Topic: jetting vm30's for cb350 twin?

[crazypj]

The principle of enlarging the cross drilling holes works as well for Mikuni as it does for Kei-Hin.
Unfortunately,  I still don't have my mini-mill up and running or I would make you some (need it for precision cross drilling)

PJ

[johnspeck]

Yea man, these needle jets aren't cheap. 15 bucks a pop from sudco.

What do you think is a safe needle jet to try going leaner from p-5 without having to buy 5 of them?

I wish there was a way to tell the approx effect from going down one step.

couldn't tell you, i have no experience with anything but a P-5 in the #159's.

[johnspeck]

The principle of enlarging the cross drilling holes works as well for Mikuni as it does for Kei-Hin.
Unfortunately,  I still don't have my mini-mill up and running or I would make you some (need it for precision cross drilling)

PJ

pj, i saw from another of your posts, recommending enlarging certain cross holes and jetting up the mains.  does this give the same results as enlarging the ID of the needle jet (going up in #)?

[Saml01]

couldn't tell you, i have no experience with anything but a P-5 in the #159's.

Youre guess would be better then mine. What do you suggest? P4? P2? P0?

Why cant someone just tell me how much leanness one step down a jet would introduce, I know on my car(standalone EFI), changing the fuel figure at any given point by 5 will produce a .3 change in AFR.

BTW. When you run your bike up to WOT on clean plugs, your 150 main jets produce an adequate mix?

[johnspeck]

couldn't tell you, i have no experience with anything but a P-5 in the #159's.

BTW. When you run your bike up to WOT on clean plugs, your 150 main jets produce an adequate mix?

yeah, seems to be the best so far.  have tried 140's, 160's...  150's seem to be about right and don't get any better with choke added.  140's liked a little choke when i really got on them...they would stutter a bit, i'd flip the choke on the left carb, it'd go a little harder.  160's just felt 'boggy'.
understand, i have to run my needles in the leanest position, or they give it too much gas.  i messed with trying them one position richer, and it was too much.
once i get the needle jets dialed in, i'd like to experiment with a needle that gives me a little more tweakability.  the other needles i have are just too rich, even at leanest settings (6L1's).

[Saml01]

I ordered a set of 150 mains and 159 P0 needle jets as per the recommendation of the guy who owns the shop.

Ill let you know how it turns out.

[johnspeck]

i would think you'd want a few different sizes, starting with one size smaller than your current needle jets.  that jump down to a P-0 might be too drastic.  then you have to wait again for another order.

[Saml01]

i would think you'd want a few different sizes, starting with one size smaller than your current needle jets.  that jump down to a P-0 might be too drastic.  then you have to wait again for another order.

Too expensive. 28 dollars for a pair, if I get 3 or 4 pairs its ridiculous.

I spoke to the guy at the shop, he said going to a P0 is a safe jump. From there I can decide if I need to go up or down more.

Remember, I cant return any of these after I buy em. So id rather buy one now, see where it leaves me, then by more to fine tune instead of buying everything from P5 to P0 and then concluding I need even leaner. At least this way I can nail down the extremes.

[johnspeck]

Too expensive. 28 dollars for a pair, if I get 3 or 4 pairs its ridiculous.

I spoke to the guy at the shop, he said going to a P0 is a safe jump. From there I can decide if I need to go up or down more.

Remember, I cant return any of these after I buy em. So id rather buy one now, see where it leaves me, then by more to fine tune instead of buying everything from P5 to P0 and then concluding I need even leaner. At least this way I can nail down the extremes.

i understand, but it's kind of a damned-if-you-do, damned-if-you-don't thing.  you end up spending the money one way or another.

i've received a LOT of advice from shop guys, online experts, etc.  i have main jets i can't possibly use in these carbs (too small), needles, pilots, air correction jets, needle jets...

unless the guy at the shop giving you advice has used #159's in vm30's on a cb350 twin, with firsthand knowledge of the lean-to-rich spectrum from a P-0 to a P-5, it's conjecture, and you just spent the money anyway.
i have a set of bleed jets that were one size bigger, but too lean.  couldn't make them work.  i either need to modify them, or just buy new ones.
so leaning yours out 4 steps might give you the same results as my bleeds, and you're still out the $$$.

if you actually go back and read through this thread from start to finish, noting dates, you'll see the long painful trial and error lesson.
i spent three weeks alone trying to dial in those stupid bleeds...new pilots, needles, air correction jets, mains, etc.  finally i realized the needle jets themselves weren't working, seeing as i tried different sizes of everything else, methodically and maddeningly.

