Cv carb jetting

Deadfrednc

Deadfrednc

New Member
Hey guys I've got an 81 cb650c with cv carbs it's running rich and it's not very smooth through the power band kind of jumpy. It had been rejetted prior to me owning the bike and currently has 130's in the 2 inners and 127.5's in the outers. I've changed them before with the jets supplied in the rebuild kits which I believe were 110's an it didn't have any power I barley made it to the end if my driveway. As it is now I ride pretty regularly I recently made a 370 mile round trip and it did good but when my brother in law rides behind me he says it's a pretty rough smell. I'm not sure where to start or how many sizes to drop on the jets. Thanks for any help
 
Theres really no best answer for this type of question. Personally, I would just buy one of every jet size between the ones that are too rich and the ones that are too lean, then swap them out one at a time until you find the one that works best. If you want to save some money, you can just start in the middle (so 120's in this case) and go up or down from there.

I would also question why youre running 2 different jet sizes, unless work has only been done to 2 of the cylinders, I would assume they should all be the same.
 
It could be other things. The CV uses emulsifiers, they are the longish tubes with holes in the side. They aerate the fuel after it passes through the jet so the droplets are smaller and vaporize better. If the emulifiers are clogged (frequent issue), the fuel is not aerates. Larger drops are let into the carb and while the fuel amount is correct, the vaporized amount is less. So you end up with unburnt fuel in the exhaust, a lean burn in the cylinder....Only the vaporized fuel burns.

Removing the emulsifiers and clearing/checking the holes may improve the operation for you.

Here's an example...see the little holes on the side?
 

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What does the bike feel like from:
0 to 1/4 throttle?
1/4 to 1/2?
1/2 to 3/4?
3/4 to full?

Answering those questions should point you in the right direction as far as jetting is concerned. But it assumes everything else (rubber boots, air filters, fuel filters, etc) are all in good working order.
 
nrcb125 said:
What does the bike feel like from:
0 to 1/4 throttle?
1/4 to 1/2?
1/2 to 3/4?
3/4 to full?

Answering those questions should point you in the right direction as far as jetting is concerned. But it assumes everything else (rubber boots, air filters, fuel filters, etc) are all in good working order.
Click to expand...

CV carbs, not slide carbs. With CVs, revs matter more than throttle position.
 
They 2 inner jets are larger because those cylinders run hotter. As for the emulsifiers they are new and clear. I did not know what they were called or what they did but I did not neglect to make sure they were clean. That's good to know though thanks. I'm also running unipods and the exhaust is a factory 4 into 2 but the baffle is home made and does have exhaust packing I also cut about 2-3 inchs off the total length. Before the engine gets to running temp it has a low rpm hesitation that does effect performance very noticeably. At running temp I'm sure it effects performance but not to a point that stops me from riding on a daily basis. I'll have to test the 1/4-full throttle thing I've never paid attention to how that affected it.
 
Deadfrednc said:
They 2 inner jets are larger because those cylinders run hotter. As for the emulsifiers they are new and clear. I did not know what they were called or what they did but I did not neglect to make sure they were clean. That's good to know though thanks. I'm also running unipods and the exhaust is a factory 4 into 2 but the baffle is home made and does have exhaust packing I also cut about 2-3 inchs off the total length. Before the engine gets to running temp it has a low rpm hesitation that does effect performance very noticeably. At running temp I'm sure it effects performance but not to a point that stops me from riding on a daily basis. I'll have to test the 1/4-full throttle thing I've never paid attention to how that affected it.
Click to expand...

Pod filters and CV carbs don't usually play nice. Listen to Sonreir about CV carbs and watching the revs rather than throttle position for tuning, he knows more than I do.
 
Pod filters, particularly the cheapo Emgo style ones, are very problematic. Read this thread (I posted it on another site):

http://www.caferacer.net/forum/technical/19639-pod-filter-thread-geeto.html

It explains why they are problematic. It is based on VM mecahnical slide carbs, but applies even more to CV carbs, because they are more dependant on air velocity and vacuum to operate. Also read the thread, on this site, discussing it. Alot of good info in the discussion:

http://www.dotheton.com/forum/index.php?topic=45863.0
 
so from what I gathered in the link you posted is that the step in the rubber boot restricts air flow to the orifice that supplies air to the vacuum diaphram that operates the air needle. Which in return means that air/fuel ratio cannot be corrected regardless what size jets are used. I've read about people having problems with pods before but it was never explained. From what I had heard before pods were not an option but your saying they are fine as long as you use the correct ones. Right? Also If thats correct I imagine I should install the correct filters then resize jets if still required?
 
Remember that the carbs in the thread are not CV carbs, like yours. It is easier to get VM carbs to work with individual filters than it is to get CV carbs to work. And, as a point of fact, the port that I had circled in the picture of CV carbs, is the atmospheric port, not the vacuum port, but semantics aside, yes, blocking that port restricts the operation of the slide. This quote from kopsicle, in the second thread (on this site), goes into detail regarding CV carbs and pods, and the tuning issues. Basically, his take is that you can make them work, but you have to chose between WOT performance and partial throttle performance. You can't have both. Personally, I never went through the trouble to try to get CV carbs to work without an airbox. All the ones I have tuned had the box in place for the reasons he outlines.

kopcicle said:
That's got CV's? Take a #2 phillips screwdriver, heat the tip with a torch until cherry red and plunge it into your eye. It will hurt less. Really. Bikes with CVs have airboxes. Bikes with slide needle carbs have pods. Even Mark Dobeck, former owner of Dynajet, cops to a loss of midrange using his kits.

And for the other eye heat a 3/4" drill bit chucked up in an air drill ...

