Bonneville America Forums Forums Technical Forums Performance Mods Keihin CVK40 - Where The Parts Do Their Part

I plan to change my carbs main jets down from 145 to 140 in a few days and was wondering if I would need to adjust my air fuel ratios.

Now considering what a diagram called, "Where the parts do their part" in the

Keihin CVK40 website Part II - Tuning and Options section shows, it appears no adjustments would be necessary as the main jet doesn't really take effect until the throttle has been twisted between half and three quarters.

I was just interested what other's experiences were after changing their main jets.

Any ideas

No answers. Well that's a first

I am not sure I really understand your question. If you change your jets the the A/F will change.
there is this quide to tuning CVKs for our bikes http://newbonneville.pagesperso-orange.fr/telechargement/mise%20au%20point%20carburation.pdf

hope this helps.

BTW do you have a A/F meter? or access to one?

Staffo you can set your idle mixture by ear or with a vacuum gauges. Gauges are better,but a good ear can do a great job of idle mixture setting also.

Micah and Michael, you guys obviously know your stuff cos the way I was thinking, when changing the main jets down from 145 to 140, I wasn't sure if I would need to adjust the air fuel or not. Well, looking at when the mains take effect it stands to reason doesn't it.

Anyway, I changed them and definitely needed to adjust the air fuel thumb screws out quite a bit to get Ferdie running sweet. Btw, I have a method of adjusting them individually that works real well. No gauges, just patience adjusting one cylinder, then the other and so on till he's running nice.

Anyway, thanks for your responses guys. Goes to prove that whilst all the various components have their primary responsibility, they still all work together as a unit to give overall good performance across the range.

Btw, Ferdie was running slightly rich last dyno I did, plus I wasn't happy with the fuel economy. It wasn't over the top or anything but after a while we get to know when a little tweaking is in order. I must be pretty close now I'd say but I'm still considering buying an A/F Ratio Gauge cos I work with a good welder who could do a good job with the plug.

Cheers boys and Happy New Year to you all.

Staffo

Thats really only theoretically true as the needle slides up and down through the needle jet holder inwhich the main screws into in other words the needle goes down through the holder and into the main. So the relationship between the main jet and the needle is highly important through out the throttle. the full diameter of the main only kicks in at nearly WOT.
A big main jet and a thinner needle ( like the thruxton needle ) equals a lot of gas.
thruxton needles are longer and thinner with a more graduated taper. where the stock america needles are short and fat with a steeper taper. the speedie needles fall some where in the middle.
This plays directly into the main jet size requirement.
for example I have in thruxton needles and only 130 mains. That is because the thinner and more tapered needle allows more fuel into the venturi through the main jet.

plus I am tinking I am slightly lean.

Thanks for that reply Chad. You too have got a good grasp of how our carbies work, whilst I'm only a hack in the "how each part work together" department.

I've got no issues with replacing jets and needles though. I really liked how you mentioned the difference between the 3 types of needles.

Personally, I really like the thruxton needles and I have tinkered with all three types, including adding shims etc. As soon as I put the thruxton needles in, the pulling power was impressive and now I don't want to change them.

The other part of your response I liked was how you mentioned you use 130 mains. I've had a suspicion for some time that I've been using larger than needed mains. That's why I started by going down to 140 from 145.

I do have 135 and 130 mains to change to but that may require me to move the 42 pilots up to 45, I'm not sure.

That's not a problem either btw, but at the moment I might keep the setting as they are cos I'm riding up north on my bi-annual sebatical to visit my daughter 1500klm's away in two weeks.

Just picking your brains again, can you tell me what your setup is? You can see my current settings from my signature but my main concern with any tweaking is running lean. That's particularly important given where I'm riding will be hot, and running hot and lean isn't a good mix is it?

From what I can gather, I reckon you may well be on the right mark if you have 130 mains with the thruxtons. I'm also assuming you have 45 pilots too with perhaps K&N filters, but I'm only guessing and of course don't know what pipes you're running.

Any chance you letting me know a bit more about your settings, how you came about tweaking to that point and why you think you are pretty well sorted.

I'd appreciate any other experience you have with your setup given you sound you're like me and like the thruxtons.

thanks for your time. I always appreciate others experience with tweaking and why.

staffo

I am running a single unifilter with a drilled airbox, open exhaust, 42 pilots, thruxton needle, 130 mains, and I should go up to a 45 pilot. I am a wee bit lean on decel in other words with the throttle closed at high rpms I am ot getting enough gas causing leaness on deceleration or pop,pop,pop .I looked at what you got you are so close by my calculations

oh I am going to disconnect the TPS so that on decel the closed throttle signal doesnt cause the timing to be to retarded this may also cause the popping.

Staffo, I think you would do better with TBS needles & bigger mains. I'll comment more later. Gotta get some sleep.

why would you think that say that.

As you may know from other posts I run a wide-band A/F meter, which provides me with real-time feedback in actual on-road conditions.

