Bonneville America Forums Forums Technical Forums Performance Mods CV Carb Tuning

It's time to link my Exhaust "torque tuner" thread to this CV carb tuning thread:

Exhaust Torque Tuners

Given the latest, I see 48 pilot jets in my future. I am still dialing in the G-Pop torque tuners.

With the pilot screws at 2-3/4 turns out, AFR is an the low-mid 12's at idle. Once just off idle, when the 4 pilot bypass holes are just uncovered by the throttle plates, AFR goes lean. That is not a slide bounce issue, it is a pilot jet issue.

Once the 48's go in I predict that situation will improve. Then there will probably be a bit more tweeking of the torque tuners and float heights.

God@mn I love that AFR meter!

Been running 48s for years now and they work perfectly for me.

Installed the 48 pilot jets today (got 'em really quickly from BellaCorse, thanks Mike!).

Initial results look great. As I prefer to do performance testing during daylight, fine tuning will need to wait.

High RPM light-throttle cruise improved substantially.

The initial pilot screw setting of approx. 1-1/2 turns out was a guess on my part; warm motor idle was in the low 12's AFR.

I reset the pilot screws to 1-1/4 out & will test 'em some other time.

The float heights are approx. 18.5mm, which has the fuel level about gasket height.

Now it should be a matter of fine-tuning the pilot screws and baffles...

Pilot screws are now set just over 1 turn out. Warm motor idle is in the mid 13's AFR.

So far the 48 pilot jets appear to be a better compromise overall than the 45's.

Still testing the baffle angles.

Here's the latest:

2007 865/270 SM
TPUSA 813 Cams
TPUSA +3 Igniter
Uni 4229 Pod filters
Chevy 605 LC pipes (ends removed, 2 baffles out, 3rd & 4th drilled)
Med G-Pop adjustable baffles installed in pipes
PVL (Nology) coils
TPUSA Magnecor Wires
BC AI Removal Kit
NGK IX Plugs
152 Keihin Mains (the brand matters)
48 Pilots
Pilot Screws approx. 1+ out
N3RF (TBS) Needles
Float heights approx. 18.5mm
Polished Slides with stock 2.5mm transfer port
Yamaha FZ1 slide springs
1.38mm air bleed holes in throttle plates

Now closing in on about as good as this may get. WOT & upper RPM is good (mostly in & about the low to mid 13's AFR), & the previously problematic 4 - 6k RPM WOT rich dip is gone.

In 55 degree F weather, warm motor idles approx mid-13's AFR.

65 MPH (indicated) cruise is just over 14 AFR.

Light throttle, hi-RPM hovers just under 14 AFR.

Light throttle, low-RPM dips down in the 11 - 12 AFR range, however the bike is rarely operated there.

Overall, the 48 pilots work better for me than the 45's did, however I don't think it's the optimum size. A 46.5 -47 would be better, if available.

When not "on the throttle", the pilots play a large role. Getting that right can pay big dividends in overall driveability & satisfaction.

The motor feels & sounds the strongest yet. I suspect the stellar mileage I was getting with the 45 pilots & air bleed mod will be negated somewhat by the 48's, however I have no plans to go back to the 45's.

The ironic thing is if I didn't have an on-board AF meter I would be absolutely THRILLED with the way this thing runs, however I think there's a bit more to be had with optimal tuning.

I may still experiment more with the needles...

I haven't done all the mods you have but I found similar results regarding the 48 pilots. I wonder if drilling a 45 would work.

A hand held pin vise would be ideal for the control you would need to do an accurate rebore. Brass is soft enough, should be easy.

A watchmaker s lathe would really kick azz for duties like that. I see them on E Bay all the time, some go fairly cheap too.

The bike runs awesome.

That said, light throttle low RPM (just off-idle to approx. 3k, & ONLY at light throttle, i.e. barely cracked) in 2nd gear is running a bit rich, down in the 11's, before coming back up. To be clear, I'm not talking about roll-on throttle acceleration, I'm talking constant throttle position, throttle plate off-idle, however barely cracked open.

