RZ350 carburetor conundrum

[85RZwade]

Always lead with a picture, right? Some of you may remember when I bought these modified RZ350 carbs from RB
designs in Portland, but I don't. They're taper bored to, I think, 29mm and have dividers added. Supposedly jetted  and ready to use, and I figured I'd use them someday.
Well, I think it's someday. The RZ has developed a symptom that I don't want to live with any longer. Right at  1/4 throttle, 6000 RPM it slows, as in, loses speed. Crack the throttle open and it takes off. The symptoms are worse when it's hot outside, and I can just notice it on a cool morning, so I believe it to be a rich condition. My theory is that either the needle jets or the jet needles have worn and have screwed with my ratio. I'm ready to pull the carbs off, examine the needles and jets to check my diagnosis, then chuck the scientific theory out the window and put the modded carbs on. I will open them up and check the jetting first, and the fuel level. This should be fun. 

[Kawtriplefreak]

Good luck. I gave my set of bridged EPA carbs away to Evans Ward this year at the Gap. I put them in a drawer in my shop 5 years ago and bought a set of SpecII prepped VM30s after spending an hour doing a needle clip position change. That being said they can be made to perform very well. I just don't have the patience for them. Oh yes and I forgot to add mine had the jet kit and adjustable needles as well.

[1976RD400C]

I bought the Dynojet kit to try to make mine run right. It still had that stumble at 5500 and 1/4 throttle. The Dynojet needle was at least adjustable, so I started to set the clip leaner and it got a bit better. Went full lean on the needle and it definitely helped but I ran out of adjustment. It still does it some. Mind you it is a major ordeal to pull carbs and adjust. I think next up is go down one size on the needle jet tube. Seems like the needle may have the wrong taper.

[85RZwade]

The thing is, it's been fine since 1988. I put the Toomey needles in with the pipes, jetted up when I bored the cylinders and have cleaned the carbs a couple of times over the years. That's it. This condition has developed over the last year or so. 

[1976RD400C]

I wonder if it has to do with how/when/where the power valve is opening??

[85RZwade]

Good question, I hadn't considered that. Again, 

[m in sc]

worn jet tube, i almost guarantee it. they DO wear over time and cause a weird un-tunabale rich spot. had an r5 that gave me fits over it about 15 yrs ago.

[Kawtriplefreak]

I concur.

[85RZwade]

These are the RB carburetors; clean as the proverbial whistle inside. 420 mains and what appear to be 20 pilots. 20 seems way too small, but I may not be seeing the numbers correctly, even with my assistant.



I wonder what needle jets and jet needles are in there 

[SoCal250]

I have a similar lighted magnifier on my bench, but what you need for jets is one of these:
https://www.harborfreight.com/5-piece-loupe-set-98722.html
The 10x is my go-to for inspecting jet numbers and condition.

[m in sc]

sounds like what toomey used to do/does w the kit.

 

[1976RD400C]

The service manual says the OEM Mikuni carbs came with 20 pilots and 230 mains. I think mains in the 400 range are aftermarket and sized differently than Mikuni. I'm ordering new jet tubes for mine.

[m in sc]

they arent. its the needle and jet tubes those kits use.

thats a common jetting setup.

[85RZwade]

I brought a set of parts donor carburetors back from Dennis's place after the Tiddler ride, and today I pulled my carbs off. I disassembled them to verify jetting; 420s, same as the RB mixers. I don't see any difference between the 39,160 mile needle jets and the donor carb jets, and the needles look fine as well. Maybe the wear is too small to be seen with the naked eyebulb. It can't be a cleanliness issue...



There were a couple of hiccups in the disassembly process:





So there I was, prepared to transfer needle jets, reassemble, check fuel heights, possibly adjust floats, reassemble, check again...and the scientific method took it in the shorts.



Started it up, throttle tuned it a little and adjusted the idle. Tomorrow I'll take it to work with a little screwdriver in my pocket to play with air screws. More later.

[SoCal250]

Good luck.  
That's a very early production engine you have there. #75 of the run