Bonneville America Forums Forums Technical Forums Performance Mods CV Carb Tuning

With the warmer weather it can maybe use a little less on the pilot screws to get max fuel economy, however the bike is running so well right now I'm reluctant to mess with it ...maybe that's just a phase I'm going through...

Current tuning:
07 SM 865/270
Stock heads & pistons
TPUSA 813 cams (Gen 1)
TTP Fire Starter 865 Option 2 Igniter
PVL (Nology) coils
IX plugs & TPUSA Magnecor wires
Short tapered TORS, inlets derestricted
BC intake manifolds, with heads port-matched to intake manifolds
Uni UP4229 pods mounted on modified Mikuni RS "50mm" de-horned velocity stacks (41mm OAL on the modded v-stacks)
TPS connected
ACV enabled
Keihin OEM 150 mains
NBZT needles, no shims
Keyster 40 pilot jets
Pilot screws about 2&7/8 turns out port side, about 4 turns out starboard side
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
18.5mm (or so) float heights (fuel level below gasket surface)
Sunoco 87 non-E gas + a splash of Lucas Octane Booster

The motor runs smooth and strong. My reasonably experienced butt dyno says about 70 HP/50 TQ...we'll see what the DJ Dyno has to say about it, hopefully soon.

Bike went to the dyno today, not an optimal dyno-weather day, however I had the time & the shop was available.

At this point I think the carbs are tuned about as optimal as they are going to get, and I have now hit the wall with these carbs & the current exhaust.

The bike runs so darn well...it starts instantly with no drama; cruises nice & smooth, has a good or better throttle response than any other CV carbs I've experienced, no decel-popping, no bad habits, will purr like a kitten or growl like a tiger.

Depending upon which DJ dyno it is on, the upper limit for this combo is about 69-70 RWHP & about 50 Tq, SAE corrected. It has done better uncorrected on a good weather dyno day, however I consider the corrected numbers to be more useful for tuning comparison purposes.

The only changes that I have since made is to nudge the port side PMS a bit more closed & the starboard side PMS a bit more open.

I have another exhaust that I want to try to see if I can get a bit more power out of it, however IMO at this point the CV carbs are the bottle neck. We'll see.

Gotta say you've got some staying power, that's around 5 years of CVK tuning. You modified those carbs every which way to get them optimally setup for your bike.

Are you really thinking about changing them now?

Forgot to mention above that the ends of the throttle plate screws were also filed/smoothed for improved WOT airflow at the throttle shaft...

Given that all decel popping is gone, why not plug that ACV again...doing so can only improve the already good fuel economy...hmmmm...

ACV is now disabled, no down side except for some minor decel popping on closed throttle. The ACV disable mod is super easy to do, however it requires making a proper sized plug, of which to my knowledge none are commercially available.

Current tuning:
07 SM 865/270
Stock heads & pistons
TPUSA 813 cams (Gen 1)
TTP Fire Starter 865 Option 2 Igniter
PVL (Nology) coils
IX plugs & TPUSA Magnecor wires
Short tapered TORS, inlets derestricted
BC intake manifolds, with heads port-matched to intake manifolds
Uni UP4229 pods mounted on modified Mikuni RS "50mm" de-horned velocity stacks (41mm OAL on the modded v-stacks)
TPS connected
ACV disabled (rubber plug used in place of vacuum channel grommet)
Keihin OEM 150 mains
NBZT needles, no shims
Keyster 40 pilot jets
Pilot screws about 2&7/8 turns out port side, about 4 turns out starboard side
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
18.5mm (or so) float heights (fuel level below gasket surface)
Sunoco 87 non-E gas + a splash of Lucas Octane Booster

Here's a pic (from left to right) of the OEM AVC grommet, the donor auto body rubber "bumper" that was used to make the ACV plug, and my first attempt at making the ACV custom plug:



I ended up using another custom plug that turned out better (it's currently installed), however this pic provides an idea of what the finished product looks like.

The custom plug simply replaces the OEM grommet. The donor "bumper" was sourced from a Sears Hardware "hard to find parts" bin.

I did a mixed-route ride today of 2-lane, 4-lane with traffic signals, some stop & go, & highway (60 - 75 mph) starting & ending at the same gas station & the same pump (107.4 miles total pump to pump), neither racing nor hypermiling - 58.5 MPG - not too shabby...

Ken, I'm surprised a racing team hasn't tried to conscript you.

The bike started running rough, it came on suddenly but not as fast as an electrical issue...

Clue # 1: probably fuel related.

So...I flipped the fuel valve to Reserve. No change. Crap.

I was near a gas station, so I figured why not pump a gallon and see what happens. No joy.

Clue # 2: It's not the fuel itself.

The O2 sensor is located on the starboard side crossover tube, so the AFR readings on my AFM are starboard side biased...the meter peg-bounced to 18+ AFR, but wavered...

