I've seen more than one set of Bosch platinum plugs my friend installed in customers vehicles with 350 small blocks as part of a complete tune up,make the engine run like it had a dead cylinder,and replacing the plug didn't cure the issue...he tried swapping the old plug wire back on,same deal,it wasn't the wires..when he'd put the OLD spark plug back in,the engine ran perfectly again..
He never could pin down why the Bosch plugs did this--it wasn't an issue with other brands,but he figures its something to do with them being platinum and the engines were not equipped with them as original equipment..
Maybe there was just a lot of defective ones too,he now won't install any Bosch plugs in GM engines,if the customer supplied them,then he'd say I'll have to charge you if the thing misfires or runs lousy and I have to put a different brand of plugs in..
Champions seem to be the most likely to break off in the block if left in too long too,on top of never firing again once they fouled--AC and Autolite's will usually come back to life after you flooded the engine once the engine warmed up...once a Champion fouls,its dead for good..
I have had poor luck with NGK plugs in motorcycles and some small engines--surprisingly Champions seem to work OK in small engines ,other than never recovering from a flooded or fouling issue..
I prefer AC or Autolite plugs in my tractor engines,but use whatever I have on hand too--when you have only one cylinder and spark plug,it HAS to be a "good" one,as there isn't any other cylinders to share the duty of making it run..
I tried those "Split-Fire" plugs in small engines--they work great for a few hours,then cause all kinds of weird running issues ,if you can get the engine to start..magnetos can be fussy about certain plugs,some do not like resistor spark plugs..
I just stick to Autolite 295 or AC 45 plugs,or Champion J-8C or
J-17LM or J19LM plugs in my tractor & mower engines..
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