Bonneville America Forums Forums Technical Forums Performance Mods CV Carb Tuning

A certain Mikuni (sp?) jet fits too, another possibility for off sizes. I have them in Wendys bike, can't remember what model carb it was anymore though.

I broke my usual rule of one change at a time by making three simultaneous changes.

After dinner I installed the Keyster 44 pilot jets, set the pilot screws to 3 turns out, and installed aftermarket 152.5 main jets (marked "AB" on the top).

On a warm motor the idle is about 13 AFR, & high RPM WOT AFR stays in the powerband in the low 13's to the limiter.

That said, highway cruise AFR is still a tad richer than what I want. I have yet to find commercially available 43 pilots, so I may have to make one by reaming out a set of 42's. Then again, 42's might just work...

Here's a recap of the current situation:

07 SM 865/270
Stock heads
Stock pistons
TPUSA cams
+3 TTP Pieman Igniter
PVL (Nology) coils
IX plugs & TPUSA Magnecor wires
Chevy 605 pipes with medium G-Pop baffles
Very mild intake spacer port-match cleanup
Uni UP4229 pods
152.5 AB mains
NBZT needles, no shims
FZ1 Vacuum piston (slide) springs
Polished slides
1.58mm throttle plate air bleed holes
18mm float height
44 Keyster pilot jets
Pilot screws 3 turns out

The bike runs exceptionally well, & has instant throttle response with great driveability.

CV carbs can be tuned to perform quite well. I'm convinced that the mains and needles must be optimized before the pilot jets, pilot screws, float height, & throttle plate air bleed holes can be optimally configured.

At this point the carbs are within a pilot jet size and pilot screw adjustment away of optimization.

Then I'm going to experiment more with slide transfer port size and vacuum piston spring configurations

BTW, this tool KICKS ASS for changing pilot jets without pulling the carbs:

http://www.debrix.com/Keihin-Slow-Jet-Driver-Tool-for-Harley-Big-Twin-p/16-0546-vt.htm

My wife is no mechanic, however even she appreciates a quality tool that fits properly!

After dinner I swapped the 44 Keyster pilot jets for 42 Keihins, and as a baseline set the pilot screws to about 3.25 turns out.

Hmmm...this is getting pretty close; the light-throttle AFR's are zoning in where I want them.

Hiway cruise AFR on a warm motor was generally in the very high 13's pushing near 14. Ideally, I want that to be slighty above 14.

Warm motor idle hovered about 13. I think the pilot screws can come in a bit without compromising idle quality, and maybe that will be enough to get the hiway cruise AFR where I want it.

The throttle response is excellent. The motor pulls cleanly from 2k up. Needle to main transition is excellent. High RPM WOT is where I want it.

I never would have thought that a needle change from N3RF to NBZT would have necessitated such different pilot circuit tuning. But it did!

Getting the low speed circuits dialed in is basically a matter of balancing the throttle plate bleed hole diameter, pilot jet size, pilot screw settings, and matching it all to the needle.

I plan to pick up an extra set of Keyster 42 pilot jets. I may then custom drill or ream the Keihin 42's to something larger than 42, but smaller than 43. Not sure just yet. I'm gonna run the Keihin 42's as is for awhile & tweak the pilot screws.

Well, the hiway cruise is good, but not yet where I want it to be, so it looks like a set of custom 41 pilot jets are in my future. As 41's are not commercially available, I'll need to make them.

It's now a delicate balance among the throttle plate bleed hole, pilot jet size, & pilot screw settings.

What were your stock pilots? Like I mentioned in one of my post, mine were 40 and I always thought they were 42. If you still have those you may be able to drill or ream them to that 41 you are looking for. BTW mine is an 07 SM also.

The spare carbs had 42 pilots, so I picked up a set of Keyster 40's.

At 4.5 turns out on the pilot screws a fully warmed motor idles in the low to mid 12's, & highway cruise is pushing 15 in some situations, so at cruise it is a bit leaner than my target. That said, no carb farts either. Keep in mind that my bike needs to run more out on the pilot mixture screws as a consequence of the throttle plate air bleed holes.

No bad overall, however not optimal, either. That's OK, as I now know what the outer lean limit is on the pilot jets, on my motor, with the NBZT's. This isn't a bad combo for max cruise economy, however it isn't quite what I want.