Clue # 3: Probably the starboard side.

I noticed that the motor would smooth out a bit and blast along just fine once it was on the needle heading to the mains...also confirmed by the AFM.

Clue # 4: probably the pilot circuit.

So, I next drained both float bowls. The starboard side had some fine-grit type "dirt" in it; port side was clear.

Clue # 5: Probably something clogging the starboard-side pilot circuit, most likely the pilot jet.

So, I dropped the starboard side bowl & pulled the main jet and pilot jet; the main looked clean & unobstructed, however the pilot jet looked slightly occluded. Bingo. I used a plastic dental pick to gently clean out the innards of the pilot jet.

Clue # 6: The clear plastic inline fuel filter was a bit discolored.

I then replaced the fuel filter with a larger clear filter - when in doubt, change it out.

I put it all back together, & it fired up happy again. Took it for a ride and all was well.

The bike still runs great, CVK carbs & all...

I have followed this thread for 10 years and have enjoyed all the effort and dedication you have put into your ride to 'Get it right'. It seems like you haven't performed any more changes over the last 4 years so things are good. Have you ever considered boring out your CVK carbs then start the whole process over again???? Probably add a few ponies to the engine.

Might as well keep this old thread alive.

Having bored out a set of CVK's, I agree with you, not worth it.

The 39mm FCR's are a better way to go but a real PITA to get set up correctly, plus expensive. But when they are set right, hang on!

I now have a set of the 45mm HSR's that really have some potential. As son as it gets warmer I will be getting those dialed in.

Micah, whoever bought your FCR's got a screamin' deal!

I do not know who ended up with them. I sold them to fast eddy and he passed them on to someone else. Not in the states as I recall.
It would be interesting to hear how they worked out.

He sold them to a customer here in the UK. I have just fitted them along with a 904s big bore kit, it all worked out fine. The spacers on the inlet side were a pain in the arse to remove so they fitted the standard intake rubbers.

Sorry about that, they were shrunk onto the carbs to fit the pod filters. I never thought about anyone taking them off!

Glad to hear they have good to good use.

@B02S4

Greetings Sir,
Coolfogman here from the TriumphRAT forum. I want to thank you for your response to my query regarding my carb troubles!
Following your recommendation, I up-sized the pilot jets in my CVK's to #45 (keihin). Idle circuit thumbscrews are each ~2.5 turns out. (Also cleaned and re-oiled my K&N pods.) Took her out for a 30-mile test ride at varying speeds... found that the at-cruise miss/stumbling is much improved but still there as well as the power-robbing wind gust symptom - although the intensity of that also seemed lessened. 🤨

Thinking that the denser foam element of Uni's might have a positive effect in regards to the wind-gust issue - I purchased and installed a pair of Uni pod filters (#4229), from a local shop and took her out again... performance-wise the bike responds much the same as having the K&N pods on it (with slightly quieter intake suction noise).

Here's where things are at currently:

'06 Speedmaster 865
8042 miles on odometer
Stock motor, head and cams
Airbox completely removed
Uni Pods (#4229)
Air Injection removed
EVAP/Emissions removed
Sceptre pipes
TTP Stage I Igniter
Nology coils
New Bonneville "Premium Spark Plug Wires"
NGK BPR8EIX-9 Iridium plugs gapped to the .8 - .9mm spec
#45 Pilot jets (Keihin)
#148 Main jets (Keihin)
Idle Screws 2.5 +/- turns out each
Thruxton (NBZT) needles w/1 shim
Stock slide springs
Vacuum slides - wings sanded to remove surface variations (unknown cause), using 3M automotive premium wet-sanding paper from 800 - 3000 grit (sanding 90° to previous grit), finishing in direction of movement and then polishing wings and bodies with Flitz polish
Vacuum slide bleed holes at 3mm (drilled by previous owner)

The bike starts easily with no choke and idles well with no popping.
Acceleration is smooth up through the gears with no perceptible hesitation or flat spots
At highway cruise (~4200 rpm/70mph), there is a slight stumbling happening, but if I roll into the throttle it accelerates without hesitation

In addition to the at-cruise stumble, the other symptom I'm still experiencing is: when I get hit with a wind gust - from any location other than head-on or directly astern - the gust immediately seems to suck the power out of the motor and I have to roll into the throttle to maintain speed!

As I stated previously, the above two symptoms are much improved with the installation of the larger pilot jets but still there.


I'm hopeful that you or another forum member will have a resolution?!

Thank you and much appreciation in advance to all who may have input!

Best Regards,
John Wentworth
"Coolfogman"

Well, for starters (pun intended), if your bike fires right up w/o the enrichener, then it is getting more than enough fuel through the pilot circuit. Probably too much if it runs smoothly on a cold motor, no "choke" (enrichener).