[Saml01]

I looked at your progress, and its astounding. You sir have beyond patience.

However, out of everything you have tried, you have not tried leaner needle jets in the 159 same series.  So its only reasonable to try that, along with other technical reasons.

I also agree with you in terms of "damned-if-you-do, damned-if-you-don't thing.  you end up spending the money one way or another". However, there has to be some sort of logical approach to spending the money in order to lose as little as possible.

I totally agree with "firsthand knowledge of the lean-to-rich spectrum from a P-0 to a P-5, it's conjecture", in fact the guy said the same thing to me "You wont know till you try and unless you find someone with experience in your entire identical setup that has tried it first hand, you are on your own". So we talked about it and he says that if it was him, he wouldnt buy a series, but just make a safe jump down(but whos to say whats safe) and decide if you need more or less from there. I was thinking of going to O-5, he said that might be too drastic, I agreed.

Blindly buying 5 leaner jets could leave me right where I started, an overly rich middle that still needs leaner jets.

Look on the bright side John, now if this works, it could help you too.

[crazypj]

Cross drilling allows more air to mix with fuel so it leans out mid range.
 The lower down on tube, the higher rpm/ airflow will be needed to lean out.
 In conjunction with needle taper you can get just about any mixture at any rpm (but the amount of work is unbelievable so compromise is good  )
 Its a cheaper way to go, after all, if you go too far you can still jet for 3/4 throttle but it will be rich at top end, or, you can fit different air bleeds to intake side to make it richer with 'correct' main jet (maybe  )
 Start by drilling top set of holes only with #68 drill (1/32") #60 drill is 1mm and that's the biggest you ever go.
 If things improve, drill just one set of holes below and test again. You already have air bleeds removed so there is no restriction to emulsion tubes
 Stock cross holes seem to be in the #72~#78 range
 You can also drill lower holes only if the problem is above a certain rpm, the very lowest holes (closest to main jet) will be for max rpm/airflow.

PJ

[pampadori]

i'm just going to sit back and wait for you guys to nail the specs and then get a matching like set of needles and what not.   

[crazypj]

Just posted this response on Caferacer.net so ignore first line, its easier to copy and paste so I dont have to type everything twice and anyone who doesnt use Caferacer can find info here or honda twins 

OK, its pressure difference and how restricted the intake becomes. (couple of things you may or may not have heard of, venturi principle and Bernoulli principle

http://en.wikipedia.org/wiki/Venturi_principle

http://en.wikipedia.org/wiki/Bernoulli%27s_principle

 With a smaller orifice the airflow is faster which creates a higher vacuum over discharge port so 'sucking' more fuel, or sucking more efficiently so jet becomes restriction to fuel flow.
 The emulsion tube automatically introduces more air as fuel level drops so you get good mid range atomisation and better acceleration.
 Using a 'primary' type needle jet means the 'automatic' leaning out doesn't happen and you get rich mid range (JUST WHAT EVERYONE IS HAVING TROUBLE WITH)
 You can make a primary type work, but it never works properly.
 For racing it isn't so much of an issue but for street use, it will and does cause problems

PJ

[Saml01]

The 159 P-0 was definitely the step in the right direction. With the needle in the second from the bottom position, the cylinder that works* gets the correct spark color, responds well to throttle input and I can actually accelerate past 3k rpm now.

Turns out one of my coil packs dies when the bike warms up, lol.

[Saml01]

In case anyone ever asks. This is what you need to run in Mikuni VM32's to get a running bike.

27.5 Pilot, Quarter turn out
159 P0 Needle Jet
150 Main Jet
6DH4 Needle
3.0 slide - im curious to try a 2.5 and see if it makes the throttle response crisper.
2.0 Air Jet

Sorry for the #$%*ty shaky video, I made it to demonstrate engine operation.

