What causes four stroking

[Robbi347]

Just had engine out and fitted new crank seals as it was burning gear oil. Everything back together but when starting it's 50/50 whether the bike will start ok or four stroke. My question is what causes four stroking?

[yanw]

Have you checked the timing?

[Robbi347]

Can that go out? Flywheel is keyed and pick up is not adjustable?

[tzar250]

Maybe mixture too rich?  Did you change the oil pump seal as well?  I think that can also leak (especially if after-market) and let gear oil mix with the 2 stroke oil, if you had been noticing a drop in gear oil level.

[Robbi347]

Yeah could be too rich. Would moving needle position be enough or is it a jet change do you think ?

[Mad Jack]

Check that your chokes are turning off properly as I've had this happen on one of my 2 strokers years ago

[Robbi347]

Check that your chokes are turning off properly as I've had this happen on one of my 2 strokers years ago

Cheers , I'll check that

[Jim Lyon]

Was it running clean before the gear oil problem? If so, it's doubtful that the carb. settings are the problem ! Has gear oil in the wrong place fouled your plugs up? - I don't know how long you've had the bike, but when it comes to carb. related problems, the float bowl height is ignored too often.  While apart, it might be worth considering running the carbs through a vapour bath? Certainly easier to check than another engine rebuild !