Bonneville America Forums Forums Technical Forums Performance Mods CV Carb Tuning

You've done the research, field testing and the work. Now write a tuning handbook :-)

I want to experiment next with velocity stacks.

and after the stacks... TURBO!

I picked up a set of 50mm long Mikuni RS stacks. Next, I need to get a set of Uni pods to fit the stacks. More to follow.

I had the bike on the dyno today. The stacks have not yet been installed. I wanted to see what the dyno had to say before experimenting with the stacks.

Keep in mind this run was on a stale blend of 87 pure-gas & 91 cat-pee (E10).

DJ Dyno, WOT sweep test

SAE corrected (.95), smoothing 5: Max HP = 69.64; Max Torque = 49.48

SAE uncorrected, smoothing 5: Max HP = 73.21; Max Torque = 52.02

Max. HP is approx. 7750 rpm & max. torque is approx. 4850 rpm.

Run conditions: 54.78F; 29.93 BP; Humidity 17%

So why is there a difference between "corrected" & "uncorrected"? DJ automatically adjusts the "corrected" output to reference temp/pressure/humidity values so that different runs by different operators on different dynos can theoretically be compared. The "uncorrected" output was what the bike actually achieved on that dyno with that operator on that run with those specific run conditions.

The "corrected" HP is a bit higher than what I expected to see, & it pulls hard all the way to redline.

The "corrected" torque, however, is a bit lower than what I expected; that said, there is 45lbs or more torque all the way from about 3200 rpm to 8000rpm.

In the run there's a minor torque dip from about 4950 rpm to 5600 rpm, with the bottom of the dip about 5400 rpm; I see that on the road with my AFM (AFR goes rich, into the mid-12's as it sweeps through that RPM band). I think that the torque dip can be improved with a combination of larger G-Pops (covered in exhaust thread) and the velocity stacks mentioned above. That said, with prior tuning iterations that gap spanned 4000 to 6000 rpm & AFR went into the high-10's, so things are going in the right direction.

For those into show & tell, here's the SAE corrected run:


Also keep in mind that my bike is intentionally tuned for real-world driveability, not dyno supremacy. IMO, the two are different.

I think there's more to be had, & relatively inexpensively. More to follow!

good numbers I think

I have decided to modify the stacks to run Uni UP-4229 pods. The filters will need to be modified also in order to fit the stacks. Awaiting receipt of new Uni filters from Amazon.

My v-stack experiment appears to be a success. The stacks were modified to fit the filters. I only took a few pics, because I was more interested in riding the thing than admiring my work.

Basically, a pair of Mikuni RS 50mm velocity stacks were de-horned to fit inside of a new set of UP-4229 filters. I don't run Uni's dry, so they got a light coating of Uni foam oil, outside only, & that was blotted off. I used a combination of free-hand hacksaw, Dremel grinding, and hand-filing to remove & shape the horns.

Here's a pic of the horns removed:



The end result has a 41mm stack length; since the v-stack horns were removed, they are functionally more like snorkles than true v-stacks...no matter, it extends the OAL of the intake tract, which is what I wanted.

This is a close-up of the v-stack mounting flange with the stack inside the filter & the clamp removed:



Here's a pic of what it looks like inside the filter:



A couple of more views of what the starboard side looks like mounted when mounted up; note the limited clearance to the sidecover mount:



Side view:



Back to back test runs in similar weather with & without the stacks showed that the 4900 - 5600 rich dip is gone, & overall things are leaner at idle & cruise, and the above-mentioned rich band is gone.

With this set-up it appears that I can now run pure gas. Gotta test that theory.

Time will tell what the dyno has to say, however once I run the bike in different weather conditions, try pure gas or a pure-gas blend, & check the carb-sync, its off to the dyno for measurement.

So far, I like this v-stack mod.

Latest gas fill included 2.5 gal of pure gas. Weather was relatively cold & AFR's were pretty good overall.

When the throttle is run like a flat slide, all is well. However, if the throttle is whacked or rolled on too quickly, the AFR can still go situationally rich. One theory is that the slide isn't opening optimally under some conditions.

I may have found better CV carb slide springs for this application. Gotta test 'em out, though. More to follow when I get some time, daylight, & dry roads.

Some preliminary testing with slide springs intended for a turbo snowmobile application have been promising. The spring rate looks good, however as supplied the spring is too long. Clipping coils to shorten the spring will simultaneously reduce the preload and increase the spring rate. The art is in balancing the correct preload and spring rate for a specific application. I like what I see so far, however I want daylight before doing any serious performance testing.

I'm not sure yet if the snowmobile springs are going to work out. The springs are 109mm OAL as supplied. I first trimmed them to 102mm OAL & the throttle response was on the slow side. Next up was 96.25mm OAL and throttle response improved, however it clearly wasn't optimal. Today I shortened them to 87mm OAL & throttle response improved a bit more, yet is still not optimal. That said, no lean-bounce at 87mm, so they can go shorter still.