My initial thoughts...the NBZT needles should NOT need to be shimmed if the mains ate properly sized...the mains are prolly undersized by 1 or 2 steps...the vacuum piston (slide) air bleed hole is too large (although if you are running OEM slide springs, I doubt that is the cause.

Could it be an intermittent vacuum leak? Yes, it could. Check everything with the methods of your choice.

If mine, here is what I'd do:

Check all caps & connections for splits/tears/leaks...
Pull the needle shims...
Install 150 or 152 Keihin mains

Not letting me edit posts...sure wish that could be remedied...

Ordinarily 1 change at a time, but in your case, I suggest all 3...better, the same or worse?

@B02S4

Thanks for the reply!
There are NO vacuum leaks... ALL the caps are new!
I've checked for leaks with both carb cleaner and starting fluid... there was NO change in idle using either type of spray.
I'll pull the shims this weekend and see if that makes a difference.
I'll have to order the larger jets as I don't currently have those sizes.

A question...
Previously - using a float height gauge, I had adjusted my floats to the recommended 17mm height. When I checked the fuel levels - using the sight-tube method - fuel was just above the top of the gasket flange (~2.5mm above the flange face).

I've since adjusted my floats so the fuel level in the bowls is right at gasket flange.

Do you think the fuel level in the bowls is contributing to the symptoms I'm experiencing?

Regards!

Very helpful that you ruled out vac leaks.

Although there are different schools of thought how to best go about tuning, I prefer "top down"... that means, get the mains close to dialed-in first, if possible. That can save unnecessary tail chasing, as different pilot & needle combinations can compensate for sub-optimal mains, to a surprising degree. And that can cause unintended consequences.

I don't recall what the fuel height is on my bike, although it is lower than OEM spec; I also use the sight tube method to verify actual fuel height. The above said, I doubt that is the source of the surging.

I suggest using some tape to mark your throttle twist grip, in 1/8, 1/4, 1/2, & 3/4 increments. You might be amazed at how little throttle is used JRA (just riding along). At any rate, take note of the specific throttle position when the surging issue starts & stops.

Have you pulled the float bowls to check for sediment? If not, I suggest doing so. And pull the pilot jets to make sure they are not partially occluded. If those jets are suspect, replace them. They are small enough to be easily damaged unless they are cleaned by someone with the correct knowledge & tools. To check if any of the other pilot circuit orifices are occluded, the carbs will need to be pulled to do it right, so just stop there for now.

I suggest not pulling the shims until you also have the larger mains on hand. Then, pull the shims & install the larger mains. Go test. Better or worse? Report your findings.

I used to have a great illustration showing to what degree and when the different carb circuits come into play and the over lap. Since photobucket deleted all my stuff because I refused to pay for it I can't post it anymore.

CFM, I just realized that you prolly used new 45 pilot jets, so disregard the "sediment" comment if you were recently in there...

@B02S4

No sediment in the bowls. Also, when I last cleaned the carbs they were completely torn down and cleaned ultrasonically (~1hr each). All ports and passages were blown out using compressed air (run through a filtered regulator, water trap and desiccant dryer assembly that I use for painting and to protect my air tools). In addition to cleaning the carbs, I pulled the petcock and made sure there was no debris clogging its screen as well as the little hidden filter in the "T" crossover pipe.

After cleaning I polished the fuel bowl needle seats to ensure a good seal and installed new float needles, all other parts/jets as listed and new bowl gaskets.

I received the new larger #150 and #152 main jets yesterday so I'll be removing the shims and installing the 150's and testing in the next couple of days. If the problem is still there, I'll install the #152 mains and test again.

You mentioned before that (with the #45 pilots), my not having to use the choke on startup meant I was probably getting enough fuel (down low)... with larger mains do you think I'll need to drop the pilot jet size back to #42?

Thanks for everyone's help!

Regards,
John

I suggest trying the 152 mains & no shims, with no other changes, first...test on a fully warmed up motor.. take note of throttle position & any symptoms you identify...give us some specifics & we'll get it sorted out.

Did a plug-chop check on the way home today. 4300rpm/74mph. Ambient temp 102°F. Engine temp. 180°F
Definitely looks like she's running lean!

OK... I REALLY don't like the fact that I can't just upload/attach a picture to my post and am required to host them on some website, embedding a link to the picture in my post for it to show up here!
Is there a work-around?

John

Update:
Removed the shims and installed the #152 mains Thursday evening. The test ride was my commute in to work Friday morning...
In town low speed low RPMs running up through the gears everything was fine! The larger jets seemed to have the bike running better - like it had a more available power and no issues with acceleration or run quality.

