www.iwt.com.au/images/effects3.gif

 

Do you happen to remember which needle they went with using the p4? Oddly, the range you describe for the needle jet was where my miss firing was. Raising the needle fixed it mostly, but perhaps at the expense of making mid range too rich. Not enough that it stumbles or anything, but there is definit black soot coming from the exhaust as I'm riding. Can also smell it on me a bit.

If what you say holds true, I would likely be best off going back to p8 with stock needle position perhaps running a 132.5 main to fix the flat spot I started with on top. Though likely going with the y1-51 needle as its supposed to be a tad leaner. There was evidence of running extremely rich with almost spray showing from the exhaust and the smell that goes with it.

Confusing part with tuning this carb is that changing the needle and needle jet had a huge influence on the main. I knkw it isn't supposed to, but wfo is behaving a lot better with the combination I have now compared to having no power at all before with having a richer main to boot.

Running a p6 with y1-50 in 4th slot from top, 17.5 pilot 2.25 turns out, 127.5 main, I need to fix the starting issue although idle is strong. It seems rich as choke is only needed for a few seconds before shutting it off (35deg f) but still has a bit of a lean stumble at 1/8th throttle more present when not warmed up. Mid range I think is rich based on the level of black soot seen on license plate/fender. Main is a step lean for the cold as verified with the flatness going away once warmed up.

Main is an easy change, back to the 130. But then ill mess that up trying to fix 1/8th throttle again. I dont really know how to keep mid throttle from being so rich without leaning out 1/8th. I was thinking the needle jet would fix it but it appears that dropping the needle jet to a p4 will make 1/8th even leaner when it is already lean. Then what to speak of the pilot circuit being rich, I cant richen the pilot circuit to fix 1/8th due to that. Though leaning out the pilot circuit. Currently makes it backfire more.

Lordy these carbs are a pain, one change anywhere throws everything off.

 

Alright, ill go back to the drawing board. The p8 needle seems almost necessary to keep that low throttle input happy. The bike was actually gokd starting, idling (had to make it idle lower than 1500, but that was likely pilot needle settings) and actually pulled good till the full throttle flat spot. I had the stock needle second slot then.

Something was definitely rich at that point, I'm thinking 1/4th to 3/4th was a bit fat causing my rich symptoms and was carried on when I switched to the richer needle. What makes the needles richer/leaner and for what range of throttle would it be?

All in all I'm just trying too hard to find where this rich spot is. I was running good before and just needed a larger main to keep happy.

 

Try a 15 pilot jet? That's what I have btw

What air jet if any do you have in it?

The taper angle and the diameter at any matching/measured given point along the taper determine richness of the needles. The 51 needle is .01mm larger diameter than the 50 making it leaner

 

Richard, whose chart is that? The chart for the 40mm pumper shows a similar graph, but in two separate locations in the actual text, the description for the needle jet is 1/16th to 1/4. I think I'd go with the text.

http://www.mikuni.com/pdf/hs40_manual.pdf

 

It's a diagram from this page

I see the graph in the HS40 manual now and it's also showing quite a range for the needle and needle jet. Since we know the two work together to meter fuel at throttle openings needing more fuel than the pilot system and less fuel than the main jet in these variable venturi carbs, it's safe to assume the graphs rather than the text are correct.

Actually I don't think the jetting graphs for the HS40 would be much different than any other Mikuni or even any other modern non-CV m/c carb since they all have the same parts?

 

Lol, I really wasn't too good at tuning before all this started but I'm bound to be worth my salt soon.

If I recall the taper of the y01-50 was leaner in the sense that 3/4 throttle should be leaner k believe. Is the y01-51 the same grind but just a leaner initial taper?

 

I can never find any official measurements taken at intervals along the length of the 9 series needles like you can for lower numbered series

Since all the numbers and letter(s) for the two needles except the 50 and 51 suffix are the same I'd say they are essentially the same taper angle just that the 50 is thinner/richer at each increment along its length?

 

Interesting, then perhaps the best bet is to use the p8 e ith the 51 needle.

 

The 51 needle has a more 'dramatic' taper. When all else is proper, it can be felt in the powerband. Butt Dyno. I rode yesterday and used said method. I also went with the p6. Works for my elevation.