Large Model Association Forum

I have a 3w 70cc twin.The Tillotson carb is mounted below the crankcase and pointing rearwards.When mounted on a test stand,with the carb on the top,engine runs OK.When mounted in a model,with the carb pointing down,the engine will not run correctly and there is a constent drip of fuel from the carb.If left unattended the fuel in the tank will all drain away.There is gravity feed from the tank,because the carb is below the fus bottom line.All new gaskets,etc have been fitted to the carb.I was of the opinion that a pumper type was in effect a sealed unit from the tank until the pump operated,ie it should not drip.Any ideas folks and no I cannot have the carb at the top,that would mean the exhausts coming out of the top.


Barrie B

Hello Barrie, I would try two things on this engine. Firstly, the difference in the way it runs will almost certainly be the reed seats (I assume it has a reed block assy) The thing to do is to turn over the reeds so they are pressing harder against their seats. The engine should make a "parping" sound as it is rocked back and forth. This is the sound of a decent seal of the reeds in the block. Secondly, check the seating of the metering needle. Is is linked correctly to the diaphram ? I am guessing this is where the fuel is getting through. Tho old 3w 70 engine was always a bit of a dripper and it always ran better overall when inverted.However it should only drip a carbful and certainly not drain the tank. Hope you sort it . .
regards Glenn 1222

Thanks Glen,the carb inards look OK,I have found another carb of the same type,I will try that.I have not checked the reed seal yet but will do so this weekend.


Barrie B

I didnt think you could turn over the reeds because they have offset mounting holes?,if the reed sits open all you have to do is put the heatgun on them briefly to warm them and they will easy to straighten,but it would be harder to pull fuel if the reeds didnt seat correctly,i would say id definately the metering needle in the diaphragm part of the carb,either not shutting off or spring has worn slightly as to not shut off the fuel,either buy a carb kit for it and replace the needle and fork,you can try bendng the fork very slightly downwards a little till it seals.
thats my 2 pence worth

I am not that up to speed on the 3w but can you not remove the barrels and rotate them this would mean you could effectively keep the carb on top but have the exhausts at the bottom, if you do this ( if barrels can be rotated ) please remember that you may well have to rotate the piston by 180 degrees too as the ring groove needs to clear the ports, i know this is doable on quite a few engines and in particular with the old Tartan.

Tillotson parts are available from the manufacturers in Ireland and they offer great service.

Thanks for help folks,I have the engine sorted,fitted another carb.
Barrie B

Hi alan,
ports wont match up with the crankcase.
andy