vision stumble

[bmeyer6472]

I don't know if anyone on this board has done this - please excuse me if so, I'm new.
My 82 had been plagued with the vision stumble for years, in spite of being fitted with yamaha's retrofit. After I discovered this site, I did the things that are recommended to cure the stumble, and was rewarded with a much smoother running bike, but it still had a slight bog when cracking the throttle in the midrange. I decided to work on the accelerator pump to see if that would finally cure it. What I did was to remove the cotter pin at the end of the pump rod, that holds the spring on the rod, and install several washers over the rod, between the spring and the cotter pin, in effect strengthening the spring, which gives a bit more pump shot. I did this after observing that the pump lever was barely moving when twisting the throttle, even with a pretty big twist. I am happy to say that this totally fixed the problem on my bike. I guess it's possible that my spring was weak in the first place, and that others might not have the same problem, but it sure worked for me. I ended up with about 3/8" of spacer washers all together - after a few trials. You need to find the right amount; too much is almost as bad as not enough. Hope this may help someone with a similar problem.
Bob  

[Lucky]

You'll be interested to know that there was a Yamaha TSB (tech service bullitin) that was SUPPOSED to have been done along with the retrofit.  The adjustment involved turning the outer collar on the threaded part of the rod to lengthen the rod so that the total rod length was 60 mm max.

in a few of the carbs i've rebuilt the rod was considerably shorter, and I adjust them as needed as part of the rebuild, so it may be that this was never done on yours, or as you say, that the spring may just have weekened over time.

Either way, thanks for sharing your tip!

--Lucky

[Dave T.]

We have "sort of" been down this road before. I remember a person here that put a spring from a "clicker" ball point pen in place of the stock spring. I tried it and it did not work for me. I have just removed the limiter screw from the plunger and then fiddled with the stroke length until little or no bog took place.

Lately though, I have found that spraying carb cleaner down the main air jets helps tremendously. But, they seem to plug up fairly easily after a few hundred miles. All you have to to though is remove the air filter and shoot a few short bursts down the MAJ's. I have also changed my front MAJ to a 132.5. The MAJ goes from the top of the carb to the 4 small holes right below the throttle plate, which is where the V has the most problems working with the vacuum operated flapper.

[Lucky]

DT, you seem to have a rare "late" 82 carb. I've never seen one with a limit screw on the accel pump. ?Jason has one on his and those are the only two i've even heard about. ?Anyone else have one?

[Dave T.]

I was just looking at my manuals and seen that the Main Air Jet that I was talking about is actually the Pilot Air Jet. I figured clear up my mistake. Picture 11.11a page 103 in the Haynes manual proves it. According to my factory manual, the one "replaceable" jet atop the carb is the pilot air jet. Since the circuit goes to the single orifice "pilot outlet" after passing by the "by-pass hole" (4 small holes by the throttle plate) and the pilot screw. Looks like the MAJ is the "fixed" one next to it. Also the Haynes manual says the MAJ is "1.8" whatever that means. Also it says the PAJ should be a 130. Mine came with 120F and 135R PAJ. I changed the front to a 132.5. Maybe a 130F 130R combo would work best?

I also see a "power nozzle" in my section view. Looks like it's teed off the pilot fuel system. It's the small brass pipe that comes up the upper body of the carb. Also looks like raw fuel comes up and back down the carb throat. I don't quite know how that system works. Could cause stumble. ????

This doesn't make sense. The pilot air screw will take care of what the PAJ does, hence it should be fixed, not the MAJ. It looks like the idle circuit air is adjusted twice, once by the pilot jet and again by the PAS.

The bypass holes are where the "stumble" comes in. Looks like these holes control the midrange throttle response. If these holes get plugged, you get a major stumble. I see the pilot jet controls the amount of fuel that gets to the pilot outlet and the by-pass holes.

I'm glad i did this, now I find that my Yamaha manual on page 4.1 says that the PAJ is the Main Jet. More confusing bullshit. ?:P I wish I wasn't so slow in my old age.

As far as the limiter screw Lucky mentioned, it isn't included on the schematic in my factory '82 manual. But I do see it on figure 4.1 in the Haynes manual (page 101). I should see if I have 1983 carbs. But my '82 doesn't have the "accelerator control valve" like figure 4.1 has.

[rick_nowak]

the best motorcycle mechanic i know, Al at Sturgis Yamaha, who works on my visions did several iterations of drillings in the bypass (air bleed tower) of 83carbs on my 82.  it got so lean on the next to last pass that we went back.  it is his contention that the midrrange on these carbs is set too rich by factory and a slight opening of the air bleed holes will (does) cure the stumble.  also very very sensitive to float height setting.  rember,these are basicly car carburetors, use car type circuits, and are modified by mikuni for motorcycle use.  i don't know (Lucky probably does) is this sort of carb was ever tried again on any bike.  one of those visionary things by yamaha.

