Full throttle problem

[Gannet]

My 40 Minus (SJM) starts first pull, runs smoothly and has plenty of thrust up to about 3/4 throttle. Opening the throttle to 3/4 and beyond produces a reduction in speed with the engine sounding as if it is going to stall.
I cannot replicate this in my small test tank - (propeller left on for this test).
I dont know what to change or check. Also, of course, I cannot test the change without putting it back on the dinghy.
I have dropped the float chamber bowl to confirm that there is a good flow of fuel.
Could it be an ignition problem? If so, what? The plug is an AB 6. Ignition is the normal Villiers.

Looking forward to the advice from the great pool of experience in this forum.

Thanks

Jeremy

[charlesp]

First, what fuel mixture are you using? Are you mixing it fairly accurately?

An SJM would have been manufactured using 10:1. I suppose it's possible that the needle may have been changed over the years.

[Gannet]

Thanks for your quick reply.
I am using 10:1.
It is mixed accurately. 100ml of TC-W3 added to 1 litre of 'new' petrol.
I have just removed the needle. It is marked '3'. I believe that to be correct for 10:1. The needle looks a bit 'worn' over the last 0.35 inch. The plating is missing over this length. Is this significant?
Jeremy

[Taspiper]

All sounds ok, try enriching the mixture a bit, probably leaning out. If there's no fuel it's not going to go any faster.

[Gannet]

Hi Rolf,
Thanks for your advice.
The reduction in engine speed as the throttle is slowly opened beyond 3/4 is quite marked; to an extent that it sounds as if its going to stall at full throttle.
Shall I adjust the needle screw adustment by 1 turn out?
Cheers
Jeremy

[Taspiper]

Most certainly give it a try you should also check for any other problems in the fuel system too.
If the motor has been running ok with the screw as is it's probably some other blockage.
We tend to fiddle with things that are adjustable when something goes wrong when in actual fact it could be just crap in the system. Check all your gauze filters.

[charlesp]

The gauze filters are indeed next thing to check. First check the one on the carburettor banjo, then the one on the fuel tap.

[Gannet]

Yes,
Thanks for your advice.
I have checked the two filters - visually ok. Additionally, with the float chamber bowl removed there is a good flow of fuel (tank is half full). Also checked that the bottom of the loop of the pipe twixt tank and carb is clear.
I did notice that the 'Y' shaped piece that closes the needle valve is set so that the fuel level is slightly lower than I have seen on another Villiers carb. Is this significant?
Thanks
Jeremy

[Taspiper]

It could be if it's very low. Probably pay to set back to standard by bending the tab a little. I'm not sure of the specs but it's on here somewhere if you use the search function... Float height or something.
Also while apart make sure the centre piece is clear of any rubbish. Just because filters are clear old fuel deposits can still form in the bowl and clog your jet.

[Gannet]

Rolf,
Thanks for your advice. I will bend it a bit to increase the fuel height.
Cheers,
Jeremy