The problems began to exhibit when I got onto the highway and got up to about 65 miles an hour where, once again, the engine run quality was very bad - cutting out and sputtering like it wasn't firing right and the rpm's falling off like it was out of fuel. At one point about halfway to work it was running so badly I backed off the throttle to slow down below 50-55mph (where it had seemed to run better). But when I twisted the throttle to accelerate back up to 55mph - the engine completely bogged out like it was out of fuel!
I slowed to the point that I began to pull out of the right lane onto the shoulder when it started running normally again (although still not well), so I continued on to work at a reduced pace.

At the end of the day I took the uni filters off (to verify the slides were moving up and down normally), and also disconnected the throttle position sensor at the connector (thinking that might have had something to do with how the bike was not running well at speed). Well, that didn't have any effect (!) because on the way home it started running so poorly that I had to limp it along at 50 mph down the freeway - with diesel truck and car traffic passing me - like I was on a moped! 😖😤

Got it home and put the fans on it to cool it down. Once it was cool I put it up on the jack and pulled the tank to remove the tops of the carbs and check that I hadn't inadvertently covered up the vacuum slide bleed holes with the little footed piece that goes over the top of the needles... causing the "even worse" running issue... that wasn't the case as both holes were unblocked and open.

I put everything back together, put gas in my temporary fuel bottle and hung it from the rafters; connected the carbs and fired the bike up and turned up the idle knob so the motor was turning ~4500rpm. Using carb cleaner with an extension tube - I sprayed around the manifolds and spacers; around the rubber boots that connect the carbs to the manifolds; at the vacuum caps; and, all around the carb bodies high and low on all sides.

At one point - spraying the inner side of the L/H carb - the engine bogged down!

What I believe I've discovered is: there a significant vacuum leak which only occurs at higher RPMs and steady throttle! It's possibly at the carb fuel bowl vent T-pipe where it passes into the body of the L/H carb. So either the tube is cracked or there's a leak at the U-clip where the tube connects with the body the carb!
Without removing the carbs again and completely tearing them down I'm not sure if that's exactly the case (I've ordered a new T-pipe and new U-clips). So, when everything gets here sometime next week it'll be another carb teardown!

After thinking about this some more...
It's possible the leak may not be at the vent t-pipe at all. It could be the throttle shaft!

Is there some type of o-ring seal on the throttle linkage shaft where it passes through the body of the carb to the butterfly plate?
If so, is this a serviceable item (I don't see a throttle shaft O-ring/seal in the carb parts diagrams)?

I'm now thinking a leak at the throttle shaft is the more likely culprit for what I've been experiencing.

What do you think?

Regards,
CoolFogMan (John)

The symptoms do seem to point to a vac leak. That said, a vac leak at the butterfly shaft would be very unusual. The jetting is definitely in the ballpark. I do think it is a vac leak of some sort, & doubt that it is the shaft.

Does the rough running only happen on a warmed up motor?

Depending on how you respond, there are possibilities other than a vac leak.

I originally posted this yesterday on the your thread looking for a carb set. What you are describing sounds like something I had with my America associated with the carb float bowl vent, creating a vacuum in the float bowls and a fuel starving situation at speed and when there were wind gusts. I'll repost it here...

Your problem about a vacuum leak at speed got me thinking about an issue I had while travelling at speed some years ago. First off, there are 2 vent hoses that can cause problems with our bikes. The vent line for the fuel tank and one for the carb float bowls. The vent line for the fuel tank can cause your bike to quit if it gets kinked or driving in heavy rain. The fix was to cut the hose at a 45 degree angle to prevent water plugging the hose. I thought I did this to my fuel tank vent but instead I cut the carb float bowl vent just before a trip across Canada years ago. These hoses are just behind the engine and are easily mixed up.

My bike started to run really lean at highway speeds especially when trying to pass a transport or heading into a crosswind. I guess by shorting the wrong hose, it ended up in such a location that a vacuum was created in the hose/float bowl when the bike was at speed. I ended up with a loss of power along with real good gas mileage. I didn't solve the problem until I got to my destination. I couldn't figure out why my bike was running so lean at speed. I thought I had a major vacuum leak. Pulled the carbs, checked the hoses, carb mount rubbers, float levels, hidden filter, etc etc. Then I remembered the mod I did to the hose and after checking, realized that I had caused the hose to sit in some form of vacuum pocket. I relocated the hose and the problem was gone.

Food for thought before you change carbs unless you have confirmed the throttle shaft is leaking...

This should be in it's own post for future consideration. After reading Coolfogman's problem it exactly describes what I had. Relocate the float bowl vent hose which is behind the engine or install a foam plug over it.

Here's my hypothesis - there is a vacuum formed just behind the engine at speed that gets transferred to the top of the fuel in the float bowl through that hose. This vacuum prevents proper fuel flow inside the carbs. Usually you have atmospheric pressure pushing on the fuel in the carb just before the pilot/main/idle jets. If you end up with a vacuum in the bowl at speed, fuel transfer will be curtailed. It is quite rare but has happed in the past with a couple of members, myself included.