These machines are mentioned on EARLY RACE DEVELOPMENT pages, but they could not be omitted from pages about Yamaha's history at Daytona.
After the 1958 Catalina Grand Prix, Yamaha maintained a presence in the US by sponsoring local riders racing out of their base at Marty's Foreign Motorcycles in California, as mentioned on the CATALINA GRAND PRIX pages of this site.  
It wasn't until the 1961 that they sent over another team of new machines and riders from Japan, this time for the USGP to be held at Daytona.    
 
The new machines were the direct descendants of the Catalina YD machines, code named YX24 250S and also known as the RR 250. The frames were a development of the YD and first seen on the YX18, Yamaha's first dedicated road racer, which only made it to prototype stage. The new YX24 250S / RR machines featured large alloy drum brakes with a 2LS front and SLS rear and were also fitted with fairings from the YX18. There was no provision for a rear guard (fender), but a front was available although not fitted for the Daytona race.   
Engines were YX24 spec units based on the 5-speed YDS1 engine. The cylinders were now alloy construction gaining a weight saving over the older YDS1R engine but pistons still retained 2 rings at this point. Ignition was by Hitachi MC-2RY magneto and carburation was provided by 2 x Mikuni-Amal 276 27mm carburettors mounted on long alloy inlet manifolds with float chambers bolted the rear of the engine via anti-vibration mountings. 
The RR machines were not the 1st choice for the Daytona race as they were hoping to run the new RD48 disk valve machines, ahead of the World Championship races soon after Daytona, but they were not ready in time.  
Fumio Ito was to lead the team as with Yoshikazu Sunako in support, both veterans from Asama and both would make up part of Yamaha's team in the 1961 World Championship. It was of course Ito's 2nd trip to the US after leading the team to at Catalina. 
Ito was #3 and Sunako #4.  They finished a creditable 7th & 11th

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