Topic: jetting vm30's for cb350 twin?

[johnspeck]

i've got a brand-new set of mikuni vm30's on my recent cb350 rebuild.

i ordered these set up for a basically stock bike, with the exception of k&n filters.
i didn't take them apart to check jet sizes, etc.

trying to tune the bike...it idles perfectly, throttle responds really well, revs smooth, no problems.
go to ride it, and after 5000 rpm it basically just dies.  just no power, nothing.

the main jets that came with the carbs are 200's (!).

my exhaust is pretty ratty, one muffler is a little rotted, the other has something loose and rattling around in the muffler.
so i've ordered shorty reverse megaphones.

i think it's either carbs or exhaust, as i know the timing, points, ignition, coils, throttle cabling, etc., etc., are right.

any suggestions as where to start with jet, needle, etc. settings with mikuni vm30's on a cb350?  stock compression, porting, bores/pistons.  k&n filters and (with the new mufflers) opened-up exhaust.

thanks!

[mlinder]

You need probably 140's or 150's for main jets for that. You are chokin the crap out of that bike with 200's.

[johnspeck]

thanks for the input.
any other jetting suggestions?  i know most of the notes i see on cb350's are for racing motors, with vm32's.

[crazypj]

Where did you get carbs from (Sporting for less?)
Just about all people selling them say they are correctly jetted, and they are,  for a 1974 RM125 or similar.
 You're going to need all the brass changed, its about $50~60.00 for new emulsion tubes, needles, pilot and main jets.
 The slide is re-useable but not ideal probably a 2.0 cutaway
Don't have any real recommendations for 30's but I would start with a 110 main jet as your running way rich at present.
You probably have 'two-stroke' emulsion tubes which richen things up as airflow increases.
 I've posted diagrams and pictures about it few years ago.
PJ
 

[johnspeck]

Where did you get carbs from (Sporting for less?)
Just about all people selling them say they are correctly jetted, and they are,  for a 1974 RM125 or similar.
 You're going to need all the brass changed, its about $50~60.00 for new emulsion tubes, needles, pilot and main jets.
 The slide is re-useable but not ideal probably a 2.0 cutaway
Don't have any real recommendations for 30's but I would start with a 110 main jet as your running way rich at present.
You probably have 'two-stroke' emulsion tubes which richen things up as airflow increases.
 I've posted diagrams and pictures about it few years ago.
PJ
 

i got them from speedmotoco.  they have sudco stickers on them.
i did a search for your posts you mentioned, and can't seem to find them.  do you happen to have a link, please?

(you mentioned the slide size, which do you recommend?)

the bike is idling well, so i assume that circuit is jetted ok? it will idle steadily at 400 rpm if you set it there.
i'd love some pointers on needle and tube sizes.  is this stuff i might be able to pick up at local shops, or specialized online suppliers?

thanks, i've searched and found nothing specific to these twins with vm30's.  i really appreciate your help.

[crazypj]

Here's photobucket link, will at least show you what you need looks like and maybe why you need it
http://s91.photobucket.com/albums/k315/1crazypj/Carburettors/
I'm really not impressed with Sudco's jetting, seems they are just buying in carbs then shipping them straight out again.
Maybe they only want to do smoothbores or CR?

PJ

[johnspeck]

thanks for your input.

we tried the smallest jet we had (190), and dropped the needle down as far as it would go, and it now pulls hard to 9000 rpm.  it's still rich, so today i'll be picking up some smaller jets.  i also receive my new mufflers today.

we need to get this thing to a point where i can actually ride it further than the neighborhood, and get some plug readings.

[mlinder]

190's wont help you get plug readings.

PJ is probably right, 120's or so. I was thinking of the jetting I'm doing on the 350 racebikes I'm working on.

You need to get a range of jets from 105 to probably 125's, and the correct needle, and start somewhere in the middle and work form there.
190's are rich. You already know you have to go down. Plug chops wont help you with 190's.

