a continental O-200 is a beautiful engine. and being a horizontally opposed flat 4 it is inherently balanced. it should run like a sewing machine.
balance is a subtle thing.
starting at the spinner. are all the screws in the spinner the same length? having screws of different lengths and with different washers under the heads will throw out the balance.
are the engine mount rubbers (lord mounts) in good condition? there are two rubbers in each of the 4 mount points. your top rubbers can be quite serviceable and easily seen. the bottom rubbers can be totally stuffed and you wouldn't know it. they are only about $15 each so they are the cheapest you can get in the 150.
the rubbers won't affect balance but they will transmit lots of vibration.
the prop has been to a shop. but is it balanced??? you can check this yourself with a few spacers on a shaft on some blocks on the dining room table.
is the spinner symmetrical and balanced?
go and buy 8 new spark plugs. they are about $40 each. roughly $320 in total.
don't drop them and install them with care. spark plugs that are failing will cause uneven combustion and make some cylinders seem like duds.
are the magnetos in good condition? I had a failing oil seal in the bottom of one of mine and the uneven combustion it caused in the engine made it seem as though the engine was totally clapped out.
are the plug leads in good condition? if these are shorting or breaking down you engine will run roughly.
Are your magneto P leads in good condition. one of these leads intermittently shorting to earth will cause your engine all sorts of rough running.
is your engine oil screen nice and clean? a failing component will often lodge parts of itself in the screen. do some detective work.
are the bolts holding your propeller in good condition? a broken or stretched bolt can give the illusion of all sorts of vibrational problems. equally a set of loose prop bolts can cause all sorts of vibrational illusions. do you have identical numbers of washers under each bolt? ( i even use identical length lock wire between bolts)
get a dial gauge. firmly attach it to the engine and check the propeller flange.
when you turn over the engine by hand does the flange track true?
if it doesn't then you have a bent crankshaft.
with the engine ticking over at idle look at the prop tip arc. is it tracking true?
a prop mounted on crooked will give all sorts of trouble with blade angle of attack being different each side.
ok if all of that checks out then you need to look inside the engine.
the easiest first point of checking is to take off the oil sump, the "football".
with the sump off you can look up into the interior of the engine with a powerful spotlight. can you see any damage? can you see anything that doesn't look to be in the right place?
turn over the engine and check out the big end bolts on each each conrod.
are these all tight and secure? is there any corrosion evident?
putting the football back on will mean a new gasket and new oil.
are all the sections above the carburettor all tight and secure. no air leaks evident in the induction system or looseness introducing mixture changes randomly?
if everything so far checks out then the cylinders need to come off for checking such things as broken rings, cracked pistons, identically sized combustion chambers, valve seating and the like.
if you do this bit yourself you must make sure that as the cylinder is taken off the piston doesn't drop down and put a dent in the conrod. dent a conrod and you've stuffed it.
there is lots of other stuff that can be checked internally in the engine. basically you are looking for damage or out of tolerance setup.
if you don't have them already get yourself a copy of these two manuals.
Continental Aircraft Engine Overhaul Manual for models C75,C85,C90 and O-200. the identification on mine is Form X-30010.
Continental Aircrat Engines Series C-75, 85, 90 & O-200 Service parts catalogue. the identification on mine is Form X-30011. this gives exploded views of what is present in each assembly.
now the caveat here is that I am not a qualified mechanic. I am a private owner of an O-200 engined aircraft and I have been sitting here thinking "what would I check if I had this problem." I've been maintaining my own aeroplane for about a decade.
so your mileage may vary.
get your hands greasy and use a strong light.
