My Mikuni VM34 on 850 mk2a

As you may recall I fitted a VM34 to my 850 rather a long time ago and I promised a report on how it goes. Additionally you may also recall I told you I’d had a heart attack recently as well and this curtailed any hope I had to get out riding.

Until today that is. It is lovely and sunny and almost warm it was almost SWMBO who suggested I go out. I just happened to be about to tell her as she said it to me. I’ve worked it out that it was at least 7 months since the last time I was on 2 wheels and I have missed it terribly. I couldn’t turn down the opportunity. Thankfully I am feeling reasonably OK while I wait for my bypass op early in May. After which I know I won’t be permitted to ride for at least 3 months. I’ve got to get out now I can.

So, all togged up, tyres checked, oil checked, fuel checked. Let’s go.

Fuel on, choke lever lifted, ignition on, flick the starter button and……….. vroom! It starts immediately, but I knew it would. The CNW starter allied to the Shorai lithium battery is superb! I let it fast idle and walked back to close the gates, and then turned off the choke. Lovely tickover although a tad slow as it’s still cold.

Then on the road we go, and it’s like renewing an old friendship and the get up and go was still there. I took it out through our back roads to the main road, and then onto the main fast road. It’s nicely warmed up now, and time to see how good, bad or indifferent it is.

It is great, pulls like a train and very torquey as usual. I took it up to 80 (allegedly officer) and there was more to come if I wanted. Well I didn’t, the bike’s almost 50 years old and why stress it? If I wanted blistering acceleration and high speeds I would have kept the Busa and ZZR. For now I want comfortable and easy riding. This is it. It works fine.

I did perhaps 25 miles and pulled in for a pit stop and switched it off for a bit, so I could relax (I am very anxious not to upset my heart right now) and have a look over just to check. All is well.

Helmet and gloves back on, fuel and ignition on and it starts up without any hesitation. I then rode home a further 25 miles or so and it was sheer bliss. I’ve been riding bikes for 50 years and today was one of the best ever.

I arrived home and parked up, the tickover was a touch high at 1100rpm or so, so I’ve turned it down to 1000ish. I’d not made any adjustments since unpacking it since it arrived from Matt. I will pop out the plugs and see what they look like (like I’d really know ) other than that all is well.

Finally, is there as much ‘urge’ as a pair of well setup 32mm Amals would be? Probably not from what I recall of my not-very-well-setup 32s, but I don’t care. This works so well right out of the box.

Great news ! Glad to hear you got out so early , never surprised that the Mikuni delivers great all round performance with little drama …. good luck in May , follow the rules early on , then it all up to good luck and yourself ….

The Mikuni is a revelation and a single Mikuni a smart way to break in a new engine along with used header pipes. Once I have 600+ on my rebuild I will get a pair of Amals on and move the Mikuni to the next rebuild.

Great carbs that just work period. Gotta love it!

With a single carb you only have one of everything, so tuning is easier. And with a Mikuni, you have a much better range of jets and needles to play with. If you have a look at the manuals for Japanese buikes of the 70s, every model uses different jets and needles. They probably change things every time the make a minor change to the ports, cams and exhaust systems.
In comparison, Amal carbs on a Commando is usually pretty crude. But the motor delivers so much torque that it probably does not matter much.
I have been surprised how much faster my Seeley 850 has gone, after I have worked hard on the jetting - mainly the needle taper.
The only problem with MIkunis is you cannot make the complex needle jets which are used in some of them - can get expensive. when you buy a few.
I have a friend who has a very nice RZ350 Yamaha. He searched high and low for a pair of new old-stock needle jets - found some in America. The bike is an absolute blur. It is probably as fast as a TR3.

Good to hear you enjoyed your ride....

But you fitted a Mikuni and you are surprised it works? Really?

Just enjoy and take care

I’m more surprised it works because I fitted it But I had been asked to report how I’d found it, and it was effortless.

Thanks Flatspot, a great writeup. Keep well.

P.S. personally I would leave the tickover at 1100. (Kinder on the camshaft).

Thanks I’ll bear that in mind next time I’m out.

cliffa said:

Thanks Flatspot, a great writeup. Keep well.

P.S. personally I would leave the tickover at 1100. (Kinder on the camshaft).

Click to expand...

Well yes, but personally...the cam will suffer if it doesn't get oiled, and the difference between 1000 and 1100rpm isn't enough to keep it oiled.

I understand many don't want to be thought a boy racer blipping the throttle all the time, but if is sat stationary a long time you should be occasionally blipping to 2500rpm to get some splash going.

Welcome advice, but I don’t and wouldn’t allow it to tickover for long. It’s either about to get ridden, or about to get turned off, and then it’ll tickover

The way the carb needs to be jetted for best performance, is affected buy the size of the inlet port, comp. ratio, exhaust system and the fuel you use. Amal carbs do not have as much facility for adjustment as MIkunis. But if you are jetted slightly rich everywhere, your motor will still perform well and you will be safe