The plan at this point is to custom drill/ream the 40's into 41's. That should do it.

The 41 custom drilled/reamed pilot jet experiment is a success. I did this in two steps; first step, .4mm drill bit(in pin vice); second step, .41mm (actually .016") reamer (also in pin vice); this brought the Keyster 40 to a custom 41.

At this point, at a very relaxed pace I can swap out pilot jets in less than 40 minutes, start to finish. It took a bit longer this time in order to drill and ream the pilot jets, though.

In wild anticipation of a test ride I forgot to scribe the pilot jets as being modified...oh well, maybe next time, if I ever need to remove them.

I went with 3.75 turns out on the pilot mixture screws as a starting point/baseline. As a general rule, a 1 step size change (single digit change, in my case, from 42 to 41) will require a .75 turn change on the pilot mixture screw to maintain comparable idle AFR's all other things equal.

Initial test weather was clear & dry, temps in the low 60's, low humidity, & gusting winds. On a fully warmed motor it idles in the low to mid 12's; flat, light throttle highway cruise is now in the low 14's.

This combination is better, and should reclaim some cruise economy. I really want the cruise AFR's to be in the low to mid 14's. I think the pilot screws can come in just a bit, maybe an eigth of a turn, maybe a little more.

When I'm happy with the pilot settings, I'm going to concentrate on vaccum piston (slide) transfer port size and G-pop baffle angle (exhaust)...

Whoops, there's a typo in my 41 custom pilot jet message above; the custom 41 was made from a Keyster 40, not from a 42.

3&1/3 turns out on the pilot screws proved to be slightly too rich, & 3.25 was slightly too lean for what I want. The pilot screws are now just over 3.25 turns out and the cruise AFR is about where I want it. Just gotta make sure there is no carb spitting. At 3.25 turns out on a fully warmed up motor the idle AFR was hovering about 14, and that's lean enough at idle to induce carb spitting with the hotter 813 cams. At 3.34 turns out (I can't really get that precise, but I like saying that better than 3&1/3) the idle AFR was in the low 13's with no carb spitting, but the cruise was still a bit richer than optimal. Just goes to show you what a small sweet spot there is when the needle, pilot jet, pilot mixture screw, float height, and throttle plate bleed hole diameter work & play well together.

WOT high RPM is a bit leaner than I want, so up next will be a set of custom main jets; it has a set of 152.5 aftermarket jets in it now; I believe that the aftermarket jets are marked true to the actual bore size. Keihin main jets are not true to actual bore size (with Keihin mains, the actual bore size is generally smaller than the number marked on the jet).

Anyway, I made a set of "153" main jets (with a 4-flute #53 wire gauge reamer) using a set of Keihin 152's as donors. FWIW, the #53 reamer wont pass through a set of Keihin 155's (just every so slightly too tight) but pass clean right through Keihin 158's). If for whatever reason the "153's" prove to be too large, then I can always drop back to Keihin 155's.

Just gotta get some time to install the modded mains & some daylight dry weather so I can test 'em.

If my theory is right, and presuming that the larger mains do what I expect them to do at high RPM WOT, then the pilot mixture screws may be able to be leaned out slightly more...

I also want to try my 2.8mm transfer port vacuum pistons (slides) again...but after the main jet experiment.

I marked and installed the custom mains, pulled the custom pilot jets, marked them, & reinstalled them.

Well, another interesting discovery; the aftermarket "AB" 152.5 main jets swallowed the #53 reamer (actually a snug fit), so I expect my modded Keihin jets to perform very similarly to the AB's they are replacing.

The weather sucks right now for performance testing (light rain, low pressure, high humidity, getting dark) so I'll need to wait another day.

I believe that the "AB" jets are marketed by RD Precision Engineering/Freeman Automotive group; they are dimensionally longer than similar Keihin main jets, the inlet chamfer is different, the bore diameter is different, and the bore length may also be different.

I'm looking forward to testing the custom mains.

The modded Keihn #53/152 main jets run OK, however I like the AB 152.5 mains better overall; so next I'm gonna try the Keihin 155's just for a comparison.

I tested the #53/152's, Keihin 155's, and Freeman "AB" 152.5's. All are similar, yet different.

