Small Engines (Lawn Mowers, etc.): surging, manifold gaskets, engine stalls

Question
QUESTION: i have a B&S 20hp v-twin model# 407777 type-0167e1 that is surging and backfireing through carb and muffler. carb was cleaned and valves were adjusted i have narrowed it down to a bad cylinder when i pull the wire on the good side engine stalls(runs perfect on good side no backfire or surging). i pulled the head cleaned everything valves seem to be ok. i did notice some oil in the cylinder so i thought bad rings, cylinder wall looks good and still has crosshatch pattern. i did a compression test and got 130psi on bad side and 115psi on good side so now I'm stumped. should i still try new rings or is there something else that could be causing the problem?

ANSWER: Do you have one or two ignition modules?  If you have two, have you tried swapping the modules?  Is there a small wire connecting the two modules?  Does the harness have diodes in it?

Compression should be fine, rule of thumb is as long as the cylinders are within 10-15% the compression is considered good.

Have you installed new spark plug?

When you cleaned the carb did you blow compressed air from an air compress through all the carb orfices?  Were both intake manifold gaskets in good condition?

Let me know.

Eric

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

QUESTION: Eric,there are two ignition modules and there are diodes inline with each wire leading to each module but no wire runs between the two. tried swapping modules and still no change, diodes look fine not burnt or anything. i did find a paper thin homemade gasket on the bad side if the intake manifold so i bought a new one and still no change, thought for sure that was the problem. new plugs on both sides and i am getting a good spark on both. i totally disassembled carb cleaned with carb cleaner and blew out with compressor repeated process again today and also ran a fine wire trough everything, still no change. i am completely at a loss this damn thing is driving me crazy any other advice would be greatly appreciated, thanks.

ANSWER: Did you use a Briggs and Stratton gasket?  Do you have a leakdown tester?

I once had a Tecumseh with a weird problem.  Turned out the head gaskat was leaking slightly, not enough to tell on a cold engine but when warmed up the leak was just barely detactable.

Have you checked the torque on all the head bolts?

Eric

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

QUESTION: intake gasket is B&S, head gaskets look good, i have an adapter in my compression tester kit that allows you to hook a compressor to it so you can do vale work without taking off the head. i did use it to fix one of the springs and air leaked into the crankcase pretty fast(on the good side). also checked the flywheel key just to be sure it isn't a timing issue and key is perfect.that being said i left out one important issue along with surging i also have backfiring mostly trough carb and sometimes trough muffler only when both cylinders are firing or if just the bad cylinder is firing. i took the head back off the bad side to check the valves once again, and again there was what seems to be a lot of oil coating the cylinder/combustion chamber. still don't see anything wrong with the valves so i pulled the pistons and checked the end gap on the rings the biggest feeler gauge i have is .050 and there was still play I'm guessing its about .060. the spec sheet on this motor says max end gap is .030 so i am way out of spec. when i did the compression test i cranked the engine with the with the ignition key but i just read on B&S engines you are supposed to crank backwards by hand, don't know how true that is but maybe thats why my test came up with good #'s. so i am thinking that to much back pressure is building up in the crankcase because of loose rings and also not enough spark because to much oil in combustion chamber which is causing backfiring. although i have a decent amount of knowledge with cars this is my first attempt at small engines so let me know if i am on the right track or if there is something else i could be missing thanks again,

john

Answer
When you checked the rings did you make sure they were square in the cylinder by using the piston to push the ring inside the cylinder?  Did you check the ring gap at the top, middle and bottom of the cylinder?

Are you planning on re-ringin only or boring and replacing pistons/rings?

The backfiring through the carb is generally a timing problem.  Are the push rod straight and true?  Most ignition models have an electronic advance  but I have only had one coil ever cause a performance issue...most of the time the coils either produce spark or they don't.

If you just re-ring you might want to use chrome rings as they tend to seal a little better if the cylinder it wore.

Eric