Bonneville America Forums Forums Technical Forums Performance Mods CV Carb Tuning

I've been experimenting with different spring rates & slide configurations, with the theory that running pod filters (in my case Uni UP4229's) reduces the vacuum demand & therefore lighter springs can be run.

Here are some preliminary thoughts:

OEM springs are too heavy & make for lazy throttle opening.
The Kawasaki GPZ springs are too light; the slides open too fast.
Stretched GPZ springs work better than stock GPZ springs, but are not optimal.
Yamaha FZ1 springs (made for a 37mm Mikuni) work better than stretched GPZ springs.
OEM springs clipped 9mm improve the low end, but the spring rate is too high above 5k.

The OEM slide opening (approx. 2.5mm) is sufficient with the FZ1 springs. Drilling the opening out to 2.8mm is too much with the FZ1 springs.

Still experimenting, so stay tuned...

I cut my stock springs down ever so slightly (9mm sounds about right) and it did help the initial throttle response. Be careful not to cut too much - a little goes a long way.

my best throttle response speed with the CVK carbs was drilled 3mm and the stock spring and the shrouding for the atmosphere side of the diaphragm removed (sort of like porting the carb).

Quote:

---

Still experimenting, so stay tuned...

---

Good one! And thanks for the post... I'm in learning mode.

Very informative.. Ill be watching this thread for any updates.. nearly winter here so there will be plenty of time for playing with the bike

I also beveled the front & rear edges of slide cutout & polished the slide body (hand polish with 800/1000/1500 grit autobody paper & Simichrome) when I drilled the 2.8mm air port. Whether or not any of that made any difference whatsoever I don't know, but it couldn't hurt.

I tested OEM springs cut to 90mm (when the spring is cut a slight down-bend is helpful at the cut end to help it seat at the cap).

It now accelerates HARD, & the stability is acceptable, with the balance tipping slightly to response.

This tells me the optimal spring length with the 2.8mm slide air port is in the 90mm - 94mm (free length) OAL range, with modified new OEM springs.

For reference, my current setup is an 07 SM, TPUSA 813 cams, long TORS, Uni UP 4229 pods, 150 mains, OEM springs cut to 90mm OAL, polished & chamfered slides, slide air ports drilled to 2.8mm, TBS needles no shims, 45 pilots, TPUSA +3 ignitor remap/8250 limiter, IX plugs.

I think I'm going to run it like this for awhile. The optimal OAL spring length may be 92mm, perhaps I'll give that a go.

TIP: if you cut the springs, make sure that you give the free cut end a small down-bend. Two smaller needle nose pliers will help you get that bend.

Bonus TIP: My measurements were taken with new OEM replacement springs; compression springs such as these may or may not take a "set" over time.

I have Chevy 605 long cannons ready to install (thanks Don), however I'm going to hold off on that install until I can get the bike back on the dyno & see what my carb tuning hath wrought...

I'm pleased that the 92mm springs & 2.8 air ports are a good combo; they provide a good balance of response v. stability.

Next, I'm going to try a 1mm lower fuel level (18mm float height) to mitigate a minor low RPM cruise richness condition.

I did the 18mm float adjustment today. They were set at approx. 16.9mm prior to the adjustment, which was apparently how they left the Keihin factory.

Pulling the carb rack is so much easier with the Uni UP4229's than it was with the K&N pods; there's sufficient clearance on the port side without needing to remove the battery.

This latest tweak improved the low throttle cruise. Overall driveability is excellent.

Next, I'll check the vacuum balance. Then back to the dyno.

I also pulled the hidden filter when I did the float adjustment. I plan to add an inline filter downstream the fuel valve.

This might be a dumb question since I'm still learning the intricacies of carb tuning. Just to clarify, the Yamaha FZ1 springs you're using is the one labeled "13" on this diagram right? I've got a spare set of carbs I'm thinking of tinkering with.

Yes, I have the part number and will post it when I get a chance.

I believe Bike Bandit lists it as part #1213409.

2003 FZ1 - Yamaha Part # 5LV-14933-00 (for 37mm Mikuni)

These are more stable than the Kwacker GPZ springs. I only suggest the Yammie springs for OEM Keihin CVK slides with the stock 2.5 mm air port.

