Mikuni float arm orientation

Anyone know which way this arm goes in?

The arm is from a modern Mikuni 36MM off my '77CR250. PO had done the Mikuni conversion with a Sudco Mikuni. After cleaning it I noticed that the way the arm was in the carb was the opposite of what the parts diagram shows for the older OEM Mikunis. OEM diagram shows the horizontal arm closer to the bottom of the carb, while this Sudco Mikuni had it the other way around. Here is the OEM diagram:




The OEM Mikuni on my '78 looks exactly the same inside as the Sudco modern Mikuni, so it seems odd they would have switched the orientation of that arm. Obviously this makes a big difference in float height.

It should go in just the way you are holding it, just like the diagram. You may be able to see some small wear marks from the float pin on it. I've pulled some used ones apart and they were installed upside down. I've seen some with a little tang that should hit the carb body as a stop, and some that don't have it.

Thanks, yes it had the wear mark, I've used those marks in the past on other carbs when I cant remember. But the wear mark is on the wrong side, so this arm must have been put in wrong by either Sudco or PO.

I would say PO. I have also pulled some apart where the floats were put in upside down to compensate for the arm being in the wrong orientation.

Hold the phone - Visiteur's picture shows the arm the other way!!!???

That is interesting. I'm going to go home here in a few and check one out. I have a gaggle of them.

I've got boxes of Mikuni's and I'll be danged, I just looked at some diagrams and they came both ways. I never knew that so I learned something new today. I would guess you would have to know what carb you have to determine which is which.

Probably able to go either way dependant on carb model and clearance for floats, if you have a spare bottom plug (the one that gives you access to the main) drill it and fix a clear hose poke it along side the carb and this will show you the fill level.

I think jo360 is right different ways on different models. In looking at float heights of the Sudco modern mikuni, one way gives proper float height without the arm being too far off parallel with the carb body, but if I flip it the float height is good while arm is relatively parallel. I'm going to go with the direction that gives me good float height and close to parallel arm. Good idea on the bottom plug float height "tool".

More info - I contacted Sudco. They have seen this issue before and they confirmed the modern Mikuni they sell should have the arm such that the long horizontal arms are closer to the top of the carb - just like Visiteur's pics. He also said he has seen a few parts diagrams where it is drawn upside down.

My 82 has the arms like Visiteur's picture

Thanks. After doing the research I am convinced the arms go in just like Visiteurs picture - long arms towards top. I believe the husky parts diagram is wrong. Couple of reasons why I say that:

1. Per Sudco, modern Mikunis are almost identical to old OEM. Floats are identical. All modern sudco have the arm like Visiteurs per Sudco. If older OEM arms were to go the opposite, then the floats would not work without really bending that tang.
2. If you look in the factory serviced manual, there are a few photographs that clearly show the arm position is opposite of what is shown in the drawn diagrams.
3. To achieve proper float height, the arms would not be too far off parallel if they were not like Visiteurs picture.

Just a theory, but I think that incorrect husky parts diagram is to blame for a generation of upside down arms. Think about it - the average joe goes to clean his carb, uses the diagram to put it back together and then you have a whole sea of upside down arms and guys saying "well diagram says it goes in that way". Then that caused a wave of improper float heights for decades.

Is this true for the 38mm Mikuni that came with the 250 WR too? (1983 model).

It was just my theory, but I would trust the factory manual pictures more than the parts diagram. If you cant get it dialed in, try it the other way.