[johnspeck]

32's or 30's?

this is a thread on 30's in 350 twins.  there's lots of info on jetting for 32's elsewhere, i was hoping to keep this on topic.

i'll be picking up my needle jets from the machinist this morning.  if i get any time to tweak, i'll post results asap.  i'm hoping for some dyno runs this coming weekend.

[Saml01]

32's or 30's?

this is a thread on 30's in 350 twins.  there's lots of info on jetting for 32's elsewhere, i was hoping to keep this on topic.

i'll be picking up my needle jets from the machinist this morning.  if i get any time to tweak, i'll post results asap.  i'm hoping for some dyno runs this coming weekend.

32's.

I cant find any of this "lots of info" you speak of and I have searched a lot, no one knows #$%* about this bike and these carbs. The universal response is "that kit is junk, those carbs are too big, and it will never work right".

In your case the jets probably wouldnt make much difference in a 30 vs 32.

Anyway, im done. The bike runs like a top. If anyone, in the future, wants to get their CB360/350 running with new carbs, my final combination is the ideal place to start.

[johnspeck]

when you say "runs like a top", what are your riding characteristics?  have you taken the bike on any long distance high speed rides?  have you done plug chops at various points in the throttle to see if it's rich or lean at a given point?
your video doesn't show much more than a blast down the block.  not trying to say anything negative, i'm all for good info.  but it seems like a pretty lean needle jet, when mine is running (rich, but running *hard*) with a P-5 of the same type.
i regularly ride 80+ mph, for around 10 miles, twice a day.  i'd be curious if you're lean on your top end.  but you do have a slightly richer needle, if i remember correctly.

[Saml01]

when you say "runs like a top", what are your riding characteristics?  have you taken the bike on any long distance high speed rides?  have you done plug chops at various points in the throttle to see if it's rich or lean at a given point?
your video doesn't show much more than a blast down the block.  not trying to say anything negative, i'm all for good info.  but it seems like a pretty lean needle jet, when mine is running (rich, but running *hard*) with a P-5 of the same type.
i regularly ride 80+ mph, for around 10 miles, twice a day.  i'd be curious if you're lean on your top end.  but you do have a slightly richer needle, if i remember correctly.

Havent gone on long distance rides yet. Speed is not a factor really, its load and throttle position that's important.

Plugs come back with the correct color. But I have a lot of experience tuning cars and I can tell when and engine isnt running right and where.

"pretty lean needle jet" means nothing. What are you basing that on?  Starting with a Q8 and installing a P5 can also be construed as a pretty lean needle jet. One thing I have learned from this exercise, is that until you try a jet you cant know how it will perform and trying to guess is useless. However, when you install the leanest needle available and you are still running rich, that should be an indicator that you need a leaner needle jet and removing airjets is not a solution.

"running rich and running hard". Have you rode your bike with a perfect tune? What you think is hard might not be. If you run too rich you kill power, run to lean and kill power as well.

As for the top end, we are running the same main jet, the difference in engine size is negligible. However, compare the orifice created with my needle and jet vs yours at various positions and that can give you an idea of whos running how.

I tried an even leaner 6DP17 needle and the bike wouldnt run in the mid range at all, 6DH4 is one down from my H7 and its perfect.

[johnspeck]

well, i've built a '68 bonneville, '79 kz650, '74 rd350, all pretty close to 'perfectly tuned' (with my bike mechanic pals back home mentoring me along the way), so my seat-of-the-pants can kind of tell me when something's right or not.  having rode my bike in both rich and lean carb conditions, i can feel the difference.  i know my bike's off because when i'm at WOT under load/against wind on the freeway, i hit a wall around 8500 when it should want to rev to 10k.

but i ordered some smaller 159-type needle jets, as my initial install of the enlarged 176's is still not right, although i didn't have a lot of time to mess with variables.  so i'll likely go your route, with smaller 159's.  knowing the way my bike is running with P-5's, i doubt i'll end up with a P-0, but you never know!

i would, however, disagree a bit with your assessment that it's ALL about load and throttle position.  i think you have to run through the entire range of throttle with these carbs (or put the bike on a dyno) to really know if you've got them dialed in.

but at the end of the day, it's all about what you want from the bike.  i want a bike that i can ride around town AND ride long distances at prolonged high speeds.  whether it's out to the hill country or dallas via freeways, or on the track.
to that end, i've had my engine apart for a complete top-end rebuild, and i'm in the process of replacing/refurbing every component on my bike.  engine (overbore/new valvetrain/headwork/exhaust this winter), ignition/charging, suspension, brakes/wheels, etc.

best of luck and good riding!