I have tested various combinations of jets, needles, & shims in my 865/270. That said, my current setup is: 07 865/270; TPUSA +3/8250 reprogrammed Gill Igniter; TPUSA 813 cams; tuned CVK carbs (a/k/a “carbies” for our Aussie friends), with polished & chamfered slides, 2.8mm air port, OEM slide springs cut to 94mm OAL & clipped end bent to 92mm OAL, 18mm float height, 45 pilots, pilot screws 1 turn out, N3RF needles - no shims, 150 mains, TPS connected; UP4229 pods; NGK IX plugs: long TORS. My bike normally operates 400’ to 1500’ ASL.

I’ve tested just about every main jet combo from 145 thru 160 (with never a need to try smaller or larger); NBZY, NAGB, NBZT, & N3RF needles, with various shim combos; & various slide spring rate & length combinations.

The bottom line is that CVK carb circuits are complimentary, not mutually exclusive. The main selected will have a bearing on the needle performance, & pilot size will impact screw settings, float height will influence pilot size & screw settings, & airbox elimination will influence air port &/or slide spring rate/length. That’s just for starters.

With the stock 865/270 cams I really liked the NBZT needles. I could not, however, get them to work & play well with the TPUSA 813 cams. The NBZT needles ran rich in the middle (4k – 6k range) & that could not be improved without also destroying the top end A/F ratio ( that is, running too-small mains). The N3RF needles are similar in profile to the NBZT needles, however the N3RF needles are longer. The N3RF is the best needle that I have tried with the 813 cams on my bike.

When I ran the stock cams the smallest mains I ran were 145’s, & at that they ran lean, even with the crappy stock 865/270 cams. As a general rule stock-cammed CVK-carb’d 865/270’s tend to run rich near/at the limiter, presuming proper tuning; 813-cammed CVK carb’d 865/270’s, however, stay linear at the limiter if properly tuned (& that limit is higher with a reprogrammed or programmable igniter!).

Staffo, my opinion is that your bike currently needs the small 140 mains to get the midrange right with the NBZT needles you are currently running. Those needles run the richest of any of the above referenced. I think you would do better with a 145 – 148 – 150 main size range & N3RF needles, personally. In other words, the top end is compromised on the lean side in order to get a satisfying midrange, where you arguably spend most of the time running anyhow.

If, however, you had a wide-band A/F meter on-board, I think you would see that the top end is most likely running lean @ WOT with the 140 mains, & you are leaving power on the table, even with the poor-performing stock 865/270 cams.

As a general rule the 865/270 will run “OK” with overly rich mains, however, mains that are too large will sap torque & thereby suppress power. Most shade-tree tuners seem to run mains that are too big, & don’t understand how the carb circuits work together, or what order to tune them in to achieve maximum overall effectiveness.

Ken & Chad, I reckon you guys are probably right. I'm at work on the iPhone so it'll take too long to tap away why and besides, it's snooze time waiting for the beast to erupt somewhere So I'll reply on my laptop tomorrow. Excellent prognosis though and btw, an air / fuel setup is on my must have list.

One thing though you didn't mention is what size pilots you reckon I should use??? I have the 145's if need be and I realise this may need the other changes first but I'm thinking considering most are now using the 145's you may be thinking those. Anyway, I'll say more tmw but if you get a chance can you let me know what your thoughts are on the pilots?

Actually, I'm pretty excited about your suggestions not only cos i like tweaking, it gives me a chance to use my new stand.

So Thanks, I appreciate you both taking the time to explain the reasons why. Given I'm riding north again in two weeks I'd really like Ferdie dialed in nicely for the trip.

it seems to me that these bikes seem to run better with a richer bottom end (had a dyno guy tell me the same thing after coming to that conclusion thru my trial and error)

IMHO for your set up Staffo I would probably go with the 45 pilots. the only down side is they will not help with fuel economy as they are always drinking thoughout the entire range (not just on the bottom end).

if you really, really like playing with the carbies I recommend getting the FCRs. lots of adjustment possibilities.

I agree with the richer bottom end. I run 48 pilots and on the dyno they are a little rich but not so much that I feel the need to change them. I run 155 mains and stock needles with no shims. I tried the TBS needles and didn't like them so I switched back. 45 pilots would probably have put me pretty close to a perfect A/F mixture. With the 48 pilots once my needles (mains) come into play my A/F mix is just about perfect for where I live.(Elevation/humidity) I could probably go 45 pilots and 1 shim but I am happy where it is now.

Hi all,

Provided I get my sh#t in order, I'll be giving the following a go tomorrow:

45 pilots

TBS Needles

145 mains.

I know the TBS aren't everyone's cup of tea but neither are the Thruxtons to many. I reckon the Thruxtons are great around mid range but I do think they are thirsty and as suggested, come at the expense of quality pwr at WOT.

So I'll see how they pan out but I'm always open to everyone's suggestions from their own experiences.

Of course bum dynos I wouldn't actually call an exact science but that one depends on the owner many times.

