I'm making some progress on my newly acquired RT1, at least I've derusted the inside of the gas tank and have it setting with a 32 to 1 mix. This bike has been in storage for about 20 years I think, It is in cosmetically good shape, and I am going through it one step at a time to try to get it running first, and then take it from there. I plan to run it on mixed gas until the oil pump is evaluated and operating properly. I know that are also a number of dried / hardened rubber issues that need to be addressed among other things.
   I have moved on to the carburetor and, after reading some of the posts here, I don't believe It is the original carb. The engine number is RT1-08188. The numbers stamped on the carb flange is 275 and E4, I think it should be E1.  I ordered a repair kit for an RT1 (Keyster KY-0126) before I even looked at the carburetor, and it has a different main jet from what was in the carb. There was a 200 in the carb and the kit came with a 220, I think that is for the E1. The original needle was marked 6DP7, the one in the kit is marked with a number 27 if I remember correctly (it's in the garage). The rest of the parts match the ones that were in the carb.

I guess my question is can I use this carb? If I do, I'm thinking I could use the original needle and main jet and switch out the other parts that match. Of course that is assuming it ran with this carb 20 years ago!
Am I correct that this is not the original carb?
Also I cannot find what the float setting should be. The information in my Clymer manual does not match up with this carb.
Thank you in advance for any information!
Dick
 

Me being impatient I'd just clean the carb you have by removing the pilot jet then needle jet out through the top to check any holes in the side--,might only have one big side on air hole not multiple small hole but anyway check the air bleed to it from the carb mouth. Poke or blow the choke fixed jet in the bowl cavity. Remove the main jet from the holder as things get stuck behind the jet. Most importantly is the miniature Oring that seals the needle jet extension into the bowl. If it's brittle or undersize then fuel can bypass the main jet & won't rev out just go blubbering rich. I wouldn't worry if it initially starts up running bad if it's been parked up for a long time. Just warm it up & increase revs bit by bit until the neighbors complain then time to head if down a country road & give it beans to clear its throat. I'm lost on engine numbers but if it's a true RT1 it will be piston port not reed inlet. Love those first ones. Yamaha used RT1 prefix up to RT3.

Don't use the Keyster calibrated parts (jets and needles)

Thanks for the replies Mark and RT. I have it all disassembled and clean. I will put it together with the original parts (they look ok) and give it a go. I have a new miniature o-ring for down inside the bowl. I can use the gaskets and other non calibrated parts from the kit.
It made me a little uneasy that I could not cross reference the needle that came with the kit. It has a Y23 on it, not 27 like I said in the original post.
The 70 - 71 RT1 (MX) service manual on this site had float adjustment information that was needed.

The aftermarket carb kits use their own references. 

I think that some of the kits MIGHT work ok you use all of the parts supplied, but they are unlikely to work very well if some parts are mixed and matched with original Mikuni.

A member of another Yamaha forum reports 100% success on fixing sticking float needles by just cleaning the inside with something such as Autosol.

Yamaha used two different methods of float level measurement plus they published some incorrect specs over the years. 

Float arms should be parallel to float bowl gasket surface when done or something is wrong.  I tend to be very picky about following specs but now I just set them parallel and call it good. 

Keyster has their own proprietary "calibration" and for some reason it doesn't work well on these bikes.  I've fixed several poor running Enduros by throwing away the Keyster jets and installing the original Mikuni.    (In contrast, I've heard some Hondas run better with Keyster calibration)

Yamaha had several jet revisions for the same models.  If a revision was used on the production bike, the "E" number of the carb changed, presumably so it was easy for the dealer to know what jetting the carb came with and/or if the latest revision was installed.

I might have a chart of revisions for your bike...  IMPORTANT to know that jetting works as a SYSTEM.  Generally if a revision had a new main jet size, it also had a different needle number or something.  Someone might have installed a larger main in your carb during custom jetting...but it would be a mistake to just swap it for a smaller size unless you check and verify the slide needle and slide needle jet match the sizes shown for the carb specs that specify the smaller main. 
 

Here is the chart...  E4 was the last production revision.  I would start by identifying what components you have in YOUR carb and then as a starting point, choose a column that has the same needle and copy the specs in that column and use those jets. 

You just don't know if someone rode your bike at their cabin up in the mountains and jetted it for altitude or something?

Note the "-" number after the needle ID number is the clip position from top.  So "-3" means clip third groove down from top.

Thanks Mark that is the information I'm looking for but I'm not seeing the chart. Did you attach it, or am I missing something?
Dick
 

DOH! 

Here you go.

Note that there are several bulletins listed....  Yamaha had a lot of "surging" complaints off-throttle with their 360's.  So there were jetting, timing, and even porting suggestions to try but not one "fix all" cure for everyone who felt they had this issue. 

Chart is what they put in production bikes and the updated 1971 specs are fine for the 1970.  If you have the 6CF1 needle, just use 1971 specs.  If you have the 6DP1 needle, I'd pick the "E2" spec but E1 would be fine too.  (They raised the needle and went down on the main from E1 to E2, E4 got a different needle and back to 240 main.)