[CRXDrew]

27.5 Pilot, Quarter turn out
159 P0 Needle Jet
150 Main Jet
6DH4 Needle
3.0 slide - im curious to try a 2.5 and see if it makes the throttle response crisper.
2.0 Air Jet

Now are your main jets the Long/large hex variety (4/042) or the large round (N100.604) I figure you are using the large round because the orifice size on the hex jets are tiny in comparison. This link elaborates:

http://www.ozebook.com/compendium/t500_files/mikuni.pdf

I'm actually running:
230 hex mains
37.5 pilots 1.5 out (which are wrong i know haha)
3.0 slide
2.0 air jet
159 Q6 needle jet
6DP17 needles.

I think I'm lean still (although the choke levers instantly kill the bike probably due to huge pilots). According to my link those 230 hexs are equivalent to 140 rounds. I need to mess with it a bit now that I have my wiring all but figured out... been chasing ghosts there so I haven't had time to focus on the fuel since the bike runs... just a little weak (probably from its lean nature). I'll be back to report my findings... I would love to come up with a definitive answer. This thread has seen enough hits that google points randoms like me here, it would be great to link a complete writeup to this thread for future info seekers. 

[johnspeck]

we're both using large hex main jets.

i don't think there is a definitive answer.  depends on your bike and any mods from stock...exhaust, filters, etc.

this is 'ballpark' stuff you can try and see if it works for you.

my jetting produces a rideable bike in the city and on the highway, but it's still not 'right'.  i'm waiting on some other jets to mess with (smaller needle jets of the 159-type).  they should be here by the end of the week.  possibly doing some dyno runs next weekend if the jets come in.

saml01 states his jetting is 'right', but he hasn't taken the bike on any long runs (or high speeds), so...grain of salt.

[crazypj]

saml01 states his jetting is 'right', but he hasn't taken the bike on any long runs (or high speeds), so...grain of salt.

 And if you trawl through the dross at DTT you'll find its 'broken' (running on one cyl) may be mechanical or electrical or something caused by carbs.
 Not sure where I posted it but I have been running bike on dyno with stock carbs as many people either can't afford or don't want to mess with Mikuni. I've been posting print out of F/A mixtures (although nothing new since last November/December) When I get Kei-Hin right I'll start again with Mikuni's

PJ

[johnspeck]

 And if you trawl through the dross at DTT you'll find its 'broken' (running on one cyl) may be mechanical or electrical or something caused by carbs.
 Not sure where I posted it but I have been running bike on dyno with stock carbs as many people either can't afford or don't want to mess with Mikuni. I've been posting print out of F/A mixtures (although nothing new since last November/December) When I get Kei-Hin right I'll start again with Mikuni's

PJ

yeah, i sometimes think about just looking for a really nice pair of stockers just in case, but i almost feel like that would be 'giving up'...if i work it through, i'll have accomplished something and learned so much in the process.  although, if i wanted to race this bike, some classes require them.

i am looking casually for another 350 engine, so i can slowly do a rebuild over the winter (overbore, some head work, etc...enough for an honest mid 30's HP), so i'm thinking i'd use these mikunis on it.  being able to make them work on a bike in stock tune, i'd have a good starting point to work back up from (although it almost makes me sick to think about repeating the process, just backwards!)  

[crazypj]

Know what you mean about the giving up, beginning to wish I'd never started with the 32mm Mikuni's on 360, they worked OK on the XS but now I have some 34's for that................
 I can take bike in in next few weeks if I can get it sorted out and runnable again.
 Greg (guy in work) has got his CB450 racer to Orlando from Michigan.
He wants to keep me motivated so he'll have someone to beat. 
Its all fun 

PJ