[crazypj]

If you work out fuel flow you need about 107cc @ 11,500 rpm for a 14:1 F/A ratio on a CB350.
 Slightly richer is good for max power and also evaporative cooling of cylinders. (12.5~13:1 )
 Mikuni jets you have are measured in cc/min flow.
 Intake, exhaust and cam system have a pretty major effect on actual jet size needed.
190 is way too huge no matter what you have, your probably running better than 8:1 at full throttle
 PJ

[mlinder]

Eh, I'm freaking myself out, math is hard without coffee. PJ, can you give us the formula you used for "you need about 107cc @ 11,500 rpm for a 14:1 F/A ratio on a CB350"

[crazypj]

Eh, I'm freaking myself out, math is hard without coffee. PJ, can you give us the formula you used for "you need about 107cc @ 11,500 rpm for a 14:1 F/A ratio on a CB350"

 I'll have to look it up, I was tired, should have said its for a 13:1 F.A. ratio not a 14:1 F/A
 Anyway, this is as much of formula as I can remember at present
 You know cylinder size,
 350/2 =175cc
 you know max rpm =11,500
 its a four stroke so you half that 11,500/2= 5750 ( 'sucks' every other stroke)
 5750 x 175 = 1006250cc/min which is about 1.0cu/M, use 1.1 for safety, slightly rich is better than slightly lean  
 air density is approx 1.275Kg/cu.Meter at sea level,
 1.1 x 1.275 = 1.4Kg air/min = 1400gm/min
 fuel is about 1000Kg/cu.M, or, about1 gram/cc.


1,400 divided by 12.5 (fuel air ratio we want) = 112gm/min
 1gm = 1cc fuel  
therefore it equals a 112 main jet, if flow is in cc/min

 The original number (107) was for CB350 (326cc)

 This gives you a starting point which can vary by +/- 20%, depending on exhaust, air filters, cam, etc

 I'm sure I missed something, that seems too easy ?
It does seem to give a usable jet size though???

PJ

 OK, I looked it up


 Its from Streetfighters magazine article a few years ago,
 Phat Bob's Calc's, Part 1- Fuel

 The formula is this:-   Q (in cc/min)=C x A x [(2 x RHO x (P1-P2)]^1/2/RHO

 Volume of flow of incompressible liquid (fuel)
through jet of area A                                            = Q
 discharge coefficient,                                           = C (variable, probably 0.6~0.65)
 pressure drop across jet                                        = P1-P2
 density of fuel (approx 1000kg/cu Meter)                  = RHO
 Main jet size                                                        =A


 I think I only use part of this formula ?

[mlinder]

Sweet, thanks so much, PJ!

I knew I was doing something wrong.

[johnspeck]

well, i can say with no doubt, 115 is TOO SMALL.  waaaaaaaay lean.  ran like garbage.

went to a 115 main, needle in the middle.  barely made it to point B on the test ride.  running like it was missing.

stopped, put the 150's back in and dropped the needles again (clip at the top slot), and all of a sudden i have a rideable bike once more.

*maybe* 140's?  all i can say is, 115's definitely aren't working.

again, k&n filters (NOT STOCK), short free flowing mufflers (NOT STOCK), mikuni vm30's (NOT STOCK), so...?

[crazypj]

HMMMM, that's weird.
 I had Kei-Hin 115 main jets (1.15mm hole size) and they were way too rich.
Checking small round Mikuni 110, holes were pretty much same size.
 Don't know if the incorrect (twostroke) emulsion tube makes it act that way?
 All the jetting recommendations  I've seen with the primary choke tube seems incredibly huge to me. (180~190 in XS650)
 I made some emulsion tubes to fit 32mm Mikuni's and they only needed 125 main jets in my XS800, and that had open pipes on it ( video of first time startup, its done 450~500 miles since) http://s91.photobucket.com/albums/k315/1crazypj/XS650/?action=view&current=XS800.flv

PJ

[lordmoonpie]

I ran VM32s on my cb350 with no problems - have you done a plug chop yet to check the mix?

[johnspeck]

update:

140 mains, needle clip in the highest position, plugs read lean.  getting a little closer.

dropping the needle one position lower, the plugs are black and sooty.
so now i need to dial in the needle jet/jet needle (and tube?)

i also noticed a leak at both of the joints between the mufflers and header pipes, in the form of a little sooty residue.  the universal mufflers came with reducer collars to fit different pipe diameters.  mine are obviously not sealing 100%, i'm thinking i'll need to wrap them at that spot, to seal the leak.
i think this is why it doesn't like to cruise smoothly at minimal throttle.  it's great under acceleration, but "burbles" like the carbs are out of sync (though they aren't) when you're just holding rpm's and throttle.