The #53/152's were relatively slightly richer than the Keihin 155's.

The Keihin 155's were close to spot-on at WOT 7/8 throttle & up, however were on the rich side below.

Both of the above were relatively richer than the AB 152.5's.

Hmmm. What I decided to do was take a #53 4 flute reamer & give the 152.5's a few light turns, just by hand.

So far the lightly modded 152.5's perform the best overall with the current setup. The hiway cruise is sub-14, & with 3.25+ turns out on the pilot screws the warm motor idle AFR is in the low - mid 13's. I plan to lean the pilot screws slightly, & presuming there is no carb spitting, take a run to measure fuel consumption. If all goes well, I predict close to 50 MPG average, if not going WOT.

Beware of anyone who tells you that CV carbs "can never be a performance carb" or that the various carb circuts don't "overlap", as they certainly can, and do.

Mains influence far more than what some understand.

Needles are a complexity all by themselves, and needles will influence what pilot jets and screw settings your motor will need.

Float heights can make or break what you are trying to achieve.

Anyway, now to dial in the pilot screws before I focus on the vacuum piston transfer port diameter. Again.

The pilot circuit is pretty close to optimal now; the bike cruises about 14 AFR with just under 3.25 turns out. No carb spitting yet; it can maybe go a little leaner, but not much, perhaps just a fraction of a turn.

I think I can get a little more performance out of the midrange transition by optimizing the slide port diameter.

I nudged the pilot screws a bit leaner; the're now about 3.2 turns out, no carb spitting. I'm calling that good.

I have to say you have taken tweeking these carbs to a new level.

So when you plan on coming by to help me optimize mine?

My latest mod was to drill the vacuum piston (a/k/a slide) air hole transfer ports with a #39 Wire Gauge drill bit; a #39 WG bit is approximately 2.53mm. The stock air holes are supposed to be 2.5mm.

I drilled it by hand using a dremel chuck, as I didn't have a pin-vice large enough to accomodate the #39WG drill bit.

I'm not sure yet if this mod really accomplished much, although the throttle response is crisp. I don't like to do serious performance evaluations in the dark, although I did take the bike for a ride & it seems to perform well.

I don't see a need to go any larger on the slide air ports, especially with the FZ1 slide springs. I previously ran another set of slides that were drilled out with a 7/64" bit (approx. 2.78mm), & at the time thought that was too much for anything other than stock springs, or modified stock springs. I still do.

Under NO circumstances would I suggest drilling the air holes any larger than 7/64"; really, #39WG is large enough for the FZ1 springs. I would consider 7/64" as the upper limit, under any circumstance. It really doesn't take much to clean up & optimize the air hole. IMO Jenks is an idiot if he thinks 3.0mm air holes are called for.

Anyway, I'm gonna test this out a bit more when I get some time, daylight, & decent weather.

I like the slide air port hole size (#39WG), there is no need to go larger IMO, at least with the FZ1 springs.

The motor had a slightly rich range at very light throttle from 2.5k - 3k RPM; I suspected it might be float height, so I dropped the fuel level another 1 mm or so.

I'm not a fan of the "measure the float height with the carbs pulled" method, as when the carbs are reinstalled there is no guarantee that the actual fuel level will be where you want it. I therefore prefere the "sight tube" method that shows the actual fuel level in the float bowl, with the carbs connected.

The starboard side adjustment was a piece of cake. The port side, however, was an absolute ******, though. I think the port side can be tweaked another half-mm.

Anyway, my theory was correct & the issue resolved.

The motor has a great balance of performance & economy; it is a kitten at low-rpm light throttle, a lap-cat at cruise, & a tiger at WOT. It is responsive, smooth & strong.

When I get some time I'll take it for a dyno run to satisfy my curiosity. I have a pretty good idea what the numbers are likely to be. That said, so what, the driveabilty is excellent & that's what I'm after.

I still don't think you done yet.
But how bout a general summery of what you've done and where its at?

Thanks Ken, You sir have turned this into an artwork. I've been truly inspired by your dedication here. Hopefully in the near future I can acquire a reliable AF gauge and head down this same rabbit hole in search of optimum balance of performance and fuel economy.

Matt, it's a sickness, I tell ya...

