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I'm at my wits end now with the engine. 2 weeks ago it broke down with suspected head gasket failure, all the signs pointed to this, milky white oil, coolant system pressurising too much and white smoke billowing out the exhaust.

I stripped her down and had the head pressure tested and skimmed. Purchased genuine headgasket and new head bolts, checked the liners were flat and rebuilt it.

She fired up straight away and seemed a lot better. Went for a short ride with a mate following me and he said it'd give a bit of white smoke low in the revs like when taking a junction and also when I changed gears. Said it was getting worse as the journey progressed and smoked a little higher in the revs. Only did about 8 miles and went home.

I don't know what else it could be now and I'm tempted to either buy a secondhand engine or strip the bike for parts.

Can anyone suggest anything else that could cause this? Any common problems that I may of overlooked?

A cracked liner could be letting water by.

its likely that you still have some of your emulsified engine oil circulating, as replacing the oil does not get rid of 100% of the old oil, there is always some hanging around...could just be that you are evaporating all the water in the small amount of emulsified oil left in the engine.

would explain why it seemed to get worse as you went along, as the engine gets hotter more of the water will be driven off.

if the new oil is not getting cloudy, and you appear to have no other signs of a HG going - so loss of coolant, excessive coolant buildup in the expansion tank, bike bubbling after you turn off the ignition, i would try taking it out for another ride, 15-20 miles, warm the bike up fully before starting your ride, and see if it manifests itself again.

have you got a compression tester to measure the cylinder pressure after you did the rebuild?

A cracked liner would make sense, however when I stripped it the first time there was coolant in all 4 cylinders and I did use the drain screws for the block. Although one spark plug wasn't working, so maybe cylinder 1 could be the culprit. Would a compression test show signs of a cracked liner?

I do not have a cylinder compression tester, however I could always get one and test each cylinder