Advice sought, Yamaha 8hp won’t throttle down

[Twyfordbridge]

I have a Yamaha FT8 outboard. After using it yesterday quite normally for a couple of hours I went to approach my swing mooring. On going to reduce speed it suddenly increased revs dramatically. There was no control over revs at all, it was full speed or cut out when throttle was fully turned down to what would normally be the idle position. Fortunately I was able to get out of the swing mooring area into clear water without hitting anything. Slowly drifing, I restarted the motor. Same thing,in neutral, crazy high revs. I could increase revs even more but couldn’t slow it down. Thankfully the harbour boat were able to tow me back to my mooring Buoy.
I have watched several YouTube videos and examined the parts diagrams and feel quite confident that I can strip and clean out the carburettor myself. My question is: Is there a particular component part that would cause such an issue?
All linkages and cables are in good working order.
Any advice would be appreciated. Thank you.

 

[Rappey]

The problem must surely lie between the twisty hand throttle and the throttle butterfly valve that you can see in your scematic. Something like the throttle cable sticking and the butterfly valve not returning

 

[VicS]

I agree with Rappey.

Disconnect the the throttle cables from the carb and check thet the butterfly throttle valve can be opened and closed easily by hand to eliminate any problem there. It should be closed by a light spring and be easily opened by hand.

Assuming the butterfly operates OK the problem lies somewhere in this lot

 

[Twyfordbridge]

The problem must surely lie between the twisty hand throttle and the throttle butterfly valve that you can see in your scematic. Something like the throttle cable sticking and the butterfly valve not returning

Thanks Rappey, I haven’t yet removed the carb and have only looked at it externally. All the linkages and cables seem to work very well. All are regularly lubricated and move freely... although I don’t know how or if they are operating inside the carb itself.

 

[Twyfordbridge]

I agree with Rappey.

Disconnect the the throttle cables from the carb and check thet the butterfly throttle valve can be opened and closed easily by hand to eliminate any problem there. It should be closed by a light spring and be easily opened by hand.

Assuming the butterfly operates OK the problem lies somewhere in this lot

View attachment 197525

Thanks very much Vic and Rappey. I will try that. Hopefully it’s nothing serious but quite alarming to suddenly happen especially in crowded mooring area
I’ll check that out when I go back to boat tomorrow

 

[Rappey]

You don't need to remove carb . You should be able to locate the arm on the carb that rotates the butterfly valve and watch that it is returning to rest position when you turn the throttle. This can be the only thing that can cause the engine to rev up to full rpm.

 

[Rappey]

This is the butterfly valve in tickover position. Yours will be similar. The brass butterfly valve is on a shaft that is spring loaded outside the carb body to return the b.valve to tickover . At the right hand end of the shaft a cable or other form of mechanical connection will rotate the valve to fully open. It should snap shut as quick as you back off the throttle.

 

[Twyfordbridge]

This is the butterfly valve in tickover position. Yours will be similar. The brass butterfly valve is on a shaft that is spring loaded outside the carb body to return the b.valve to tickover . At the right hand end of the shaft a cable or other form of mechanical connection will rotate the valve to fully open. It should snap shut as quick as you back off the throttle. View attachment 197541

That’s very useful, thank you