Some friends have dropped a CR85 off here for me to sort out a no go issue.
It turned out the engine and exhaust was full of water from what I presumed was a blown head gasket.
I replaced the head gasket and the bike fired up straight away and went like a beauty.For about 10 minutes, then it started missing and not running so well. At this stage water was dribbling out of the weep hole in the water pump, Because the system is being pressurised by combustion pressure, gases entering the cooling system I removed the radiator cap and all looked good in there but when I revved it,the coolant “foamed” a bit Could just be coolant flow or combustion gas but the water had stopped coming from the weep hole Because you are stopping the combustion gas building pressure with the cap off as soon as the radiator cap was replaced it started dribbling again Pressure build up again I removed the spark plug and there didn’t appear to be any water being expelled through the sparkplug hole, because the combustion pressure his much much higher then the 1.8 bar of coolant system pressure the cap maintains, you removed the cap thus letting the pressure out had you not removed the cap before this and turned the engine of with no cylinder combustion pressure (engine not running remember) the cooling system pressure maintained by the cap until it cools would have pushed coolant into the passage that the combustion gas is escaping through when you turned the engine over like it was prior to me replacing the head gasket. I then pulled the head off again and all looked good in the cylinder ( a few scuffs around the exhaust port but no scores), with no coolant in there. I inspected the head and cylinder mating surfaces and it looks all good there too. Should I consider replacing the head as I put a straight edge on it and I could see light through the gap Yes but what is that gap? What is the spec because you being a mechanic by trade know a gasket is designed to compensate for non perfect matched surfaces, if you understand what I mean. Seeing as though it isn’t my bike I am dubious about spending too much on it, as the owners plan to sell it in the near future. I am presuming that there is some cylinder pressure escaping into the cooling system still hence the leaking weep seal. Anyone want to shed some light into this dilemma? Can the heads on these bikes be machined flat again or are they a disposable item? No I would think it can be machined to a spec but again whats the warpage it has. Any cylinder head can but the squish in the combustion chamber will be affected especially on a two stroke. I suggested you deck the head and you said that they are around $65 or there abouts and not worth mucking around with
A head gasket normally doesnt just blow, there is a cause, loose bolt, coolant level low engine overheats head distorts, electrolysis on the mating surface reduce sealing mating areas and many more reasons. You said the cylinder heads are very cheap, but if it was me to rule out the head in the short term I would purchase a new head gasket, get a large piece of glass and a couple of sheets of 300 grade wet and dry and deck the head. Then I would check the radiator cap is releasing pressure at the spec its supposed to for a CR85, if its faulty and stuck when it needs to purge coolant through expansion and cant that coolant pressure will need to go elsewhere as you know. The other thing we discussed earlier in the week is some form of electrolysis, if it has electrolysis a cooling passage could be eroded enough that it is opening up with heat when running allow combustion pressure into the cooling system and vice versa when the engine is stopped [/b] [/u]
