Hi-Top vs. Low-Top Carburetors: RD250 & RD250

[Diablo007]

I've seen questions regarding high vs low top carburetors at least a couple of times.  I don't recall any replies mentioning the attached Yamaha technical bulletin.  I happened to find it by accident while skimming the parts manual while on hold on the phone.  The TB shows the difference to be an attempt, "...to improve the waterproof qualities of the carburetors and to prevent sticking slides..."

I always figured the 'boot' was to keep water out.  As for the increase in slide length, I've heard more than one explanation.  Now that I've seen this TB it makes sense as the explanations always centered around how the slide moves, either the up/down motions, or from vibrations.  Basically that the longer slide allows the slide to move more smoothly.

[m in sc]

except they went back to low tops on the 400's.. but implemented a splitter clip to hold it to the frame. (so the cables wouldn't jam in the fins...)    think about it.. 

[Diablo007]

I posted this as I have never seen any post containing this TB.  Was there a subsequent TB that recanted this one, or mentioned proper cable routing in its stead?

As for proper cable routing, that's something I learned early on when I first started wrenching.  I find it interesting that Yamaha would implement changes to the design of a carburetor and release a TB regarding the reason for the changes to address cable routing issues.  If the problem being corrected was related to cabling wouldn't cabling have been the topic of the TB?

Now, to open the can of worms that I wasn't going to but is directly related to the paragraph above...  More than one person I've talked to about the hi vs. low top carburetors for the RD250/350 has explained that this was an attempt to either smooth out the movement of the slide or reduce 'rattling' of the slide.  I took smoothing out the movement of the slide to mean while moving in an up/down motion (rolling on or off the throttle), and reducing 'rattling' to mean the slide vibrations side to side while the throttle was being held steady.  No one seemed to think the design change made a significant difference.

I've made posts with amusing typos and/or grammar I've found while reading the old Japanese shop manuals.  I have a feeling that the statement 'sticking slides' in the TB was in reference to the  movement of the slide, not to cables sticking, and could have been worded better.

[m in sc]

again.. they reverted back on the 400's. as an engineer it makes no sense to make a change like that in a design only to revert it back unless they rectified the issue other ways. I think the 'water entry' was a 'smoke show' reason for the engineering change. it avoided a possible perceived safety concern. They never went back to tall carbs after than as far as i know. even the canadian rzs had low tops. 

the long slides were there so that they remained in the slide alignment pin of the smaller body to accommodate the taller tops and cable routing. that's my thought.