[Ken Williams]

I have an alternative to rick nowak?s theory of Vision stumble.  A tuner I know, Doug Lofgrin, www.visi.com/~moperfserv/  told me the Ducati Paso and Vision both share similar carburetor designs and the off idle lean condition that causes the Paso/Vision stumble.  The fix is to richen the mixture by covering one or more of the upper holes in the emulsion tube (air bleed tower).  I noticed a definite improvement years ago when I soldered shut one of the upper holes in my 83 Vision.

The small holes in the carburetor bore and the pilot screw are especially prone to plugging by varnish and byproducts of combustion.  If periodic application of carb cleaner to the fuel and DT?s spray down the idle air jet cleaning methods don?t work, with the carbs off the bike, you can physically clear the holes and pilot screw passage with the bristle of a paintbrush or a broom straw.

[Lucky]

break off that straw or bristle in there and have fun getting it out.

the proper way to rebuild them is to dip them & set them to factory specs and for 82's make both Pilot air jets 135.

I've rebuilt a couple dozen of these now and i've yet to hear any complaints on how the bike starts & runs

I can lift the front tire now in the first 3 gears, np

--Lucky

[Dave T.]

I'd pay to see you get front wheel up in 3rd, let alone 2nd. I don't mean just a couple inches, I mean balance point.   :o

I could try a little harder, go about 30, redine it in second, clutch it and preload the front end. 1st isn't a problem, tried second a few times, no way.

[Humber]

I have a limiter on my carb.... and my spare bike doesn't have it....

[Lucky]

not more than a couple of inches in 3rd, but it came up...might have gone over a bump at the time, but hey, it counted! :)  i'm sure I could do it again though

[Ken Williams]

A serious dip is the best method for cleaning carburetors.  If you need yours cleaned and adjusted, send them to Lucky.  

In my experience some Vision stumble remains even with clean, adjusted to specification carburetors.  Increasing fuel pump flow helps.  Decreasing the size of the pilot air jets helps.  However, even with pilot air jets sized so idle is grossly over rich, some Vision stumble remains.  The emulsion tube modification I recommend richens the midrange mixture, especially when vacuum is low.  Vacuum is lowest as the throttle is opened from closed or near closed positions.

[Dave T.]

Is the emulsion tube the same as the main bleed pipe? I see the main bleed pipe mixes the air and the fuel for the Main circuit. I was thinking of getting a set of carbs off E-bay and "dig in" to the carbs a little more. A 36mm set off an '83 would be nice. I could compare the difference between the 34 & 36 mm carbs.

The lean condition makes sense. I may change my 132.5F, 135R pilot air jets to 120F 120R and see how this effects stumble. Usually when a motorcycle "cuts out" it's related to the combustion chambers not getting enough fuel. But since you think just changing the jets will not help, I may check the emulsion tubes out.

How about using a tip cleaner and "reaming" the bleed hole(s) out a little? This could increase air/fuel. Start with one or two holes. Like Lucky said though, busting something off in there would suck.  :P

I don't see how increasing fuel pump flow would help though. I heard the fuel pump is for the goofy tank and the petcock is below the carb float level.

Good Info Ken... ? :)

[Ken Williams]

The emulsion tube is the same as the main bleed pipe.  Air comes down the center of the tube and through the small holes, mixes with fuel coming up from the bottom, and joins the intake air at the outlet of the main nozzle.  To richen the off idle mixture, I recommend you obstruct one of the top holes in the side of the emulsion tube.  If you wish to add more air to the off idle mixture, make one of these holes bigger.  With good technique you should not break a drill bit or reamer while enlarging a hole.  

I ended up changing my pilot air jets from the stock 140 to 135.  130?s definitely caused the idle to be too rich.  Lucky recommends 135 pilot air jets for the 82 in his earlier post.  This is leaner than the stock 130 listed in my manual.  I don?t have experience with the 82.  If you want to try richer jets, 130 or 127.5 would be a good bet.  I expect that 120 would be excessively rich.

Oops, I meant increasing accelerator pump flow, not fuel pump flow.  Yes to your comments about fuel pump function.  

If you perform some experiments let us know the results!

[jasonm.]

D.T., You did right by putting the 132.5 in the front MAJ. These '82 carbs are too rich in the middle. I dyno'd mine and playing with the flapper(opening) and jetting proved this. I put a 135 in the front. Mine runs as it should. Redline in the gears 1-4 easy.

[Dave T.]

Well, since my V was down with a bad stator, I changed my Pilot Air Jets to 127.5 F and 130 R. I had 132.5 F and 135 R in previously. I re-set the pilot air screws to 2 1/2 turns out on both the front and rear carbs, and re-sync'd them.

I got my stator gasket this morning and put a new electrex stator in. I just got back from a 25 mile ride. Results? I think my "stumble" smoothed out very nicely. The only thing not quite right is a little bit of surging at 55mph (5k rpm). But my V did this before too.

I am happy with it. This is the best the old V has ran and I'd thought I'd share with the group.  ;D