One possibility is to go to a larger throttle butterfly plate air bleed. All other things equal that will lean out the idle, at least until the throttle plate starts to open. However, doing that mod would likley require slightly more out on the pilot screw, which may make the highway cruise slightly richer.

Another possibility is to experiment with another needle, such as the OEM NBZY perhaps with shims.

Yet another possibility is to lower the fuel height, although the float height measurement (not the fuel level) is already over 18mm (close to 18.5mm now), so I'm not sure I want to do that. It's already running a slightly lower fuel level than spec.

Then there's the possibility of drilling a 45 pilot jet to make the functional equivalent of a 46 - 47. The trick with that is to find a drill bit that will pass through a 48 but not through a 45. And I don't know that I have one that's sized properly to do that.

The bike runs great overall, & from a power/performace perspective I like the 48's better than the 45's.

The easiest is to try the NBZY needles & play with shim heights.

I get the best results with float heights by pulling the carbs, so that's gonna be a bit more work if I choose that path.

If I can find the right size drill bit to make custom-sized pilot jets then I'll still pull the carbs, as I find it easire to change pilots with the carbs out.

The most intensive is drilling the throttle plate air bleed larger. I'm saving that mod for last.

Top gear 3500 - 3600 RPM cruise is good. WOT acceleration is great. Overall, the bike runs awesome.

I think there's more to be had, though. And I'd like to get some of that 45 pilot jet cruise fuel efficiency back.

I'm open to ideas...

Tested the NBZY (stock 865 SM) needles...

1 shim was too lean.

2 shims - better - similar to N3RF (TBS) with no shims, however not quite the same...leaner in some areas, richer in others. The N3RF did better in the 3500 - 3600 RPM cruise, & at light throttle openings.

N3RF (with no shims) gets the nod as the best overall needle for my situation, & they are back in.

Next, I want to find the correct size drill bit to custom-drill a 47 pilot jet...

Took the bike out today, clear & dry with temps in the high 30's F. I definitely like the N3RF needles better than the NBZY's.

The more I think this through, I'm becoming convinced that the 48 pilots are good, however in order to get the most out of them the butterflys need slightly larger air bleed holes. At least that's my theory. If I'm right, that will take care of the very light throttle low-RPM slightly rich situation.

I'm in no hurry to do that mod, & might wait for warmer weather. As a general rule, with all other things equal carbed bikes tend to run relatively leaner in cold weather, & sub-40 degree F temps qualifies as cold from my perspective.

Perhaps I'll wait til early spring to drill the throttle plates. We'll see. Gotta do this conservatively or risk ruining the throttle plates.

This carb tuning is a risky business, I tell ya.

Yep, that's possible. Good idea.

Time to drill those throttle plates again, & this time I'll drill in the 1.5mm - 1.6mm range; if I am right that will cure the slightly rich condition at very slight open butterfly cruise, enable the carb to run a slightly more open pilot fuel screw, smooth the transition from bleed-hole air to open butterfly air, & return my stellar light cruise MPG. I plan to get to this sometime in the next week or two.

Although the weather has been very nice this weekend, other issues intervened, & preferred to spend what time I had to March Madness & riding.

More to follow.

I drilled the throttle plate bleed holes out to 1.58mm, set the pilots to about 1&3/8 to 1.5 out; this improved the light throttle low-rpm cruise AFR, however that narrow band just off idle is still a bit richer than what I want, sometimes sub-12 AFR; the 60 to 75MPH light throttle cruise is also a bit richer than what I want; that is in the 13.5 to 13.9 range, depending upon conditions. That's leaving some MPG on the table.

Everywhere else is good.

I don't believe at this point that it is necessary to drill the throttle plate air bleed any larger than 1.5mm, & if I could do it over again that's what I would have done.

