just runs at half power.  I've removied the gas cap when this occurs thinking maybe I had a vapor lock issue, but the engine power/RPM's never get faster.  It's been doing this the last 3 or 4 times I've used the machine.  Like I said, the machine runs like a swiss watch for the first 7-10 mins then as soon as the engine gets warm.....the RPM's drop and the machine looses 50% power.  Here's what I've done so far to troubleshoot the problem.  1)  Changed the oil (conventional 30wt).  When I drained the old oil, there was only about 7oz remaining in the machine and it was filthy.  I changed the oil about 6 months ago and I'm 100% certain I added 18oz. of 30wt oil.  There's no drips or leaks anywhere that I can see, so clearly the machine is burning oil, however I don't see plumes of smoke when the machine is running???  When the machine is warm after about 20 mins and I pull the oil dipstick, I get a slight whisp of smoke that comes up the dipstick tube.  I occassionaly get the same wisp of some when I pull the air cleaner.   2) Replaced the air cleaner.  I noticed there was a very small amount (about 3 drips worth) of oil across the bottom of the air cleaner housing.  It appears to be entering the air clearner housing through the crankcase vent tube.  The old air filter was fairly clean and did not have oil on any of the pleats.  The drips of oil were only on the bottom of the filter housing.  3) Replaced the old spark plug with new one.  Old plug looked pretty good.  Little bit of carbon, but nothing serious.  Certainly no oil on the plug.  Spark plug gap was perfect.  4) Removed the fuel tank, throughly flushed and cleaned the inside, checked all fuel lines for cracks/holes, blew out all the fuel line hoses and inlet nipples, tightened all fuel line clamps.  Replaced the gas cap with a new one and refilled with fresh fuel.  5)  Checked throttle spring and linkage arms.  No bent, broken, or missing parts.  6)  Pulled the carburetor (which didn't have a speck of dirt or varnish inside the bowl) and completely dissambled the bowl, main jet, float assembly, and the needle.  Forensically cleaned every square inch of the carburetor using carb spray and a thin piece of copper wire.  Blew everything out with compressed air and reassambled using all new gaskets.  Zero change in performance.  When cold, machine starts on first pull and runs at full power for the first 7-10 mins then falls on it's face to 50% power when the engine gets warm.  I don't have a compression tester or a leak down tester, but I'm wondering if the problem could be a failed headgasket???  Would a headgasket issue only show-up when the machine gets warm and pressure builds inside the engine?   Any thoughts or advice?  I love the machine, so if I have to repower it with a new engine I certainly will.  I just don't want to give-up on current engine if there's something I'm missing.  In advance, thank you for your feedback.   Rich A.   

  The oil loss seems extreme.  Oil might be going out the breather unnoticed.  It could be ring blowby.   Head leak when warm or valves when hot.   You could check the valve settings.

   A leakdown test would be good.   Harbor Freight has a low end unit for $30 but tough to justify if you’re going to use it once.   It’s possible to kluge a sort of tester with a spark plug base or find some other threaded short pipe you can fit into the plug hole.   Attach a compressor to that and regulate the pressure to 10lbs.    Get the piston to TDC, wedge the flywheel so it will not move, pump in air and listen at the carb throat (valves), the oil fill hole (rings) and around the head (gasket/warped head when hot).   If there are no leaks and you unwedge he flywheel the engine will probably turn over.  You might have to bump the pressure but 10 is usually plenty to get the flywheel to move on a tight low hp engine.