Been following this as I'll soon have a 1950's Bonnie engine to sort out, (my daughters blokes toy). From past experience if you going to clean out an engine it's best not to remove any plate out (coating or varnishing) caused from the oil so that would rule out using solvents unless they are very gentle or a detergent type. As I understand it the hydo-carbons plate out on the surfaces and form a Teflon type coating and also fills in all the little machining marks etc. Over time this coating aids the flow of the oil so improving lubricant flow. By cleaning this coating off you basically have to start the process again. Cleaning sludge out no problem providing you can get it all out preferably to prevent blockages, by blowing it out with comp air. Personally, I've never had a problem flushing bike or car engines with a very thin flushing oil as some of the more mature members may recall. This was a special formulation designed to do what it said on the tin. That said I've never had to do this with a modern bike or car BTW, that's what your friendly local 'fettler' is for.
Not wanting to start an argument, as happened on another forum....the following was the general consensus. All modern oils have "solvents" in them to some degree, so using flushing oil should be absolutely ok, as long as you do put engine under load, just run engine at fast tick over till hot, without operating clutch. Its a bit like washing an oily boiler suit....you add extra detergent for the wash cycle only. I agree, if there is a large amount of sludge, and engine has not been flushed regularly, it could be displaced as some have experienced, if oil looks particularly dirty, I'd be inclined to run flushing process twice. This problem is more likely in older engines that ran on straight mineral only oil, with less efficient pumps, coarser filters, and more often than not would have sludge trap built in to design of engine, necessitating removal and cleaning.
I agree, I flushed mine as I was concerned about galleries being slightly blocked. I ran the bike up to temp and blipped the throttle to raise the oil pressure slightly without operating the clutch. This was the first time I've done this and wouldn't flush it on every oil change. Everything seems fine and I feel the engine has had a good clean...
