Engine cutting out from full throttle to idle

[Paul1098]

Head off and when checking for leaks there is a lot of liquid leaking thru' the exhaust valve, a lot less thru' inlet and still some thru' the decompressor.
Looks like it's time for a decoke as well.

20240722_190342.jpg

20240722_190402.jpg

How do you tell if the valves are burnt/warped or just need lapping in again?
Can the decompressor be sucessfully lapped or will I need to replace it?
From photo of piston it looks like theres a little bit of leakage at the front but doesn't look like it's from the cylinder, is this normal

I lined up the dots on the upper bevel drive gears befor removing the head but I'm now wondering whether it's worth looking at the bore and maybe even checking for wear. Is there a way I can do this and then get piston back to tdc with lower bevel shaft in correct position?

Cheers
Paul

[Duccout]

Hi Paul,

As long as you do not rotate the crankshaft you can remove and replace the barrel, although I would be reluctant to remove it without good reason. Your piston crown looks nice and dry so there is no serious oil burning going on. I think that you will need to remove the valves and check that they slide freely in the guides and lap them in. You will easily tell if there is a problem with the valves by turning them in the guides, as any out-of-true will show up as the heads turn eccentrically. I have my suspicions that modern fuel is causing a lot of problems.

Good luck,

Colin

[IanHood]

There’s no reason why you shouldn’t check the bore. You don’t need to remove the barrel. I use a suitably sized bits of copper pipe (same length as the height of the head) to put over two head bolts to hold the barrel down. Once that is held down you can simply rotate the engine until the piston is at the bottom of the bore and then, when you’ve finished inspecting, rotate it back to the same position you started.

Your valves aren’t burnt (you would see that with them in situ) but that does look to be quite a bit of oil burning. I would release them from the springs and check their clearances in the guides. Any significant side to side movement would suggest guide replacement.

[Dave354]

Hi, can I ask if your head it modified for twin spark Plugs?

Dave

[Paul1098]

Hi Paul,

As long as you do not rotate the crankshaft you can remove and replace the barrel, although I would be reluctant to remove it without good reason. Your piston crown looks nice and dry so there is no serious oil burning going on. I think that you will need to remove the valves and check that they slide freely in the guides and lap them in. You will easily tell if there is a problem with the valves by turning them in the guides, as any out-of-true will show up as the heads turn eccentrically. I have my suspicions that modern fuel is causing a lot of problems.

Good luck,

Colin

Hi Colin,

I wasn't sure how accurate you needed to be with the valves, hadn't thought of spinning them in the guides and doing a visual for warping. I suppose a bit of engineers blue on the seat and spinning the valve would show up warping if I wasn't convinced by the visual.

Good point about modern fuels, I think I'll start using a lead replacement additive to help protect the valve seats just incase as these engines will have been designed for leaded petrol.

Cheers
Paul

[Paul1098]

There’s no reason why you shouldn’t check the bore. You don’t need to remove the barrel. I use a suitably sized bits of copper pipe (same length as the height of the head) to put over two head bolts to hold the barrel down. Once that is held down you can simply rotate the engine until the piston is at the bottom of the bore and then, when you’ve finished inspecting, rotate it back to the same position you started.

Your valves aren’t burnt (you would see that with them in situ) but that does look to be quite a bit of oil burning. I would release them from the springs and check their clearances in the guides. Any significant side to side movement would suggest guide replacement.

Hi Ian,

Would I need to set up a pointer and degree wheel to get the piston & bevel shaft back to exactly the same place if I was moving the piston?

Cheers
Paul

[Paul1098]

Hi, can I ask if your head it modified for twin spark Plugs?

Dave

Hi Dave,

No theres only a single spark plug, the other hole is for the decompressor.

Cheers
Paul

[Dave354]

Hi,
I’ve not seen a decompressor like that before. My 350 ( and I think all other singles) uses a lever on the exhaust valve rocker cover to hold the valve open. Is yours a modification?

Regards

Dave

[Paul1098]

Hi Dave,

I was expecting a decompressor that acted on the valve and was surprised by this one.
No it's not a modification, it came out of the factory like this. Not sure whether this decompressor is because it's a desmo head or specific to R/T model or just fitted to later bikes.

Cheers
Paul

[Dave354]

Every day is a school day!