Please help

ducatijohn

XS650 Lifer
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Bought a set of Mikuni VM's. (32) Does anyone have a baseline setting to start out with? I'm down to 125 main jets and still running very rich. Left Cylinder was flooding badly, bent choke lever. I switched the needles from left to right, now the right side is wet. Suspect a bad needle, but want to order the correct ones. Stock engine with Pamco ignition and a free flowing 2 into 1 MAC exhaust. BTW, the Alpha Moto sucks. But thats another story...
 
You didn't mention checking the needle jets, so I assume that you disregarded that advice. Here's another item for you to ignore: inspect float level and set it to max specified height of 24 mm. Measure float level at the float arms at the points where they contact the float pins. Move the arms to level them with each other if needed, the set level at the tang.
 
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First off, let me say that any and all help is appreciated. Needles have been changed to 4's. I tend to agree with the float level advice, and heres why. When I shut the fuel supply off, as the bowls run low on fuel, the engine flattens out an rpm increases. In other words, as the mixture leans out, it runs properly until the fuel is used up in the bowls. I was under the impression that float height should be set such that they are parallel to the bowl mounting surface. Thanks for the actual dimension, I'll set them now.


Ignorance is curable, stupid is forever.
 
Ahhh, philosophy and carburators. Two things that go so well together. Not exactly the advice I was looking for, but point taken. You're right about Darwin's theory.

Float levels set to 24 mm. any other tricks? As it sits now, p4 needles, 175 main jets, 25 pilots, needles set to top ring. Black smoke from tailpipe. Timing checked, cam chain tensioned, valve clearance set with new adjusters. You guys should know that I've had this bike since 1980, we know each other very well.
 
The P-4s are the needle jets. What about the needles themselves? Most run a 6F9.
 
It isn't a trick, it's Mikuni book procedure. Since your trouble occurred with float level reduced to zero, so that you were way out of range, disregard going to the lean extreme of 24 mm. and go to the middle of the range, which is 23 mm. for your carbs. That's 23 mm. as measured from the gasket seating surface on the carb body to the float pin contact point on the float arms. No more tuning can be done until you get fuel level under control. You may need smaller PJ's, but that remains to be seen. You will definitely need larger mains. You'd be very well advised to order a copy of the Sudco Mikuni Tuning Manual. Also read this: www.amckayltd.com/VM34-36.pdf . Baselines given there will be a tad rich for the 32's, but tuning and setup principles are the same. BTW, it's great to hear that you "agree" with Mikuni on float settings.
 
Well, the particular display of tail feathers that you're referencing isn't so much philosophy as it is heavily diluted, de-tuned, adulterated, half-truth, front stoop, hand-me-down cognitive theory. The undeniable expertise there is motorcycles. Hence, it's usually best when people stick to espousing disciplines in which they are qualified, as there is always the risk of innocently tottering into the back yard of a career behavioral scientist and being caught midstream in an ill-informed piss. That is what has happened in this instance. Ignorance is learned - quite correct - but it can be unlearned as well. Each of us here is ignorant of something bike-related, which is why we are all here. I brought my motorcycle ignorance here with me in 2007, and still tote a lot of it about, not so proudly, but I'm trying to discard it bit by bit over time. For my part, I learn much more readily when I am not being branded as evolutionary refuse.

You have posted about 6 times as I write this, I see. Keep asking questions. But one thing I'd suggest you do is either try every fix that is suggested, or cover your rationale as to why you have not. My experience here has taught me that what frustrates prospective helpers more than just about anything is having to try to "plug holes" in what remedies someone has tried, and what they have not. In social media, where we are all captive to the written word and its inherent failings, this is mainly a function of communication proficiency and timing, but if someone mentions something such as float levels, needles or needle jets, try to cover that in your reply so the helper knows where you are. Don't ask me how I know this. :)

And welcome here!

TC
 
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6f9 needles. P-4 needle jets, 23mm float height. Do I need different needles? Hope I'm not ruffling too many tail feathers, just frustrated as I'm sure some of you have been at some point or another.
 
Experience (mine and many others') has shown the single taper 6F9 to be a fine choice for street tuning in the VM34 and VM36. Whether it's a good choice for the VM32 remains to be seen. What's in your carbs at present? Given that Speedmotoco has gone to the expense of installing appropriate needle jets, my guess would be that they may have installed an appropriate needle as well. In your carbs, the off-the-shelf needle installed by Mikuni (which usually needs to be changed) is the double taper 6DP17.

In any case, I'd advise you to start by getting fuel level under control and tuning for pilot and main jet selection with the needles you have.

Scratch the earlier advice re. mains; somehow I read "125" where you wrote "175." 175 may be a tad rich for the 32's; smaller venturi usually requires a bit leaner jetting.
 
Oops, I cross-posted with griz... sorry!

John, are the 6f9s and the P-4s what you have, or what you want to order? I have been in your shoes with my single carb conversion, and I can tell you from experience that I would take what griz and twins say to the bank. That carb guide and the Sudco Tuning Manual will at least get you to where your bike is not spitting up out the bells or otherwise running impossibly rich. Then you can tune.

One thing that griz told me I'll never forget is that you don't want to fall to the temptation of trying to tune one circuit (say, pilot) to the exclusion of others. I stepped in that poo initially, before griz kindly hosed it off my boot. :) It's like trying to lose belly fat only. Can't be done. Once you get your fuel level under control, then you can see how your bike behaves across the circuits and make more targeted changes in jetting.

Disclaimer: I know only enough to try to help, but possibly confuse... hope not. Just wanting to impart what worked for me.

TC
 
Ducatijohn, I got your first and subsequent posts crossed up. If speedmotoco put 6F9's and P4's in there, go with it; they didn't go to the expense of putting in that extra brass arbitrarily. The 175 MJ's would be where I'd start; make changes from that one step at a time. Ditto the pilots; you will definitely not need larger ones, but you may need 22.5's or even 20's. Make that determination based on testing, one step at a time.

As TC says, every change you make has secondary effects both upstream and downstream. A change in PJ's may force a change in needle position, and conversely; a change in MJ's may force a change in needle position, which may force a change in pilots. Check mixture screw settings after making a change anywhere. Work in cycles of change and compensation, making only one change of a single step at a time; you run in circles, but they tighten.
 
Still running in big circles at the moment. Think I'm gonna order some 22.5 and 20 pj's next. Nothing I seem to do regarding the mains and needles/ float height seems to have any positive effect. Stay tuned ( no pun intended )

Thanks guys
 
When you're setting up new carbs, Step One is a thorough cleaning; there's often manufacturing debris left in carburetors. While you're ordering parts, order a spare float bowl plug, drill and tap it for a fuel barb, and attach a short piece of fuel line and a piece of rigid plastic tubing for a sight tube. Clamp a carb on the bench, level it carefully on both axes, attach a remote fuel source, and observe the fuel level. It should rise no higher than the bottom of the carb body flange over the float bowl.
 
okay, so heres the update. Carbs cleaned (again), 17.5 pj's, 175 mj's. p-4 jet needles, 6f9 needles set on the uppermost undercut, 23 mm float height, (checked fuel level by drilling and tapping a drain cap method) cables adjusted as best they can be with the way its running right now. Will not idle unless the idle scrwes are turned all the way in. Still blowing black smoke, plugs fouling, no vacuum leaks (new intakes). I tried 22.5 and 20 pjs with the same results. I think these carbs are going on e-bay. next question, 34 or 36 mm? Can I use the stock mounts with the 36's?
 
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