Right on, thanks. I'd like to test them out on a spare set of stock carbs I've got.

Here's the latest:

Chevy 605 Long Cannon pipes
Uni UP4229 pod filters
TPUSA cams & +3 Igniter mod
Nology Coils
Magnecor plug wires
NGK IX plugs
152 Keihin mains
N3RF TBS needles, no shims
45 pilots, pilot screws 1.5 out
Drilled air ports (2.8mm), polished slides, chamfered edges
OEM slide springs trimmed to 92mm OAL
Float height 18mm

Sucker runs strong & happy.

I don't think the Nology coils & Magnecor wires did anything to improve power. That said, I didn't expect them to.

Just a carb-balance away from the dyno now, & that's only to satisfy my curiosity. Just need time & good weather.

Balanced the carbs today, ready for the dyno now.

Still didn't get to the dyno yet. I did, however, replace the drilled slides with another set of stock slides (2.5mm air port); the only change on the replacement slides is a light polish job by hand. The polished stock slides perform about the same as the drilled & contoured slides. The bottom line is I'm not sure it's worth drilling the transfer ports in the slides.

I might swap them back & forth a bit & see how it does.

I also want to try NBAD needles (that's 790 BA/SM for you, Stacka ) & see how they do.

What year slides did you use? Later slides already have the larger port. Drilling was to bring the older slide port up to the later slide port size of the later model.

Not exactly sure, as I bought a spare set of very low-mileage carbs from a dealer as a take-off from a then-new bike that probably had fallen over (carbs have a smashed TPS housing). One clue is the spare carbs have D screws, so they must be one of the later 790's; I did measure the air ports (came up approx. 2.46 + mm) to verify the dimensions. FWIW the carbs had 120/42's in 'em.

Not sure when they went to the D screws but I do know my 04 has them.I also don't know when the larger port was introduced from the factory just wondered if you had no change due to the slide already having the larger port.I never thought there would be enough of a gain to justify the trouble so I never even checked mine.

I tried the FZ1 springs in the 2.5mm polished slides today, & I like that combo the best so far, as it does the least objectionable things wrong. I'm gonna run this for awhile & optimize for it. There's a quick lean spike transition at roll-on, more so in the lower gears, but no hesitation, & this combo is more stable at WOT.

I may also play around with FZ1 spring length, as the springs are relatively inexpensive. The again, maybe not.

The idle AFR is in the 11's, due to the reduced vacuum demand from the free-flowing Uni 4229 pods & the more aggressive 813 cams. This is a common CV carb issue, especially with performance cams. The fix is to drill a small air bleed hole in the throttle plate. Anyone here BTDT with these carbs? I'm thinking something in the 1mm - 1.6mm range. It's easy enough to start small & go bigger. Start too big, though & the plate is F'd. Any ideas?

Personally I think you hit it just start small and work your way up until you hit the magic dia.

At what point does the A/F mix become acceptable?

Oh...got it...right off idle.

Did the throttle plate bleed hole mod...one word..WOW!

I decided to go with a 1.2mm bleed hole & see how it did. I pulled the slide & all related hardware, & taped the various openings. I then used a pin-vice (finger drill) to make the hole First pass was a 1.08mm & I followed that with a 1.2mm.

Here's the finished product...

Air filter side:



Intake boot side:



Carb rack:



Sucker idles in the high 12's now with the pilot screws 2 turns out. Hovers about 13 under hard acceleration, & cruises in the mid 14's. I'm beyond happy with this mod!

The CV's are much better balanced with the Uni 4229 pods & 813 cams now. CV carbs can be made to work well with intake & cam mods, but it takes some work.

After the carbs were reinstalled I also checked the float heights by the sight-tube method, & was disappointed to find a 1.5mm differential between the port & starboard carbs, even though both measured the same statically when the bowls were off. Bummer. Gotta address that. It sure points out the value of the sight tube method, though.

The carbs will also need to be sync'd, didn't have time to get to that yet.

Is a 1.2mm bleed hole optimal? Dunno. But is sure is WAYYYYYY better than no bleed hole. I'll leave it to tuners like TPUSA to dial it in. Get going on this, Carlos!