Hey, it's not easy to get it spot on from the beginning, but at least it's good fun tinkering.

Me, I do prefer the more scientific stats that come from dynos, A/F ratios and fuel economy etc. No not perfect by any stretch but between the lot of us, one's own instincts, some stats and figures as guidance and a little pinch of pixie dust, who knows, I may be closer to where I want to be this time.

Cheers and thanks to all who provided input, particularly Ken with his in depth comparo

Yea one can go abit crazy with all that stuff. I am looking forward to hearing you report. I don't have access to a dyno yet so I go by the seat of my pants, after a bit of research.

TBS = Thunderbird Sport?

I was thinking Triumph Bonnieville Speedmaster

Quote:

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TBS = Thunderbird Sport?

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Correct

Moe, That's not what it says on a sticker on my frame. I'll take a pic if you want but on those stickers that identify our bikes with the Vin number etc, it spells out the Speedmaster is part of the Bonneville series.

I saw it the other night and remember it cos most of us have never thought of our bikes in that way.

Now coming back from being , I'd really like to know from Ken and anyone else who has an opinion and isn't sick of this one, why I shouldn't use 45 pilots, oem needles with one shim and 150 mains, instead of the TBS needles???

Don't forget I have stock cams and would probably prefer 148 mains so considering I don't, I think 145 mains may be slightly lean.

I just want to get this tweak session pretty well right as I have a few other jobs to do on Ferdie before my ride so I'll only get one shot this time.

Ken, I know you said you have a A/F metre but you haven't mentioned any stats or notes. It's not that I don't believe you but I know I can pretty well rely on the oem needles whilst the TBS ones are untried with my current setup.

thanks for your and anyone else's ideas.

staffo

Staffo, you don't NEED N3RF needles; with the stock cams you can use stock NBZY needles, shimmed as needed, just fine. You may be able to use the NBZT needles also, with no shims.

Shim width can vary, so I suggest having the shop measure them prior to installation.

You can also do the 45 pilots, although the pilot screws will need less out/more in than with the 42's. I also ran 45 pilots with the stock cams.

The above said, I would test the mains first, get them sized right for WOT high RPM, & then focus on dialing in the needles. My gut says 148 - 150 range mains for max power for your application.

The mid range A/F will then point to which needle/shim combo you need.

Ken do you think I may be able to adjust my pilot screws out alittle rather than install 45 pilots because I am so close the only thing is some decel popping. throttle closed all the way if I crack open a small amount like 1/16 or may be less just till I feal the carb springs, no popping.

staffo, you could take the various needles and shims with you on your ride. they can easily and quickly be changed out while stopping for gas/petrol/fuel, or whatever. It would give you a chance to see what works for you between fill ups.

I would make an appointment with a dyno and have the needles, jets etc... in hand so yo can swap them in while you check the A/F on the dyno. You can be close one way or another by not checking it and still be wondering if it's right... better to be slightly rich.

I was refering to the point where you actually start to feel the carbs opening. I guess its the spring that closes the throttle if you let go of it.
You gave me a good answer though it sounds like good advice to me. I'll try it and see.
At three turns bigger pilots are in order.

Ah, OK, the throttle return spring on the plate.

Si senior.

Well I changed Ferdie's settings finally after much discussion.

Thanks to Ken and all those who provided input. I do think I was going down the wrong path, or at least I hope I was.

I guess the beauty is, provided I stay within certain parameters, we really can't get into too much grief other than in my situation perhaps being a tad rich. Well most of us know being rich simply means wasting more juice than we should but at least not doing any damage.

Anyway, I ended up changing those "thirsty thruxtons" (good name that ) with oem's with one shim. The 42 pilots to 45's and 140 mains to 150's. Btw, I'm not joking about the thruxtons being thirsty either. Up until putting them in, I used to always on average get about 230klm's before needing to change over to reserve. Not with the thruxtons though. 210kkm's before Reserve or sometimes worse.

I tell you one thing though, now that I've found a good welder who I happen to work with and owns a mobile service, on top of my prioritiy list had gotta be a plug put into my crossover pipe soI can fit an A/F gauge to take so much more of this guess work out.

Anyway, I didn't get a chance to fine tune the pilot jet thumb screws this arvo but tomorrow I will and fingers crossed it'll take me 10 mins which will tell me I'm close.

I'll let you'll know.

One last thing, using that new stand of mine was an absolute pleasure. It just made the whole procedure that much easier having the bottom of each carby at eye level whilst sat on my stool. Yep, worth every penny.

Stay tunedP

That is an impressive work stand. Nice!

yea I know right. I turned green when I saw that. some guys have all the good stuff.

You are going to find your EZ Riser invaluable. I bought one in 2004 with quite a few extra attachments and it has been one of the best purchases I have ever made. Have had many different types and styles of bikes on it. The only problem is that everyone wants to come over to use it. Not a cheap buy but IMHO well worth the cost.

why dont you just rub it in LKR. We all dont have room for one of those.