If a new throttle cable is on order for your RD350, one should take note that Yambits makes a cable for either high or low-top carbs. Economy Cycle does as well. Now, not knowing where the cables are sourced for either company leaves some of this to open speculation.
I can only attest to the Yambits cable, as that is the cable I landed on over 4 years and 3 cable options ago.
The issue I had with the Yambits high-top cable was an EXTREMELY long twist of the throttle (over 1/4 throttle) was necessary to get the carbs to even consider opening. In the meantime, the oil pump cable would open almost that same distance regardless of how much slack you attempted to remove from it. So, I got to digging. Yamaha had a few technical bulletins released, and at the time I never really considered looking into them as I just wanted to ride. Well, since going ham on my bike, I decided to dig into seeing the best way to fix this cable issue.
The TSB is actually posted on all Yamaha parts fiche's as #MB-015C.
https://kwcycle.dealerspikeparts.com/oemparts/a/yam/50041836f8700209bc784f0b/handlebar-cable
EDIT FOR CORRECT PICTURE DUMMY.
(http://www.aircooledrdclub.com/smf/attachments1/435/Images/07112011612.jpg)
This TSB was issued as a mid-production shift for the B model. Which means, most aftermarket throttle cables will have the long oil pump cable, not the shorter one, as the long cable still works for both setups. It just works poorly.
The part number required for the shorter cable is:
360-26321-00-00
Once fitted (which took a bit of finesse), I had proper slack at the handlebars and at the oil pump, so no need to get extra twisty on the throttle anymore. Also, a quick squirt of cable lube down each one, and the throttle is now silky smooth.