Mikuni VM34

The ride is an 850MKIII (US) and I live at 6200'. It's taken a few weeks to sort the new arrival from the coast .. now it's running consistently but in need of altitude correction.

It's a VM34 (B) .. the pilot jet is the first to be corrected (down to a 30-35) and I'm wondering if the air jet (1.0) should be changed as well.

At this stage the bike runs well (sooty plugs) but runs out of reasonable 'setting' of the idle mixture (and maybe down a few hp) so I'll start there. Chip in.

Here is a repost addressing your issue




RoadScholar VIP MEMBER
Joined:
Dec 28, 2008
1up3down said: ↑
I live at 6000 feet altitude here in New Mexico and my single Mikuni 34 for my 850 is jetted
stock except for the idle circuit jet which I dropped to a 35 from the CNW supplied stock 40.
I check the plugs regularly and they are clean, I doubt you have a jetting problem.
If it has sat for a long time first put in brand new plugs then take the carb float off and clean it out
of any scummy gas accumulated there. You may also want to take your two gas tank petcocks out
and insure the filters are not clogged up. If your slide is in the middle notch it is fine for your altitude. I have ridden with stock jetting in excess of 10,000 feet altitude for short periods.

Can you tell us your Mikuni settings such as what size pilot jet, what notch is your slide clip in, what size main jet?

Stock? I think not. The factory supplied VM 34, spigot carb comes with 260 main jet (way too rich), a 159 Q-2 needle jet (way too rich), a 35 pilot (could be right) and a 6DH4 needle (close)

The "stock" jetting package could be OK if you could run your Norton at air pressures equivalent to 60-90 feet under water, far from 6000 feet above sea level.

To run acceptably at 6000 feet I suggest a 220 or a 230 main jet, a 159 O-6 needle jet, a 35 or 30 pilot jet and a 6DH3 needle with clip in position 3, maybe 2, as a good starting point. I would also consider/experiment with setting the full advance timing up 2 or 3 degrees, in stages.

eskasteve said:

Here is a repost addressing your issue

RoadScholar VIP MEMBER
................

To run acceptably at 6000 feet I suggest a 220 or a 230 main jet, a 159 O-6 needle jet, a 35 or 30 pilot jet and a 6DH3 needle with clip in position 3, maybe 2, as a good starting point. I would also consider/experiment with setting the full advance timing up 2 or 3 degrees, in stages.

Click to expand...

Yes, but no mention of the air jet, which the OP asked about. Funny lots of people don't seem to know the Mikuni has one and few have changed them!

There is a post somewhere here by Fullauto who changed one to good effect! But not because of altitude. I have been working to get mine near correct, and have changed from the 2.0 as originally fitted by the factory to now a 0.9 I am fairly happy with, but I am not at altitude and have twin 34mms on a 750 Short Stroke.

Air jet will mainly affect WOT performance at higher rpm, say over 6000, has it been tested much up there? It will also affect the size of main jet that works best!

Air jets are cheap enough to get a 0.9 and a 1.1 and see if you can detect any change, you may not. Or a 0.5 and 1.5 if you want more evidence of effect, but take care about leaning it out too much at high rpm with the 1.5! You would want to finish changing the air jet before you settle on the final main jet size!

It is possible on a road bike to never really know!

You could get a copy of the Victory Library Super Tuning Mikuni VM manual, written for Mikunis on British bikes. It will tell you that 1.0 is a good start point on an 850 with a 34mm VM. And it will also clarify what the air jet does! and give broad advise on Mikuni tuning, more than most of us will ever use.

Actually, I suspect that you will get more benefit getting the pilot, needle and needle jet right!

And remember that with unleaded fuel a plug needs to be 'white to be right'.....if you can access leaded fuel it will make home tuning easier!

Or get to a dyno at your altitude!

The Victory Library resource is a good one and thanks for the direction to that. It clarifies a number of part no's., compatibilities and step sizes for those wishing to dial-in their Mikuni. Air and pilot jet to be changed out firstly. The 6DH3 needle will stay (one notch down) as will the main jet until the idle circuit is worked out .. results to follow.

update for the archives ..

The jets installed were fairly standard - main 230 / idle 35 / air 2.0 and have been changed to main 220 / idle 30 / air 1.8

The slide is a 25 .. although 30 seems common.