Overall the bike runs very well, & on the road it is mostly in the 12 - 14 AFR range, depending upon conditions; however, it is not yet optimal at cruise (slightly richer than I want), & above 7500 RPM it starts to go slightly lean (no biggie) approaching 14 AFR.

I'm working on this. I want to do a bit more experimentation with NBAD needles (790 BA/SM), so when I get some time & decent weather, that's where I'm going next...

The more I think about this I becoming convinced that the diameter & shape of the N3RF (TBS) needle is contributing to the off-idle richness. In a way the G-pop baffles and throttle plate air bleed mods require a slightly different approach to balance things out, & I sure wish I had a definitive Keihin 4 digit needle code interpretation chart.

What I believe may be correct is:

1st digit "N" = OEM identifier
2nd digit = taper profile
3rd digit = length
4th digit = diameter

The above is just speculation, however. And not all that useful without knowing what it means relative to other needle configurations. And it could be total BS. Dunno.

There isn't much to go on for CVK needles. Anyone have a reference chart?

Kawasaki Vulcan 800's also use CVK36 carbs, so there may be other needle possibilities to try...

The NBAD's performed like crap for my situation; with no shims it went way lean in the middle, & with 1 shim it was still lean, erratic,& negated any light-throttle advantage over the N3RF's.

Just eyeballing the N3RF, NBZT, NBZY, & NBAD needles, the NBZT's should perform the best for my situation, however that wasn't the case in earlier tests, although some things have changes since then.

I want to take another look at the NBZT's & test 'em again with my specific mods.

I also want to check out the N3RL needles from a Scrambler.

I had good response from the NBZT (thruxton needles) with various shimmings.
also, I recall that dynajet has a adjustable clip needle set for these carbs. I don't know what taper options there are.

I took the bike out for a short test today; weather conditions were not ideal, it was damp with gusting winds & temps in the high 40's F.

The NBZT pulls great in the midrange and provides an improved transition at WOT. quarter-throttle up to WOT it performs better than the N3RF, at least in my bike, with current tuning.

All is not optimal, however, as the NBZT cruises a bit richer than the N3RF does. And the N3RF was already richer than what I want it to be a cruise for max MPG.

Which brings me back to the pilot jets. The NBZT's don't work and play as well with the 48 pilots. Hmmm. What to do, what to do...

Ah-HA!!! I know where I'm going next. Gotta custom drill those pilots to something larger than a 45 & smaller than a 48.

The thing is I probably never would have tried the 48 pilots but for the G-Pop baffles, I would not have drilled the throttle plate air bleed holes larger but for the 48 pilots, & would not have gone back to the NBZT's but for the 48 pilots & larger air bleed holes. Ironic, eh?

So, where things are now; the mid & full throttle are now pretty well dialed in - the cruise is still richer than what I want for max cruise efficiency.

So, for my next project, I'll custom drill some 46 - 47 size pilot jets, if I can. Right back where I was in Nov 2011 with Ian's suggestion

The smallest bit that I have on hand is .48mm ; bummer.

A #77 gauge bit is supposed to be .457mm, so that looks like where I will start. Gotta get one first.

I recall from past CVK carb conversations with the now infamous 'fast fred'. that he had been using 47 pilots. And I recall ginning some up, so they must be an even wire drill size.
I have quite a collection of jets, I will look to see if there are K47 pilots and measure the hole.

Super, thanks!

Jets are usually rated at flow rate and not by the hole size alone. You can have two jets with the same size hole but rated as different jets. Granted, one might be on the high side (of the flow rate for that jet size) and the other might be on the low side of the next jet size. Having a different chamfer,(deeper, wider, etc.)affects it.

Understood, however if you need something in-between a 45 & 48, what's the alternative?

What I may do is try the 45's again, as the combination of the larger bleed holes (which will enable more open pilot screws) and the NBZT's (slightly richer than the N3RF's) may just be enough to make the 45's work...

