Hi  Everyone, can someone tell me if this is a RT1 carb please. The only markings on it are on the mounting flange and are ISO  W  and on other side 275 E1.  Also when changing the needle do you have to change the main nozzle and the jet setter to match the needle. Thanks again  Alan.

275 is the model code for the 1970 RT1, so likely the correct carb.

 

Not sure what you're asking.  Carb should have specific calibrated parts.  Pilot jet, slide needle, main nozzle (aka needle jet aka emulsion tube), and main jet are the common calibrated parts.  They have sizes/ID codes stamped in them and like should be replaced with like.  Use only Mikuni parts, aftermarket kit jets and slide needles often cause issues.  Genuine Mikuni has a square within corner of a square trademark stamp on all calibrated parts. 

If you are installing a random needle from something else that does not match taper and size calibration Yamaha used (number/letter code stamped in needle) then you'll need to start from scratch with size of main nozzle and main jet.  

Jet setter has o-ring to seal to float bowl.  It is not calibrated.  Main jet is in jet holder (aka "drain plug") in side of float bowl. 

Float needle and seat is replaced as a matched set.  They are also sized.  Aftermarket is usually okay for float needle/seat.

Hi Mark T,  Thanks for the reply, What the problem is I have a1970  RT1 with a1973 RT3 engine in it which has the RT1 carb fitted, as I have just found out. So i need to know if I can just fitt new jets,slide,needle etc for the RT3 into the RT1 body, or are both bodies different. If I need to find a RT3 carb  can you tell me what code do I look for. Thanks once again for all the help.  Alan.

Bodies for an RT1 and RT2/3 carb will be the same . RT2/3 carb should  have a 308 code stamp .
Technically you could swap out parts , but it would probably be a cheaper option to just buy
a RT3 carb complete .