Mikuni spring question

As an experiment I recently replaced my worn out Amals with a single 34mm Mikuni. I've been reading about adding a second Amal spring to my slide to help it close. My question is what has been the long term experience of those that haven't added the second spring.

I have heard about the weak Spring, but I have only ever used one spring and it has been without incedent.

Yeah , I've only ever used spring that came with the carb , never any issues , been quite a while now ....
Craig

The engineers at Mikuni fully understand what happens when an engine runs away. I'd be real surprised if they threw darts at a list of spring rates and picked the spring rate at random. The same engineers, probably, factored in a few power washes and a bit of dirt, after all the rocket scientists will tell you that a VM type is for a two stroke...

If you are used to the resistance the twin Amals impart to the twist grip then the Mikuni spring will make the twist grip feel unusually light. If you have tunnel carpel syndrome, even if you've had the surgery and/or have a touch or arthritis you may welcome the lighter effort. If your Mikuni cable is bent under the frames backbone you may need the Amal spring. Rocky Point's cables have a 45 degree metal guide tube that eliminates the backbone binding; I do recommend the cable be lubed periodically. Try the twist grip on an airhead BMW with Bing carbs, most are still using the throttle cables they were sold new with, and the effort required is Herculean; I had an R75 where my riding time was limited by my right wrist giving up (under an hour), I put one of my 32mm VM kits' on with Barnett cables (I did polish the chain drive cable lug and filled the twist grip channel with moly grease) and could ride almost all day.

There was a recent thread that had some content about the Mikuni spring, a few posters stated that they experienced a failure of the slide to return, and I have no reason to doubt them, but I'd bet that most all if not all were utilizing a cable that had a tight bend under the backbone; probably worked well enough when the cable was new, but degraded as the inner cut into the sheath, I can't say for sure.

BTW, as a side note, none of the Mikuni distributors have 34mm VM carbs, probably won't until late September/early October; I ran out yesterday...

I have a pair of 34mm VMs on my race bike and the only issue I have had is 'too heavy' throttle action. As noted above, I run the cables in as wide a sweep as possible with a dual pull grip. there are no steel bends and the info from the cable supplier says don't lube. Greasing the throttle tube ended up with grit causing more issues than grease solved!

I asked my Mikuni supplier for an alternative lighter spring which I have fitted, I really cannot tell the difference even though they did look a little different.

So far the slides have closed well.

I only ride for up to half an hour at a time, and during that time my forearms are working harder with other things like braking and bracing with a little body movement and steering, so I end up pretty whacked, so no comparison to the riding you do. I find the heavyness actually worse whilst waiting in the collecting area as I have no reliable tickover! (or maybe I just over worry about it stalling and me having to push start it!)

Thanks for your replies. I have way too many off road miles with Mikuni VMs and have never experienced any failures. Based on that I wasn't too worried but thought that I'd reach out for first hand experience. I bought my kit from CNW and the throttle cable that Matt supplies seems well thought out. A nice gentle loop and no split connecter to complicate things. I used a dry lube on the throttle tube and Tri-flow in the cable itself. Lighter action than the twin Amals for sure. Once it cools down here in AZ I'll be riding the Norton more instead of the HD. Thanks again.

Been running a single 34mm for three years/12 thousand miles, and not one hint of sticky throttle. That, and the lightness in the twist grip is a welcome perk. Out of fear of derailing the thread;

SteveA said:

I find the heavyness actually worse whilst waiting in the collecting area as I have no reliable tickover! (or maybe I just over worry about it stalling and me having to push start it!)

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You might consider a Pazon Altair, or a similar EI. For someone used to a piss-poor idle due to worn Amals, the idle stabilization feature has to be witnessed to fully appreciate just how much more comfortable stop-and-go riding can be.

Nathan

Nater_Potater said:

Been running a single 34mm for three years/12 thousand miles, and not one hint of sticky throttle. That, and the lightness in the twist grip is a welcome perk. Out of fear of derailing the thread;

SteveA said:

I find the heavyness actually worse whilst waiting in the collecting area as I have no reliable tickover! (or maybe I just over worry about it stalling and me having to push start it!)

Click to expand...

Click to expand...

You might consider a Pazon Altair, or a similar EI. For someone used to a piss-poor idle due to worn Amals, the idle stabilization feature has to be witnessed to fully appreciate just how much more comfortable stop-and-go riding can be.

I've got a Boyer on it that has worked flawlessly for ten years. My bike idles like a state employee especially with the Mikuni on it. I did try a set of my friends Premiers before ordering the Mikuni and liked the way that they worked but I've always wanted to try a single carb set up. So far I'm happy. If I ever want to go back to twin carbs I won't hesitate to put on the Premiers.

