Mikuni maintenance- choke plungers

  • Thread starter Thread starter Reggie3cl
  • Start date Start date
Yeah, those mixture screw tips are thin, but happily mine are still intact- gave the holes a quick squirt of carb cleaner all OK.

Maybe Keith makes up a rod for the choke mech?  The indents (only three) are in the exact place for the tangs to rest on the inner shoulders of the plungers- that may just be coincidence, I suppose they match the carb spacing anyway.  Rob doesn't have the springs on the rod to hold the rod and tangs in that position, and I'm sure there have never been spring loaded balls fitted.  The way it vibrates at certain revs I'm not sure if it matters on my bike where the tangs are. :o

Would I find the hole for the spring blocked by the carb bracket here- this is one of the side carbs

P9130167small.jpg


Rob: Parts diag shows springs on choke rod (111) and possibly missing ball and spring 51, 52)

mikparts.jpg


I'll ask Keith about it next time I see him, but as I say, he did warn me that I needed to treat the plungers as a service part.
 
Looking at your photo, it's difficult to see whether the hole in the carb for the spring and detent ball is there. If carbs 1, 2 and 3 have been used out of the set of 4 as Red described, then the holes should be there. If another combination of carb bodies was used (which is possible with a bit of re-jigging), you may not have the holes. But that's no problem, it's easy to make holes.

With a ⅛? drill, bore radially through the choke rod bush on the front of the carb body. Drill right through into the carb body to take ⅛? detent ball and spring.

The hole must be exactly 16mm deep from the outside of the choke rod bush, so use the depth stop on your pillar drill. The depth of the hole is what controls the spring pre-load on the detent mechanism. Be careful not to drill at an angle that will go into the choke plunger bore or through the wall of the carb body into the slide chamber. If that happens, you have a big problem. The correct angle is about 20? from vertical, but it?s easy enough to judge by eye.

⅛? bearing balls are available from bearing suppliers. The ⅛? springs are available from Mikuni. Part # B21/29.
 

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Ah, simple. :o  I'll have a looksee next time the carbs are off, but as I say I don't recall ever removing the choke rod so there would be no reason for the balls to go missing.  And Rob said that they're not fitted with a cable pull choke like wot I've got.

Looking at the vibes in the vid, I don't really see what can be done, just have to change the plungers every year.  I mean, it's even destroyed the nipple on the end of the choke cable. :o
 
Reggie, I have a 120 with Mikunis, sane as you. I don't think the carbs vibrate anything like what your video shows. The fuel in your float bowls must be little more than frothy mist with that kind of shaking. It's a wonder the bike runs at all.

I also have a cable operated choke. The choke actuating rod has the indents in it which engage with the balls (the balls in the carbs, not my balls) when the choke is in or out, just like Red described. If the choke rod in yours is rattling around loose, then I suspect you're missing the springs and balls. They're easy to lose if you pull the choke rod out. They'll just sproing off into the workshop somewhere. Being so tiny, you have little chance of finding them if your workshop floor is anything like mine.

I'll have a look in my box of Mikuni bits, I might have some spare springs and balls if you need them.
 
Are the carbs further secured to the frame or engine by a support?
If so this may add to the vibration problem. Might mean the rubber inlet mounts can't do their job properly.
 
No, carbs haven't got any additional support.  I think Keith fits something if you're using pods, but I have the standard airbox and rubbers.

The carbs seem to vibrate more at certain revs (wasn't watching) and are fairly OK at higher speeds.  Bike runs very well.

Maurice is right about 180s- I had no problems with my Miks on the 180 (non cable) and they certainly didn't go blurry at any revs I put it to in the garage.  That bike was soooo smooth.

Mrs 3CL was appalled at the vibes when she went on the back a couple of weeks ago- made her thighs go numb she said- and Rob said his missus finds his RGS the same.  I don't have a problem on the front at all, and even the mirrors are pretty steady and usable.  The crank is absolutely right and it was a Witt.  I assume the Omega forged pistons are the correct weight- Keith would definitely have checked.

I'm obviously going to have to check again for balls and springs- it really can't help the way the shaft is so loose.  Appreciate the offer of spares Cam.

First thing is to find out if the change of pilots has helped the low speed running, but today I have to drive to feckin' Dundee (on a rare day off, what a pisser)
 
No drillings that I can see for springs and ball bearings in my carbs- although the centre one is difficult to see.  Have made up a damper from closed cell foam as a temporary fix- will come up with something better over the winter (hopefully...)
 
One thing that puzzles me Richard is that the bank of carbs vibrate sideways?
I cannot think of anything in the engine that moves sideways to induce this.

How tight are the engine mounts?