First start (with choke of course) was almost immediate and a good idle, first blip of the throttle - the engine faultered a little but with a decent warm-up ran fine with the idle screw adjusted in 1/2 (a little richer) to ~ 2 turns out. A full run (at 6200' to 8000') on the road was fine with no faultering or mis-firing and after a plug check (normal riding conditions) there was a very pleasing almost white & clean plug.

I'm posting this for anyone that has had Amals and then gone to a Mikuni or that (like myself) didn't know Mikuni very well.

Findings 1) the Mikuni doesn't respond as evidently as the Amal to idle screw adjustment (this was my last bike a '73 850 MKII) with the normal twin carb setup .. and you don't get the same variation in idle speed that you expect!

Findings 2) the air jet on the Mikuni seems to be quite important! The air jet gallery feeds straight in to the needle seat and must be responsible for mixing / atomising the fuel from the main jet or at least control some functions of the main circuit.

Findings 3) the idle screw is in fact an 'air' screw and only regulates bleed air to the actual (fuel) idle jet (located in the float bowl).

Findings 4) .. still not sure about all of the galleries and pin holes on the engine side of the carb. (part of the choke / low / mid throttle operation) but this does seem to confirm that the primary jetting and performance IS only a part of the big picture and that the 'low speed' (minimum actual throttle) circuit is pretty important.

Great info, what are your feelings about performance compared to what you knew with your amals previously?

brendanq said:

Great info, what are your feelings about performance compared to what you knew with your amals previously?

Click to expand...

I can give you my experience after two years with a VM 34. It's on a very mild state of tune '73 850. Much easier starting, better idle, a bit better low end throttle response, a bit less throttle response in the upper rpm range, and no messy and dangerous tickling required. Also, when I take it out of summer storage (I live in the Phoenix area so don't ride in the heat of summer) it's much easier to get running. Not that I had any trouble with my Amals after storage since I have 45+ years or Norton experience, it just requires less messing with. I only have about 3,000 miles on it but so far it's working out fine. I have a new set of Premiers waiting to be installed if and when I become dissatisfied.

thanks eskasteve, I have a 74 850. And tempted by the premiers but I don't have the time to mess about with them as often as I do with the standard ones on it. I don't keep the bike at home, I keep it at my parents so work on it is a pain so having something I am more likely to be able to jump on and go with may be worth the slight loss of top end performance.

I’ve got to agree with eskasteve that the Mikuni is a good match for the motor. As to performance .. it’s going to be very difficult to tell ‘cos of the altitude and also that I ran a 21 tooth sprocket on the ‘73 and this ‘75 likely has a 19.

For an every-day ride I find the Mikuni to be far better on partial throttle .. less hunting and less whipping at the back end. Mind you, I’d replaced the isolastics and hub dampers with polyurethane and it was a lot tighter. The ‘73 also fractured the tank twice!

The argument of purist vs practical is a personal one .. I’ve generally gone for upgrades to electrical and ignition where possible ‘cos it keeps such a fine bike on the road (both bikes have Boyers and this one has gone to a single ‘twin fire’ coil).

Just as a side note - the ‘73 seemed to be a far better build from the factory and had a better quality frame .. not happy about the ‘75 but the improvements in the mechanical side should be worth it.

odd you would mention your 75 Mark3 would likely have a 19 tooth front sprocket as they came with 21's
only the 72 Combat came stock with a 19 as I recall

as mentioned above I am also at 6000 feet altitude here in the foothills of Albuquerque, New Mexico
I have a 73 850 with single MIkuni 34 from CNW, 35 pilot jet works fine, the needle is in the middle
I have experimented with lowering it one notch to lean out the mixture but felt the motor did not feel as strong in mid range and have gone back to the middle.
I had a set of Amal Premiers before and went back to the single Mikuni as I felt starting and idling were better, and mid range power response was more immediate and stronger. Wide open throttle above 5000 rpm is obviously not as strong as twin carb set up but honestly I just don't spend any time top ending.

1up3 .. might well be right about the sprockets (that was about 30 years ago for me) and it looks like this one will need some swing-arm attention soon so I’ll check the sprocket then.

I did use your post to set this one up and am likely to move closer to this set-up .. except I have a DH3 needle and (still) not sure about the air jet (2.0). After the snow clears I'll have a better chance to play but have a 32.5 idle at hand ... and will be necessary to ride a few lower roads without risking the engine.