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Discussion Starter #1
just installed 32mm vms on my xs400, the bike idles and drives okay (not tuned precise or anything), spark reading shows rich. the problem is whenever i screw the throttle fwide open its power disapears for a few seconds, then it sort of catches up real fast. i was told its a running lean issue. but spark plug reading shows rich. it eases up a little whenever the bike gets warmer.

is this a fuel metering issue?
or
is it something like i need to port my intact to get better suction?
 

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You can't do a plug reading on a plug that you're just riding around on. You need to get it running at a specific throttle position at specific rpm, then cut the throttle to read the plug.

Otherwise, they mean nothing.
 

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if the thing you describe gets better as it warms up

it ain't rich because it would get worse as it warms up

'port my intact to get better suction'

no comment
 

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Have you invested in the Mikuni tuning manual? Or got a pdf somewhere for free? It's explained in this much better than I can type.

But I'm with hack, I think you must first port your intact. Without that suction you aren't going to get anywhere with this bike.

Craig
 

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You've got a lot of overlapping adjustments available to you. It's not uncommon to be lean on one circuit and rich on another. Runs pretty good where they overlap.

As Krapfever says - you can't whack the throttle open with slide carbs. CV's take care of hamfistedness automatically - they slides only open up when they're ready even though the butterflies are wide open right away. You've got to do that yourself with slide carbs.

What kind of needle jet are in those VM's?

I'd suggest starting by raising the needle a clip or two (lower the circlip down, raising the needle) and see if that helps any. I'll assume you've already adjusted the airscrew for best idle? You might have to readjust after changing the needle.

Could be you need a slide with a different cutaway.

Could be you need different needle jet (primary vs bleed style).

Could be you need different needle.

Could be your pilot jet is not quite right.

But most likely you can make it pretty decent with just needle/main changes and smooth throttle application. You shouldn't need to roll it on glacially slow, but a quick whack makes too large a change in airflow for the carb to react to. That's a bit part of why FCR's have accelerator pumps. And why modern bikes come with CV's.

Michael
 

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Discussion Starter #8
I should have (but didn't) realized the quick opening of a vm would shock the system. I was worried about a more significant issue. (too big of carbs or an intact (intake) issue) jetting and adjusting needles are not a problem and i do have a tuning manual. i'll just have to learn the speed i can rip those dudes open i guess. a slow roll the bike responds great. it runs better when warm, but... dies if i choke it.
 

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Discussion Starter #9
carb specs are:
Mikuni VM32
main jet - 210
pilot jet - 35
needle jet - Q-6
throttle valve - 3.0
jet needle - 6dp17 (it's up one slot)
air jet - 2

the carb is pulling air through 3in Uni foam pods, feeding a 198cc two valve cylinder, with fairly open, wrapped, 2-1 exhaust.
 

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Hey Corey,
I don't really want to bust on you too bad, but, you are setting yourself up here for a real beating. I will try to get to you first so Geeto doesn't get after you and HackA doesn't leave you wallowing in ignorance.
First off you have shown a history of asking ignorant questions regarding carbs, porting, exhaust, bodywork, racing, etc. Now there are many actual experts in all these areas on the board but you don't seem to want or understand their answers until you have done it your way and then need even more help. So, pay attention and listen to what they tell you.
I believe that you are over carbureted for a stock motor. 32VM Mikunis are a little too big, but since you have them you can probably make them work. If you do some motor work with higher compression pistons, cam, ported head, valve springs then the VM32s should be about right. Porting the intake will not increase suction. Suction is a nonsense term with regard to engine performance. However, it does sell vacuum cleaners. In order to get good performance you need to have all of the systems in the engine, intake, cam, headwork, pistons, and exhaust, all working together. You need to have a plan in mind of what you expect and how to get it. Is this a street bike or a racebike or both? Figure out what you want, What your parameters are, re: money, ability, resources are and we( collectively) can and will help you out. Continue to throw parts and ideas at your bike and then look for a bail out will continue to get you less then helpful answers, although, we will have some entertainment value. I and many others have been through this ourselves before and made plenty of mistakes. Feel free to e-mail me or continue to ask questions here.
You have shown a fairly tough skin so far so keep up the good work.

Ken
 

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Discussion Starter #11
i am not afraid to admit i am ignorant of lots of things, especially motorcycle engine performance, and i'm certainly not afraid of a forum bully. i work in a motorcycle shop and ask lots of questions here too. it's always been my nature to source several opinions on any question and this forum is just one source.

As far as the stall whenever it's wrenched to wot, the mikuni manual doesn't cover that.

and for clarification what is the correct term for the effect of the vacuum created by a descending piston?
 

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you certainly can if you aren't using inappropriately sized slide carbs........

have too many here that don't mind it at all


I wasn't bullying you...... just not wanting to type a diatribe about how enlarging the ports would only make matters worse

it sounds much like you are lean but you don't give enough useful info to make any other suggestions

so relax already

quote:Originally posted by krapfever

Also, you can't really thwack on vm's like you could the stock cv's. You gotta kinda just roll on the throttle.
 