Thanks Ken for posting that tip. Good to see you got the result you wanted.

In the event that you drill the hole to big you can always fill it with one of the metal impregnated epoxys and start over.

I really like this mod for my application. However, not content to just leave "good enough" alone, I'm going to drill the air bleed holes larger.

IIRC, the next largest size drill bit that I have on-hand is a 1.38mm; & if my theory is correct, the slightly larger hole will perform better and enable another half-turn or so out on the pilot screws, at less throttle plate opening, for a given idle speed. It could be the sweet spot for this application. That should also make the off-idle transition even more satisfying.

Keep in mind these Keihin CV carbs have 4 pilot bypass holes in addition to the pilot screw hole. I'm also considering opening up 1 or more of the bypass holes, however I don't think that's necessary.

Anyway, on to the larger air bleeds, & I'll re-adjust the float heights while the carbs are off.

And if you haven't figured this out yet, I like tuning carbs... ...especially when it works

wow a braver man than I, but I too would like better performance out of my carbs. what is the idea behind drilling these holes in the plates?

I did the 1.38mm throttle plate air bleed mod. I simultaneously adjusted the float heights & pilot screw settings, thereby breaking my own rule of doing only 1 carb adjustment at a time. That said, I estimate that the larger air bleed alone moved the idle AFR a full point leaner.

I was shooting for a float bowl gasket height for the floats, as measured by the sight-tube method (clear hose hooked to bowl drain); the fuel level ended up being slightly higher (more fuel in bowl) than my intended target, but it's OK.

I don't see the need to drill the air bleed holes any larger. I'm sticking with this combo for now.

I also checked the carb vacuum to make sure they were properly sync'd, as I expected them to be off as a consequence of drilling the air bleed holes. To my amazement, however, they were still spot-on from the last pre-drill synch. Go figure.

To summarize, engine/power mods include the following:

TPUSA 813 Cams
TPUSA +3 Igniter
Uni 4229 Pod filters
Chevy 605 LC pipes
PVL (Nology) coils
TPUSA Magnecor Wires
BC AI Removal Kit
NGK IX Plugs
152 Keihin Mains (the brand matters)
45 Pilots
N3RF (TBS) Needles
Polished Slides with stock 2.5mm transfer port
Pilot Screws approx. 2&1/8 out
FZ1 slide springs
1.38mm air bleed holes in throttle plates

The motor is now much more responsive to the pilot screw settings with the throttle air bleed holes. Things are apparently now much better balanced for the lower vacuum demand (pods & cams), & the CV's are doing a good job of metering fuel. And I learned a few things along the journey.

This is now one happy, responsive motor.

I just returned from an 800 mile trip, first biggie since the above carb configuration. The bike ran awesome!

If I didn't mention it already, the bike now idles about 13 AFR, sometimes a little higher, seldom less.

But get this...hammering the interstate the bike got more than 50 MPG! Not bad. And better yet, just cruising along the backroads, not twisting the throttle, depending upon conditions the bike got anywhere from 55 to 57 MPG!!

I'm way impressed.

With my current carb setup, my theory is at very light throttle (butterfly) opening, the carbs are metering more off the pilots (instead of the needles), than it did before the air-bleed mod, hence the improved economy.

How about that, a hot cammed performance-tuned motor with CV carbs that is capable of excellent mileage. Hot d@mn!

Forgot to mention, over 100 miles of that trip was in light to medium rain, & maybe 15 miles in very heavy rain. The Uni 4229 pods performed just fine.

Tried NBAD needles (790 BA for you, Stacka ) today, spiked lean even with 2 shims. Back to N3RF (TBS) for me.

Drilling the butterflies was/is a trick the Holley guys would use if they had a hot cam and they had to raise the idle. The butterflies wound sometimes be exposing the transition slot(I guest similar to the needle jets) and screw up the A/F. With the hole drilled in, they can/could close the butterflies, get enough air in to keep the idle up and not expose that slot. I don't know how much "hotter" the 813 cam is over stock, but this is very interesting and a great idea. Glad it's working for you.