I know you have your A/F meters... Your going to need more playing around with them than just drilling the jets. Try a bigger chamfer first. You must be on a mission and having fun, enjoy!

It looks like a #78 or 1/64 drill. That would be 0.0156 -0.0160 inch.

I agree. It is also possible that the jet sizes conform to neither
Wire size or metric drill sizes, but are something unique
To keihn.
While the relative diameter changes are quite small, they are still a
Noticeable volume change.

How about using a reamer? http://www.newmantools.com/magafor/reamers.htm#8610

I know this much at this point:

A #78 bit shaft that measures .40mm on my crappy but very repeatable HF digital caliper will pass through the bore of a 45 pilot, & a #77 bit shaft that measures .45 will not.

The only set of 48 pilot jets that I currently have on hand are in the carbs on the bike, so this project is on hold until I can pull those jets & see if the #77 bit shaft won't pass through the bore of the 48 pilot jet.

I pulled the 48 pilot jets (this time without pulling the tank, seat, carbs, et al., after modifying a screwdriver to fit the limited clearance on the port side); good news...the #77 drill bit shaft didn't fit inside the bore of the 48 pilot jet!

Soooooo...,I drilled out the 45 pilot jets with the #77 drill bit, & created pilot jets somewhere north of a 45, but south of a 48. What are they specifically? Dunno. Let's call 'em 46.5's

I set the pilot screws at 2.25 turns out for the preliminary test. Weather conditions were not ideal, with temps about 50F, low pressure, very damp, high humidity, variable winds.

This combo is definitely better, with light throttle cruise AFR heading in the right direction.

Under power the bike is generally in the 12-14 AFR range on the road. Highway cruise AFR is north of 13 but not yet where I want it. Very light throttle low-RPM cruise has improved to the low 12's. Warm motor idle is in the mid-12's AFR, so there is room to lean out the pilot screws and increase the AFR a bit at cruise. WOT is where I want it, about 13 AFR.

Overall this is a good mod and the overall driveability is excellent.

Given that I also changed from N3RF needles to NBZT needles that involves another variable, so it is quite possible had I kept the N3RF's the low-end cruise may have been even better with them.

No matter. I've decided to keep the NBZT's in for now, & tune the low end to match them.

It is also quite possible with the NBZT's that the bike would do OK with 45 pilots. Given that I just drilled my only set of 45 pilots, I'm going to get another set and test those as well.

Changing pilot jets without removing the carbs is much quicker & less hassle, however it is a little rougher on the hands.

I'm after optimizing performance and maximizing driveability, without sacrificing fuel economy to do it. Things are heading in the right direction.



While I'm waiting for the new 45 pilot jets to come in, I'll lean out those pilot screws a bit more & recheck those cruise AFR's.

Just for kicks, I swapped out the NBZT's & reinstalled the N3RF's, & with the only other change involving the pilot screws at approx. 1/4 turn less out (& with the modded "46.5" pilot jets still in), the highway cruise AFR was up, to just over 14 AFR (motor not yet fully warmed up), which is about where I want it. Temps in the low 60's, low humidity, gusting winds.

This confirms for me that all other things equal, NBZT needs a smaller pilot jet as compared to N3RF. And I would tune to optimize the pilot screws, however I really like the NBZT's a bit better, so 45 pilot jets & NBZT's it will be.

If I need to go to a slightly smaller pilot jet, at least I know what to do.

I also ordered a higher-quality pin vice if I ever need to drill another set of pilot jets, to make, say, a "43".

what shims if any are you using with the NBZTs? each shim (.5mm) is like adding a size to the main jets. So if you have shims, take them out, leave the pilots alone. Or if no shims drop a main jet size and leave the pilots. That way you keep the needle profile and change the volume around it.

If the bit did not fit in the 48 bore did you not then drill a larger jet than the 48?