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Corey,
I am glad that you are considering many sources of information. When you come on here asking a direct question, then expect a direct answer. The answer in this case is that you are over carbureted. Your engine can not fully utilize the flow capacity of VM32s at its current stage of development. One symptom of that is that if you rapidly apply full throttle by lifting the slide to its stop, the velocity of the intake charge will drop to th point that it can't draw enough fuel from the needle jet to get a rich enough mixture, so it appears that you are lean. Changin jet sizes to cover for that will mess up jetting in that throttle opening range under normal throttle applications. It is just the physics of the intake tract.
I am not interested in trying to prove how much I know and you don't. I don't need the self praise and my self esteem doesn't suffer. We all have things to learn and I, personally, have learned a great deal from others here. You still should outline a plan for what y would like your end product o be so that you are able to ask the right questions to get the info you want. And I would refer to the vacuum created by the descending piston as pressure differential since it is really relative t the pressure in the intake tract at that moment which is not a constant and determined by more things than I completely understand such as intake length valve opening and exhaust pulses. Curious stuff indeed. Note that I did not descend to name calling such as "internet bully" as that is usually the refuge of those that can't sustain a well though out argument or debate.

Ken
P.S. If this sounds like a lecture, take into accont my 15 years in the bike business and my 15 years as an educator. I am a school principal by trade.

K.E.
 

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Discussion Starter #14
i didn't mean to imply anyone who responded here was an internet bully, i was just saying that a comment here harsh or whatever is really no skin off my back. no one on this board is that bad and i rarely if ever have seen a flamed person or thread.
 

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ok, hard to say what is ailing your bike but it really could be any number of things

I'd recommend wrapping your head around the tuning manual and following the step by step procedure

however, the PDF link posted the other day I found quite lacking and I am sure I've seen better, probably even have some here
 

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Discussion Starter #16
i printed that pdf link and count that as my manual. basically i'm not worried about the jetting and needles, its just that i thought the stall at wot was something very serious. i know rich and lean will cause issues, but i never expected something that big.

i have seen a haynes fuel systems manual for sell in parts unlimited... any good?
 

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I don't know about the Haynes manual. Their car manuals suck.

The easiest way I've found to tune VMs (and this way isn't by any means perfect) is to:

1 Check your timing and valve adjustment, "balance" both carbs by removing the pods and opening each slide enough to place a popsicle stick under it. Twist the throttle gently. Both sticks should move at the exact same time. If not, adjust the cables at the top of the carbs. Next, get 2 or three sets of clean plugs and a brass plug brush.

2. Turn both idle-air screws (the smaller ones, not the larger idle speed screws) all the way in and back both out 1 1/4 turns.

3. With a clean set of plugs in there, start the bike. Let it warm up but don't hold open the throttle in any way. You're checking idle-air mixture only here. (By the way - with these carbs, you don't need a lot of choke, especially in the summer.)

4. After you've let it run at idle for a few minutes, turn off the motor and pull the plugs - fouled = open the idle air screws up another 1/2 turn, white = turn them in 1/4 turn. Repeat with fresh sets of plugs until they look chocolate-ey.

5. Now the needles. With 32's and 210 mains, you may be rich after all, but don't fuck with the jets yet. Remove the slides and place the circlip in the HIGHEST notch -- this is the "leanest" setting for the needle.

6. After you've set the circlip on both carbs, start it up and hold the throttle open at 3,000 - 4,000 rpms for a couple of minutes. The sudco manual says to "run around the test track at half throttle for a few minutes" but I live in the city and traffic gets in the damn way. What you want is the "purest" read for this throttle setting, so don't let it idle and try not to blip the throttle either. This method isn't perfect, but its worked for me.

7. Check the plugs. Fouled = I'm gonna guess pilot jets too rich, white = bump the circlips down a notch. Keep going till the plugs look good.

8. On VMs the mains are only a factor from 3/4 to full throttle. Read that again. These aren't CV carbs where the mains seem to play a greater role throughout more of the range. You'll only know if you are rich or lean on the mains after you've run WOT for a few minutes, immediately shut the bike down and checked the plugs. Barring that, here is where it pays to know some racers who can give you a ballpark on what size mains you should be running.

I've got a 350 with a 362 kit in it, mild head work, stock cam, k&n pods and a barely-baffled Hooker 2 into one (not an ideal setup I know) and I am running 160 mains to give you an idea...

There may be other factors involved and there may be a better way to tune VM's but this method should help you get closer to where you need to be.

Good Luck.
 

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In addition to the other good advice you've gotten:

Is that Q-6 needle jet a primary type (series 159) or bleed type (series 176)? See pg 129 of the Sudco catalog Vol 34 (download at www.sudco.com) - there's a picture of both in the upper left corner. It'll be obvious which type you've got once you see the picture. The primary type is typically a two stroke part, and the bleed type is typically a four stroke part. If you've got a primary type it might be worth ordering bleed types.

Also - I don't know what slide would be typical for those carbs in your application, but that is the next to leanest slide available for those carbs.

If you're lean on the transition from closed to open throttle it'll have a pretty big stumble, even if you're rich on the mains. From closed throttle it transitions from idle circuit through slide cutaway and needle to main circuit. Ham fisted operation would make the stumble <really> bad if you're on the edge of being a bit too lean through there.

Again I don't know what would be a typical air jet size feeding the needle jet in your application, but a #2 is fully at the end of the scale size wise.

Michael
 

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I just checked on the Ohio Cafe racers site that Greg who sometimes posts here has. He lists some jetting specs. His specs for VM32 on a CB350 race motor are:

Mikuni VM32g
Main Jet 200
Pilot Jet 30
Needle jet P8
Needle 6F5
Air screw 2.0
Slide 2.5

I think your jetting should be similar to that.

Ken
 
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