47x carb upgrade?

[wolfgangh]

Hi friends, I am restoring a low mileage 47x model, and I am in the final stage of assembly. I am now asking myself whether I should keep the carb setup from the 47x or „upgrade“ to 1GE jetting? The 47X is richer in mid and high end (#195 mains vs. #165 mains in 1GE, O-0 vs. N-8 needle jet, different pilot air jet). Also, my 47x carbs are set to 21mm float height which turned out to be corrected to 24mm in later repair manuals. This gives again an additional enriching.

Do you have experiance with 1GE carb setup on a 47X? Is there any other difference that could require this richer 47X setup, or was it just for safety margin against seizures from YAMAHA?
I do have plenty of 1GE carb parts to do the upgrade of the 47X carbs.

[begbie]

Around 1985 Mikuni changed the way they measured jets.
check this out.
http://rd500lc.free.fr/technics/jet_size_en.htm

[wolfgangh]

Thanks, yes I am now aware of this, I found some old posts about it. Obviously float height should also have been 24mm always, it was set to 21mm in my carbs. I will in rease Pilot air jet to #1.3 though, but still unsure about the needle valve (N-8 or keep O-0)? I guess I will do trial and error method.

[lost1750GTV]

"Should have been"
My theory is that 21 was right for the carbs, and worked on any application where they were mounted traditionally.
RZ side mount created leak issues, requiring 24 and then different everything else to compensate for lower float.
Of course I could be quite wrong.

[MK]

Guess who did that jet measurements? You'll find that in a famous tuning manual for the RD500...

Regarding the idle air jet and the needle jet you should just do a bit of try and error.

O-0 and P-2 are just 0.05 mm different in diameter, so the difference is not that high.
The effect will only affect the throttle range where the straight part of the needle is inside the needle jet (i.e. the first maybe 10 mm of slide travel)

Float levels 21 or 24 will have a pretty huge effect, so try this first.

For the air jet: I started with 1.1 mm and drilled them up in 0.1mm steps.
Then test to run with low load and rpm (maybe 3-4k). Whack the throttle open and feel, if the engine reacts immediately. If it hesitates on quick opening, but is fine on slow opening, you're too lean and need the air jet one step smaller.
(which was 1.3mm in my case)

[MK]

"Should have been"
My theory is that 21 was right for the carbs, and worked on any application where they were mounted traditionally.
RZ side mount created leak issues, requiring 24 and then different everything else to compensate for lower float.
Of course I could be quite wrong.

I agree that you're likely wrong.
A float level of 24 instead of 21 reduces the height of fuel above the jet and thus makes everything leaner. The later models have a leaner jetting which - according to the above theory - should have been richer, if they'd wanted to compensate.

[lost1750GTV]

I read what you did/measured to check to make it right - and would guess they thought they could solve richness complaints at the same time as side stand leaking- and didn't quite get it right.
I really doubt mikuni didn't know what the correct float height for that carb body (in typical applications) should have been. And as you point out, changing it does require adjustments, but from your research and work i think they didn't spend enough time doing it.

[phildu31]

Side stand leaking ?
Incorrect float height ?
RZ side mount created leak issues ?
Richness complaints ?

I'm afraid you don't really understand what causes the leaks that occur sometimes !

[MK]

In 1985 Yamaha released some additional pages to the OEM maintenance manual from 1984.
In those pages they stated the first version contained errors which were corrected by the new version.

The revised version also mentions that the original version had DR9ES as spark plug and 1.95l of coolant which was then corrected to BR9ES and 2.3 l.

[phildu31]

One clue : o-ring in float valve ...

[lost1750GTV]

Yeah, everybody gets the o-ring wears out, and that Oldcastle edit/ repost

[lost1750GTV]

Yeah, everybody gets the o-ring wears out, and that Old carbs have wear spots etc etc.
The bike was a short lived model, not much time for factory adjustments, but there is 35 years of people sorting out jetting aside from leaking. I've read "most" of it on here.
I'll stand by mikuni knowing what float height should work (and didn't in this application), and that they made other changes to improve. Whether the same guy did both changes at the same time I sure don't know - but in any event, let's just say MK dug (and drilled) pretty deep.
Maybe the next year they would have had more revisions if the bike had stayed in production, but realistically their attention had gone to the new 750 before the 500 was even discontinued.

[hailwood1965]

I have a bike in my garage with the Bill Wilson carbs and kit on it. It works very well. I also have a bike here with stock carbs and airbox. I'd go with a Bill set up if I valued my time.