I can get BS34's to almost work on the 998-1100 with one serious caveat . They will pull hard with the throttle arbitrarily pinned OR they will mix fuel cleanly at part throttle cruise . Pick one I don't know how to give you both . Mark Dobeck and I went back and forth for most of a summer getting several of my customers bikes and my 998 urban terror two wheeler running right . The customers are probably still ****ed even though I neither recommended nor supplied this serious impediment to performance . In the end I knew neither was right (thanx for the flash runs Ron !) But the one that was the best and closest to right got me the most grief . That guy thinks I'm an idiot and in ten years hasn't wasted an opportunity to say so . Funny that the last two guys to touch it asked how I was able to get it so close . The one that isn't so good , that guy thinks I'm magic .

just a partial list

various attempts at
springs
needles
needle jets with and without discharge shields
pilots
mains
main air bleed
air horns within the air filters
closer to or further away from the head
adjustable cam sprockets
different headers ....

All because in one case the owner wanted to look like all the other cool kids
and the other thought it was a pain to get the carbs in and out with the stock aribox

coincidently both of these mental midgets went from 35-36 mpg to 30-31mpg and might have gained a couple of hp on top at the expense of having me spend untold hours filling a huge hole around 4k .

So I was asked recently if I would jet another 998 for "pods" ( is it that hard to say individual air filters ? really?) good thing I kept my notes .

Sure I would . With an eddy current dyno and by the hour ...

now for some straight info about BS34's and a Z750 . if you want more look up BS and more BS in my earlier posts

Only after valve adjust ...
The pilot needs to show lean misfire within 1 1/4 from seat and rich misfire before 3 3/4 turns out from seat . Anything else outside this and inaccurate metering will most likely cover up all your hard work .
Then , Check the sync again ...
The stock needles are fine for all but full on racing . Why people insist on a "jet kit" is beyond me . I had many conversations over a period of several years with Mark Dobeck about individual filters on constant velocity , diaphragm carbs . Here are the high an low points of that discussion .
Great full throttle response , great part throttle cruise , pick one . You just don't get both .
The larger the volume within the individual filter the better your result .
Finding a velocity stack that will fit over the carb and inside the filter is never a bad thing until its opening is shrouded or interfered with by the filter .
Most (even dynojet's long running offering for cv34's ) needles supplied with jet kits are aluminum for ease of production and not steel for durability .
You really need to find out what the stock needle was and then look for it in the SUDCO catalog .
Find the main air bleed/jet on the inlet to the carb . This will be a fixed brass jet that unmistakably leads to the main down well that the emulsion tube , main jet , and main discharge to the throttle bore reside in . Remember this is a fixed jet and difficult (not impossible) to change . Small diameter changes in this jet can cover the usual giant hole between idle/transfer and the beginning of the midrange .
Likewise for the holes in the bottom of the main slide . remember small adjustments and accurate adjustments . A slide can be easily ruined . ( yes enlarge the hole slightly )
Paul was right I do have some experience with individual filters on CV carbs . None of it good .
but as a baseline for stock internals and 1000cc "j" engine
http://www.sudco.com/CatalogJPG/126.jpg and a call to sudco about the next richer mikuni needle up from your 4cb6 ( look it up at kawasaki I could be senile) 4cb7 Canada (has multiple circlip grooves ) The as stock Y-8 (again look it up) should be sufficient needle jet for all but "why arn't you using flat slides"
sooo
40 pilot
127.5 main
open main air bleed .025 to .035"
open the hole in the bottom of the slide a like amount
start with the needle all the way down and use .015" washers as needed to lift or get the Canadian part . (you did junk the no name needles , didn't you ? )
If at all possible find the larger volume tapered oval filters .
If it won't clean out at all it's obvious , down jet
If it just won't pull redline use this simple trick . Cover the filters with cotton gym socks . No change ? it's probably something else , starts to pull on top ? you're probably lean .
Make damn sure that the fuel/float level and the needle valve/seat are near perfect as possible .
Get rid of that abortifact of a vacuum petcock and spend a dollar on pingle .
You don't need the emissions crap either . Block off the valve cover and remove the air valve .
keep in mind that the 127.5 main I used was because of a free flowing head and a Basanni that had little if no restriction . Honestly if you find yourself much over 120 you're doing it wrong
Now go adjust your valves before you do anything

A bit dated but the process is the same even if the carb and displacement are different . PM me if you have specific questions . I'll be more than willing to run up your phone bill or run minutes of your plan if you get stuck stoned or stupid . Just don't consider taking up my time with carb questions unless the valves are adjusted correctly and the timing is spot on (applies to points mostly but there are cases where the timing/coils of a electronic ignition can be crap).

I won't even talk carbs with you until the above is done and verified it's just a waste of your time and mine.

I go back to my 70's race PrOn collection a continue to be a grumpy old man

~kop
Click to expand...
 
You can try it, but those wire mesh filters don't filter shyte. I would suggest, at a MINIMUM, get decent filters, but don't expect to get the tuning to be as good as stock.
 
read this post....
its enlightening on CV carbs, you should listen to every word

http://www.mychinamoto.com/forums/showthread.php?344-CV-carburettors-They-re-different

one thing to remember when you put a POD filter vs an airbox on a CV carb is that you are slowing down air through the bore therefore less pressure and you have to compensate by larger jets
 
Thanks for all the help I'll definitely buy the correct filters for it soon but for the moment ill try cutting out my current filters to get me buy for a month or so. I've found that I will run into the same frame clearance issues on my bike as well. Any reference on where to buy shorter filters? On K&N's site they had the correct size but they were and inch to long.
 
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