Took the OAL down to 81mm. Throttle response is good. They should not go any shorter; this may be the limit. I don't know yet if they are any more effective overall than the FZ1 springs. Time will tell.

Ken, I'm statring to think this has turned into an obsession.

Looks like I was wrong about the Holtzman Engineering "black" springs; the spring rate is too high when shortened to reduce preload to an acceptable level.

I think the FZ1 springs are really the way to go. I removed the preload spacers from the FZ1 OEM length springs, as they are not necessary. I think the FZ1 springs can be shortened also. Gotta try that next.

I trimmed a set of FZ1 springs to 3.5" OAL (88.9mm) & they seem pretty close to optimal. Throttle response is excellent & they don't lean-spike on roll-on. I'm calling this good for the springs.

The ironic thing is if I didn't experiment with the turbo sled springs I likely would not have figured this out.

This is now a very "happy" motor; those who have experienced it know what I mean.

The fuel level can maybe go a bit lower, gotta check that again. After I confirm the fuel level I'll do a carb sync &, then back to the dyno...

Hey Ken,

Off-topic, but I think your private message box is full?

jh

Today I adjusted the starboard side fuel level down about 1 mm or so, to get it uniform with the port side carb. Both are now set so the fuel level is just under the float bowl gasket surface.

I also checked the carb vacuum & they were still in sync. No need to adjust that at all.

I also nuged the pilot jets a bit leaner. No time to really test it out, though.

Here's where things are now:

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 large G-Pop supplemental baffles
Very mild intake spacer port-match cleanup
Uni UP4229 pods mounted on modified Mikuni RS "50mm" de-horned velocity stacks (41mm OAL on the modded v-stacks)
Custom 152.5 AB mains (bores slightly enlarged with #53 reamer)
NBZT needles, no shims
Yamaha FZ1 Vacuum piston slide springs, cut to 88.9mm/3.5" OAL
Polished slides, slide vacuum port air holes drilled to 2.53mm (#39WG drill bit)
1.58mm throttle plate butterfly air bleed holes (this is larger than optimal, IMO)
18.5+mm (or so) float heights (fuel level below gasket surface)
Custom "41" Keyster pilot jets (Keyster 40's, drilled & .016" reamed to make "41's")
Pilot screws about 3.2 turns out

I may be running out of CV carb tricks...maybe not

Well now...took a ride this evening, temps in the high 70's, clear and dry. I think I finally figured out why my cruise AFR's have been richer than I want with the v-stacks in place & using non-E gas. It's also on the rich side up-top. And the pilot circuit is about as lean as I want to take it. And the v-stacks are likely making more efficient use of intake air. Non-E runs richer in carbs than E-10. Looks like it is time to go slightly smaller on the main jets. Yes, the mains. That's my theory, anyway. I may not have the chance to ride for awhile due to other commitments, however that theory is gonna get tested...

I am waiting for a set of 150 Keihin OEM main jets to arrive. In the interim I tested a set of 152 Keihin OEM mains and a set of Freeman Automotive 150RD mains.

The Keihin 152 mains were too rich above half throttle.

The Freeman 150AB mains were good above half throttle but still too rich at cruise. I think the pilot circuit can be tweaked to make these work, however I want to test the Keihin 150 OEM mains before making any other changes.

Jets by different manufacturers of the same number flow differently, & at different throttle positions. Not enough to feel on the butt dyno, however certainly enough to see on the AFM.

The Keihin 150 mains are good. Thanks to Mike Selman of Bellacorse for getting them to me quickly & at a fair price.

The current 1.58mm throttle plate air bleed holes & the custom Keyster 41 pilots are not playing nicely together; with the v-stacks & non-E gas I can get a good idle AFR or a good cruise AFR, but not both. The best compromise has the idle leaner than I want and the cruise richer than I want.

I then changed the 41 pilot jets for Keihin 42 pilot jets, & although the pilot screws don't need to be out as far as with the 41's, the cruise AFR's aren't where I want them. They are in fact a bit richer.

I think I know where this is going next. Smaller air bleed holes in the throttle plates. I'm not sure if I'm going to fill & redrill the existing plates or replace them with spares & drill them fresh.



And I haven't even touched the air cut valve (a/k/a "coasting/decel enricher") yet...

I'm waiting on my JIS screwdriver set to get here before removing the throttle plates. I'm gonna drill a spare set of plates & install them.

Even with JIS bits, 2 of the 4 throttle plate screw cross-point heads stripped when attemting to remove the throttle plates from the spare carbs. Major bummer. Alden Micro-Grabit #1647P bolt extractor to the rescue...

Using the Micro-Grabit is an art in itself. I now know how.

The throttle plate screws are M4x5mm narrow-head, flat-topped, shallow taper, & countersunk, so there's not much to work with. They are also peened from the other side. Glad they extracted without any shaft or plate damage.