After dinner & some lawn work I swapped the FZ1 springs & 2.53mm slides for the 92mm OAL OEM springs & 2.78mm slides; no other changes.

Took it for a ride, temps in the mid 60's, & that combo performed very well overall.

I really haven't yet decided which combination I like better...

Well just think how much better you "fuel economy" would be if you didn't have so many "test rides". LOL

I've been doing some studying of coil spring rates. I don't think that clipping the OEM springs is the way to go. Clipping a coil spring will shorten the length, thereby reducing preload, however removing active coils from a coil spring will actually increase the spring rate (lbs/inch, newtons/meter).

The FZ1 spring is very close in overall length to the OEM spring. The FZ1 spring diameter is the same. The FZ1 outer diameter is very slightly wider than the OEM spring. The FZ1 spring has 4 more active coils than the OEM spring, hence the spring rate of the FZ1 is approximately 70% of the OEM spring.

Ideally, want to find a spring with a spring rate inbetween an OEM spring and an FZ1 spring; so far, I'm still searching for that.

For now, I'm going back to the FZ1 spring/2.53 slide combo.

I may pick up more FZ1 springs for some experimentation...

With the FZ1 springs and 2.53mm slide ports I noticed a slight slid bounce on opening the throttle.

That tells me that the FZI spring rate is challenged by the 2.53 slide.

Hmmm...maybe a bit of preload might help. I tried a set of 12mm steel 2.5mm thick washers (that I had on hand) between the top of the spring & the carb cap to test the theory. It worked.

So, today I made a set of preload spacers from a set of Trackside (sourced from Cycle Gear) M12 aluminum 1.5mm thick drain plug washers. They are light, and with some drilling/filing fit the center post of the carb cap perfectly. Optimal thickness? Dunno, but the slide bounce is gone.

Gotta richen up the idle mixture a little bit though.

I also have a lead on some slide springs of the proper diameter that might have a rate less than OEM but more than FZ1.

Ken, this may seem like a strange question but has the barometric pressure (weather) been relitively the same each time you've been tuning?

Not strange at all. AFR and HP will vary with pressure, temperature and humidity. I understand the relationship and mentally adjust accordingly.

The safest way to tune is for high pressure, cold ambient temps, & low humidity. If the tuning is spot on in that scenario then it will automatically become relatively richer in warm & humid weather, all other things equal.

I have to chuckle when some say "turn the pilot screws richer in warm weather", because, all other things equal, the mixture automatically becomes richer in warm weather.

Update:

07 SM 865/270
Stock heads
Stock pistons
TPUSA 813 cams
TPUSA +3 Pieman Igniter
PVL (Nology) coils
IX plugs & TPUSA Magnecor wires
Chevy 605 pipes with medium G-Pop baffles
Very mild intake spacer port-match cleanup
Uni UP4229 pods
Custom 152.5 AB mains (bores slightly enlarged with #53 reamer)
NBZT needles, no shims
Yamaha FZ1 Vacuum piston slide springs, with custom aluminum 1.5mm preload spacers on the springs
Polished slides, slide vacuum port air holes drilled to 2.53mm (#39WG drill)
1.58mm throttle plate butterfly air bleed holes
18.5mm (or so) float heights (fuel level below gasket surface)
Custom "41" Keyster pilot jets (40's drilled & reamed)
Pilot screws 3.25 turns out

Still not done yet

I did a 250+ mile ride yesterday, mostly state routes & backroads, on and about the hills & valleys, & the bike averaged 53+ MPG (1st topoff was 53.6 & the 2nd was 53.9).

thats pretty darn impressive. What would you say you average speed was on the straight aways?

Hard to say, as speeds outside of town generally varied anywhere from 35MPH - 65MPH. I wasn't intentionally conserving fuel, & wasn't looking for every opportunity to go WOT. Most of the time it was half-throttle or less.

ok Ken what is your goal? went throughout this thread again..somewhere on the begining of it you mentioned your goal is 12,5 and around this

what is your result now? let's say....;
1/4
1/2
WOT

Had a chance to ride my bike today second time with A/F gauge ,weather wasn't best...very strong wind and wet surface(no rain)..........

my average ratio up to 1/2 throttle was in the middle 13th mostly around 13,6 ,idle above 14 , WOT went down just above 12 but that was too short run to see what is really happening