Nater_Potater said:

Been running a single 34mm for three years/12 thousand miles, and not one hint of sticky throttle. That, and the lightness in the twist grip is a welcome perk. Out of fear of derailing the thread;

SteveA said:

I find the heavyness actually worse whilst waiting in the collecting area as I have no reliable tickover! (or maybe I just over worry about it stalling and me having to push start it!)

Click to expand...

You might consider a Pazon Altair, or a similar EI. For someone used to a piss-poor idle due to worn Amals, the idle stabilization feature has to be witnessed to fully appreciate just how much more comfortable stop-and-go riding can be.

Nathan

Click to expand...

You might try my cam and you might not have a reliable tickover either :shock:

In truth I am not trying to get a tickover on it. I keep the revs over 2K when warming through and blip to 3 to keep the plugs clear. It will run at 1500 if I let it, but I don't often.

On the move it rarely drops below 4K!

Recenly fitted ignition is Pazon Surefire, it is analog, simple and fine. If I went to a more sophisticated ignition it would be crank triggered and programmable!

I have never believed in fitting Mikunis to four-strokes. I think they are designed for two-strokes where the vacuum is not so high. We fitted two Mikunis to an XS650 Yamaha years ago and could never get it to shut off, even with stupidly strong springs. I use two 34mm Mk2 Amals with the inlet ports tapered 2mm per side over the first 25mm. I do however, use Mikuni needles.

i ran 32mm VM mikunis on a 650 (then 750) triumph for thirty years and never had a problem with the stock mikuni springs returning it to idle. a year ago i swapped to 34mm VM mikunis, and they are working just as well.

i've always used the as-delivered two stroke needle jets.

I read somewhere that the Mikuni VM is a copy of a Mk 2 Amal

gripper said:

I read somewhere that the Mikuni VM is a copy of a Mk 2 Amal

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i think the story went something like amal designed a new carb and tried to get triumph,norton,bsa etc to pay for development work, but when they refused amal sold the design to mikuni and the mikuni vm was born! does anybody know if this is true??

baz said:

gripper said:

I read somewhere that the Mikuni VM is a copy of a Mk 2 Amal

Click to expand...

i think the story went something like amal designed a new carb and tried to get triumph,norton,bsa etc to pay for development work, but when they refused amal sold the design to mikuni and the mikuni vm was born! does anybody know if this is true??

Click to expand...

This does sound like a lot of bollocks to me, purely because I rode a bike with VM Mikunis in '73 and it wasn't the first to use them. I didn't get any MkII Amals until '77 when they were just about appearing after market....had to wait for them to be made!

speedrattle said:

i ran 32mm VM mikunis on a 650 (then 750) triumph for thirty years and never had a problem with the stock mikuni springs returning it to idle. a year ago i swapped to 34mm VM mikunis, and they are working just as well.

i've always used the as-delivered two stroke needle jets.

Click to expand...

Funny you should say that,I had a pair of VMs that I took from a t500 Suzuki I think they may have been 34's but I can't remember I fitted them to my t140 bonnie this transformed the bike,the only adjustment I made was to file the cutaways bigger and change the main jets,I think i remember stretching the pilot air adjusting screw springs to stop the screws falling out because I had them adjusted so far out! (The innocence of youth and no money) but they still had the two stroke spray tube in them,I rode the bike for many years like that and sold it in 1990 ,just recently my mate saw the bike parked up at the ace cafe and spoke with the owner ,the same bloke I sold it to and it still had the mikunis fitted I was amazed when he told me

Yeah I thought it might be a load of bollocks. The VM does look like a slightly more dated design but nothing wrong with that.

SteveA said:

baz said:

gripper said:

I read somewhere that the Mikuni VM is a copy of a Mk 2 Amal

Click to expand...

i think the story went something like amal designed a new carb and tried to get triumph,norton,bsa etc to pay for development work, but when they refused amal sold the design to mikuni and the mikuni vm was born! does anybody know if this is true??

Click to expand...

This does sound like a lot of bollocks to me, purely because I rode a bike with VM Mikunis in '73 and it wasn't the first to use them. I didn't get any MkII Amals until '77 when they were just about appearing after market....had to wait for them to be made!

Click to expand...

Yes, but there was also the mk 1.5 Amal concentric with a plunger mechanism instead of the tickler, they do bear a strong resemblance to a Mikuni VM. They were good carbs I had a set of 28mm on a triple.
sam

baz said:

..............I had a pair of VMs that I took from a t500 Suzuki I think they may have been 34's but I can't remember

Click to expand...

Standard size on a T500 is 32mm, a lot of T500 race bikes have 34mm fitted.

[quote="SteveA

Recenly fitted ignition is Pazon Surefire, it is analog, simple and fine. If I went to a more sophisticated ignition it would be crank triggered and programmable![/quote]

why not their digital Surefire... it has an adjustable rev limiter ?