[crazypj]

A single clip position on needle shouldn't have that much effect so either needle or jet is still off (or both)
I'm guessing there is some difference in emulsion tubes and also jet numbers.
They don't seem trustworthy any more, numbers are all over the place
Personally I still think its due to 'two stroke' primary choke 'tubes'.
This has been ongoing for at least 10 yrs, some people claim to have managed to get them to work properly. (although I've never seen dyno results with F/A printout)
 Personally, I'm going to fit some Yamaha XS650, BS38 vacuum carbs emulsion tubes and modify them for VM.
 Not an ideal solution, but, they are easily available from Mikes XS.
 I would make some but my mini machine is broken so I cant do cross holes.

PJ

[dustinlwood]

crazypj, can you email me at dustinlwood@pldi.net?
I would like to carb talk with you one-on-one. if you don't mind.

Thanks.

[Trevor from Warragul]

Hi,

I have VM28's on my Morini 3 1/2 Sport (350cc).  It runs perfectly as follows:

(1) Needle jet P-6 (series 175)

(2) Jet needle 5L1

(3) Main jet 120 (series 4/042)

(4) Pilot jet 45 (VM22/210)

(5) Throttle Valve 2.5 (VM28/56)

(6)  K&N RC1072 filters

Trevor

[crazypj]

Hi,

I have VM28's on my Morini 3 1/2 Sport (350cc).  It runs perfectly as follows:

(1) Needle jet P-6 (series 175)

(2) Jet needle 5L1

(3) Main jet 120 (series 4/042)

(4) Pilot jet 45 (VM22/210)

(5) Throttle Valve 2.5 (VM28/56)

(6)  K&N RC1072 filters

Trevor

 Thanks Trevor, they are some really useful numbers.
 Is this 'stock' parts for Morini? (except for main jet)
 Just about all the information online is trying to get them (VM carbs) working with 159 series primary choke tubes. (on various bikes, not just the CB 350)
 The VM 30 uses type 176 needle jet/emulsion tube for four stoke applications and a 6 series needle (leans out the excessive richness mid range)
 The 175 series is about 3mm shorter.
 The numbers after the series will match up though.
 4/042 are  'long' hex head jets and significantly smaller hole diameter than 'round head' Mikuni jets (N100/604, N102/221)
 I don't have any short hex jets for comparison
I'm guessing that only the hex head ones are flow rated? (I know AMAL are rated cc/min, not sure how that compares to Gram/min?)


PJ

[Trevor from Warragul]

Hi PJ,

No, they're not "stock" parts.  The 350 & 500 Morini's run Dellorto's.  The 350's run 25mm square slides, which are crap.  The 500's run 30mm round slides, which are better.  Some Morini 350 owners "convert" to the 30mm round slides, and the required jetting changes are readily available on the net.  I think Mikuni's are far superior, so I got a set of those.  I then applied some basic logic & did a direct comparison of the Dellorto & Mikuni jets, needles, etc.  For example, I looked up the details for the recommended Dellorto jet needle, such as length, taper, etc, then I found the closest Mikuni match (I got it pretty close).  I looked up the details for the needle jet, and did the same.  When all was said & done I got a very good result.  The only thing I ended up changing was the main jet (I went down one size).  My Morini runs almost perfectly with the Mikuni's.  The only thing it is reluctant to do is hold a low idle, but this is down to the Morini design, not the carbies (Heron heads, light flywheel, funny cylinder angle, small exhaust valve, etc).  Otherwise it runs great.  Absolutely no fluffiness anywhere, instant pick-up when whacking the throttle open, smooth running, etc.

Keep persevering!  I reckon the issues you are having are down to the needle jet (the 159 series are too hard to set up for a 4 stroke), jet needle (for a 350 you need a really thin needle with a very gradual taper) and main jets (modern fuel runs rich, you need to get it as lean as possible).

Good luck

Trevor

[mlinder]

This is what ohio cafe racer suggests for a built, racing cb350. The 200 mains sound a little large to me... but, what do I know, I'm just a caveman.

http://www.ohiocaferacers.com/MikuniJetting.html

[crazypj]

So far, every time I've seen huge jets used its because the 'wrong' needle jet/'emulsion tube' is in carb. (primary choke type)
 Even though Mikuni say max fuel flow is same I guess it really isnt  because there isnt enough vacuum to draw full fuel flow?
I guess  you can make it work, hell you could make bean can with holes  in work.

PJ

[mlinder]

hell you could make bean can with holes  in work.

PJ

I am so doing this on my cb350 racebike!

[johnspeck]

installed #176 P-6 needle jets.  have 6L1 needles, .5 and 1.0 air jets on order.  tried 45 pilots, seemed too rich.
the upside is i'm learning a lot about these carbs through trial and mostly error.

thanks for the input and help pj, mlinder, morini...