 
That is a very good question!  I'll double check those mounts.  I understand that a 120 crank produces a rocking motion, so that's probably what you're seeing rather than the up and down you might get with a 180 or a parallel twin.
 
I will have a look at my Jota 120, can't say I have seen the carb's move like that.
I would still have a play with the engine mounts if I were you.

The solution to the rocking couples for 120 deg triples was solved a long time ago - for instance Triumph use balance shafts. Laverda relied on a slightly lower tech solution by fitting rubber engine mounts to dampen rocking motion.
Primary and secondary forces are already perfectly balanced.
Another foil for a rocking couple is shorten the crankshaft.

180's are not balanced for primary and secondary forces, but of course no rocking couple whatsover, so no need for balance shafts or fancy engine mounts.
 
Would be interesting to see another 120 run up to see what the carbs do.  The bike is smooth when running on the road- it may shake around more just being revved- certainly no numb fingers or blurred mirrors.

Bloody things...
 
Reggie, how old are the engine rubbers? Rubber is nothing to trust on for eternity, usually even worse on some of the trash called 'OEM' that is sold these days.
 
Might be something worth to look at. Mrs. 3CL getting numb thighs after 1h ride probably not as intended by the factory. ::)

Had recently changed the 40yr old rubber engine mounts on a /8 Mercedes-Benz (parts still available at the local MB dealer BTW), astonishing how smooth and quiet the car has become after it.
 
From the video the engine doesnt appear ( mind you a bit hard to get a fix on it ) to be moving anywhere near as much as the carbs... In theory and you got to love theory dont you ... the carbs should be isolated from the engine via the manifold rubbers so they should be absorbing the vibes to an extent.

Now out of interest do the carbs vibrate as much without the air filter rubbers on and also when you have them fitted are they sitting kind of free or are they inducing some tension on the carbs and as such putting load on the inlet side rubbers.

Tell me as again hard to tell on the photos how are the carbs actually mounted to the engine are they on flanged spigots ( much like the original RGS set up or are they on spigots with then a rubber joiner ( like series 2 set ups )Are you using Red's manifolds and rubbers as not aware of anyone in Aus having this type of issue and there are some high milers.. ( oops Kilometer's sorry Vince I know how much you love your metric ) users over hear. 

You got me interested about the carbs moving so much so that on the weekend ( tank off again just for shits and giggles ) and will see what kind of movement i get in my carbs as not convinced the engine mounts are to blame here... the ones on my bikes are all the originals so as hard as nails i guess... the RGS in particular seems not to like Voxbells and has a habit of regularily tossing its toys out of the cot so to speak only to be left by the side of the road for someone else to have use of.. bloody annoying and starting to be costly so have an alternative solution am working on.
Interestingly the Corsa has never spat a horn in its life nor has the Exec for that matter... .say what rev's were you using in the video.
 
SimonR501 said:
Laverda relied on a slightly lower tech solution by fitting rubber engine mounts to dampen rocking motion.
Click to expand...

The rubber engine mounts don't necessarily dampen the engine vibration, they just isolate the engine vibes from the frame. The engine actually has some degree of freedom to jump around within the frame so its amplitude of vibration is probably greater with rubber mounts than if it were solidly mounted. I suspect the isolation becomes less over time as the rubber mounts harden.

In Reggie's video, it appears that the carbs are hitting their natural frequency and taking off on their own little excursion into vibration land. Once the engine revs go beyond that frequency they settle down again. I'm pretty sure the carbs on my 120 (identical setup to Reggie's bike) don't do that. The manifold rubbers on mine are pretty new, so perhaps that's the difference. My crank has also been lightened and balanced by Red, so the engine vibes will be different.

But before splashing out on new rubbers, I'd be tempted to try a carb steady like Keith uses. It doesn't have to be as fancy as Keith's just to experiment with the idea. I made one from fencing wire to try on my Jota (because I'm a cheap bastard). It takes much of the weight of the carbs, and nearly all of the bending moment off the manifold rubbers (the rubbers are in effect a cantilever beam with the carbs hanging off the end). Changing the stress on the rubbers will change the natural frequency of the whole setup - most likely reduce it. Carb vibes presently appear to be happening just above idle speed so it wouldn't take much of a reduction in natural frequency to put it somewhere below the idle speed, thereby eliminating the problem.

PS: I spent a substantial part of my engineering life working on vibration isolation in machinery, so the above is something of an educated guess.

Cheers,
Cam
 
The 120? engine mounts soften, rather than harden with age.  They get pummelled quite a bit, often enough to have the central bush seperate from the rubber.  Especially the RH lower rear should be inspected regularly.  Excessive chain lube and leaking engine oil from the gearbox exit can deteriorate the rubber to the extent that it literally crumbles to bits.  This will then greatly exagerate engine vibes, so much so that it becomes painfull!

piet
 
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