Hi all,
I've deleted this post - so all the YS users can breathe easier. I don't want to prejudice sales of what could be a good engine. I've just been unlucky with a minor manufacturing problem that became a major failure. As long as I get a replacement engine I will have to be happy.
Stuart

You must be well chuffed with that [&o]
What a mess.
Off to Probuild with... a very big question mark!

Hi,
Just looking at the big end bearing - the outer surface looks like chrome i.e. coated in polished aluminum & the inner surface has a layer of steel from the crank pin. So it looks like the big end bush was loose & rotated. This would block off the lubrication holes & led to the seizure. OR big end to crankpin fit was too tight & led to seizure - although the engine felt free initially, so I think it's the first option. Also I believe the conrod is fitted the correct way round - with the 2 grooves facing forward.
Stuart

Stuart

Occasionally these engines new have insufficient lube in the crankcase and, as the bottom end is mostly "waste oil" lubed, the rod bushing welds to the crank and spins in the rod before sufficient lube arrives. They will run without load in this condition for a while, but will fail as yours did when loaded.

The prevention is to add oil directly to the crankcase (via the carb) to ensure everything is lubed before the first run.

Hi Earl,
Yes -I did that, along with the cams, rockers, etc.
I've just been unlucky - but hopefully should get a new engine out of the deal -I'm not interested in repairs!
By the way - my engine has an additional needle valve, which bleeds raw fuel from the carb into the crankcase for "cooling" - but I suspect its more for lubrication. This n/v should be 1/4 tun out - mine arrived at 1/2 turn, so I left it for extra lubrication. The instructions say that any more than that 1/4 turn will cause a power loss (naturally) but there are no dire ewarnings about closing it completely.
I just wonder if YS had had problems in this department - otherwise why supply additional lubrication if its not been proved a necessity?
Stuart

From looking at the engine again, the big end bush is in 3 pieces - the inside surface is partially plated with steel from the crankpin. The outside of the bearing is completely covered in polished aluminum from the con rod - looks like chrome
I think I now understand what happened:
The big end bearing partially seized onto the crank pin; this has caused the bush to rotate in the conrod, having a polishing effect on the bearing & also blocking the lubricating holes. Then the bush has seized, breaking the bush & bottom of the con rod.
As I explained before, the engine was only being test run on very low throttle openings, following the instructions,etc & the throttle never opened more than 1/4. I also oiled the camshaft, rockers, etc before running the engine - so couldn't have done much more. Like I said - I've just been unlucky this time - & just to add to it YS are on holiday this week!
Next time! - I shall remove the backplate & well lubricate the big end, as well as oiling the cam, rockers etc. Once bitten, twice shy - as the saying goes.

Stuart