The replacement screws are super-rare. I am awaiting some RC car screws that should work. In the interim, I drilled new air bleed holes, at 90 degrees, in the replacement plates in the same basic location as before. Again, I used a pin-vice and drilled by hand. First pass was .98mm and the second pass was 1.2mm.

I'm gonna try 1.2mm to start, with OEM Keihin 42 pilot jets.

From what I know at this point, optimum air bleed diameter is most likely in the 1.2mm - 1.4mm range, depending upon tuning objectives & setup.

I really don't want to wait for the replacement screws & might roll the dice with the on-board carbs in removing the existing 1.6mm throttle plates & replacing with the 1.2's.

At the very least the reused OEM throttle plate screws will get blue loctite, maybe red; I have not decided. The screws will also be shortened slightly so they are flush with the mounts.

I will also smooth the leading edges of the throttle plate shaft "sandwich" mounts.

I think the 1.2mm air bleed holes will be OK with the 42 pilot jets. We'll see. The art is in balancing the air bleed diameter, pilot jet size, and pilot screw settings to get a decent idle and a perfect cruise...

I went ahead & pulled the on-board carbs to remove the 1.6mm air bleed throttle plates and replace them with a spare set of plates with 1.2mm air bleed holes.

None of my go-to sources - Sudco, CPW, Z1, had replacement screws. So I rolled the dice and attempted to reuse the ones in the on-board carbs. No dice. Every stinkin one had to be extracted. No way on this earth were they gonna come out with any screwdriver or bit combo. Not happening. Alden to the rescue again.

Between 2 sets of spare carbs I was able to get the 4 screws necessary to mount the 1.2 air bleed plates. The whole thing took far longer than expected.

A stainless M4 nut served as a thread-chaser for the refurbished screws. I went with blue Loctite for the remount.

While I was in there I did some hand file work to clean up the screw heads and shorten the screw shafts a bit. I also did some light file work on the sharp edges of the plate shaft mounts.

I left the 42 Keihin pilots alone for now. When all was finished it fired right up. With the pilot screws about 1&7/8 out the cruise is still a bit richer than I want, so it looks like the custom Keyster 41 pilot jets are on-deck to replace the 42's. That should do it.

Closing in on the cruise AFR's that I want. That said, if I didn't know what the cruise AFR's were, I'd be very happy with it as is. But I do, & it can be better, & its gonna be so.

I may be the only one who has ever replaced the throttle plates in CVK carbs on a modern Triumph twin. Dunno. But it sure seems that way. I learned alot from this little project.

Closing in on better AFRs with the modified v-stacks.

The custom Keyster 41 pilot jets are better. Still working on dialing in the pilot screw settings to balance idle AFR & cruise AFR. At 2.5 turns out the idle was about 12. At about 2 & 3/8 turns out the idle was low 12's & hiway cruise was nearly 14. Getting better; there is room to lean the pilot screws and improve both idle and cruise AFR's. Heading in the right direction to cruise AFR Nirvana.

Got the pilots screws down to about 1&7/8 out, however the cruise AFR still isn't where I want it & also got a few very minor carb farts over the course of a 100 mile ride in various conditions. Sooooo....looks like Keihin OEM 40 pilot jets are needed.

The v-stacks must be handling the air flow more efficiently enough to support the smaller pilots. Well I'll be...didn't see that coming.

Oh well...stay tuned

I went with Keyster 40 pilot jets, sourced locally. The pilot screws are now set about 2.25 turns out. Cruise & idle are both good. No carb spitting.

The carb sync was pretty close, even since replacing the throttle plates.

Time to dyno again. Or maybe do some air cut valve experimentation first...

Here's the most current setup:

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 large G-Pop supplemental baffles
Very mild intake spacer port-match cleanup
Uni UP4229 pods mounted on modified Mikuni RS "50mm" de-horned velocity stacks (41mm OAL on the modded v-stacks)
Keihin OEM 150 mains
NBZT needles, no shims
Yamaha FZ1 Vacuum piston slide springs, cut to 88.9mm/3.5" OAL
Polished slides, slide vacuum port air holes drilled to 2.53mm (#39WG drill bit)
1.2mm throttle plate butterfly air bleed holes (close to optimal, IMO)
18.5+mm (or so) float heights (fuel level below gasket surface)
Keyster 40 pilot jets
Pilot screws about 2.5 turns out
Sunoco 87 non-E with a splash of Lucas octane booster...shaken, not stirred...

Motor runs smooth, strong & happy...

On an 88-mile fill to fill mix of 4 lane/2 lane/stop & go, just riding along, the bike averaged 55.9 mpg.

You will have change something and get it out of tune, so you will have something to work out. I still think you should explore writing a book. I'll take a signed copy.

Good man, I wish you worked for NASA, we could use you.