Hi Everyone, Been a long time since I have been on here so hope I don’t make to big a fool of myself. The carb on my 1970 Rt1 has 275 E1 stamped on the flange,I have read that it should be 275 E4. Am I correct in this, if so what is the difference between the two.The bike runs ok, but I would like to try and get it better.  The bike came from Canada to the UK. Any help would be very helpfully.  Thanks. Alan.

The "E" number generally denotes jetting specs.  First thing to stress is that you must start by identifying exactly what parts are inside your carb before making changes.  The calibrated parts work together...  so changing one part to match a spec and not the others could result in severe engine damage!  It's always a good idea on a new-to-you bike to disassemble carb completely and make sure all the calibrated parts match what they should be. 

275E1 (up to engine number 003872) and 275E2 (003873 on) were used in 1970

275E3 specs were only used in "trial production" and not sold

1971 (engine number starting with 009101) got the 275E4 spec.

If the bike runs fine, leave it alone.  The difference between the E1 and E2 specs were to correct full throttle richness.  The main jet was changed from 240 to 220 AND the 6DP1 needle was raised by moving the clip from the third groove from the top to the fourth groove from the top.  (Yamaha stressed that if you go to the smaller main, you must raise the needle.)

The old Clymer manuals probably recommended using 1971 E4 specs on the 1970 model if I remember correctly.  The main jet is back at 240 and the slide needle was changed to a 6CF1 with the clip in the second groove from the top.  Air screw 1-3/4 turns out from 1-1/2. 

And again, best to check all the parts to make sure they are correct.  (needle jet was O-4 and pilot jet was 30 in all the above listed specs)

Oh, and make sure the calibrated parts have the Mikuni "square in the corner of a square" trademark stamped in them.  Lot's of carb out there with "rebuild kit" jets and needles in them which are often stamped with the same numbers but have completely different calibrations. 

Mark, you're a wealth of knowledge & I didn't know any of that--& I know everything, Not. Thank you for pointing out all these differences.--& possibly saving a piston or worse.

RT325, I just have the old Yamaha bulletins on some things...  and a lot of experience trying things that didn't work out as planned. 

Thank you Mark, your a fountain of knowledge. I have fully rebuilt the bike, and it has just been the finishing touch to it. Now to get out on it when the British summer get’s here. Thanks again Alan.