Not another VM34 post

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ironhawk69

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G'day!

FIrst time poster, few months lurker.

I've been working on getting a '75 XS chop road worthy and it's almost there untl I upgraded carbs last week and having issues I cannot work out.

I've upgraded to Mikuni VM34 bought from MikuniOz which has come pre jetted to Australian standards(?)
A bit of research and trying to familiarize myself with them, I installed them but cannot seem to work out what is going wrong.
The bike seems to kick over a lot easier and every second kick now, it idles and backfires a bit, but as soon as you flick the chokes off or even open the throttle with the chokes still on it cuts all revs and dies instantly.
I've played around with the air and idle screw, but no dice.
Any Aussies here that have bought through MikuniOz before and perhaps know if the pre jets/settings might be the cause?
Any advice would be greatly appreciated.
 
I bought VM 34s from MikuniOz a few years ago. They came with manifolds, rubbers, cables, and a few main jets. I threw the manifolds away as they placed the carbs right under the petcocks, but I never had any problems. My 74 runs sweet as a lolly.

It's difficult to diagnose problems with someone else's bike without being there, any diagnosis would simply be guess work. But I'll offer the following info.
Jetting - read your plugs.
Jetting - read the plugs,
jetting - read the plugs.
I hope that's clear!

The carbs from MikuniOz are pretty well jetted correctly for Australian conditions, and as Oz doesn't differ greatly in altitude that shouldn't present a problem, unless you're at the very top of Kosciuzsko. So, the slide cutaway, needle jet, needle, pilot jet and main jet should all be correct. You do have adjustment available via needle clip position and idle mixture screw and the extra main jets provided by Mikunioz.. Leave the main jet alone for now as that only comes into play from 3/4 to flat out.

I assume you have read Mikunioz's tuning tips.

First things first, clean and gap you plugs, clean and gap the points, gap the valve lash, check your timing, check exhaustmport gaskets, check the float height and bench set the VM settings as per Mikunioz's suggestions.

Always start at the beginning - idle!

Let it idle for a while, without letting it overheat (use a fan positioned in front of the engine), then read the plug. Turning the mixture screw out will make it run leaner, in will make it run richer.

Once you have that sorted, move onto needle position. raising the needle clip up will richen the mixture, lowering the clip will lean the mixture. I would suggest lowering the needle clip to start with. Again, Read the plug once you can get it running on over 1/4 throttle.

Mikuni oz gives basic info on settings: middle clip position on the needle and from memory two turns out on the mixture screw.

Other things you can do. check for leaks, check the condensers, check for a leaking head gasket, check the valve seats.

Do not snap the throttle open until it is running pretty well. Snapping the throttle open on any engine that is not tuned correctly will invariably result in the engine dying.
 
Last edited:
toglhot said:
I bought VM 34s from MikuniOz a few years ago. They came with manifolds, rubbers, cables, and a few main jets. I threw the manifolds away as they placed the carbs right under the petcocks, but I never had any problems. My 74 runs sweet as a lolly.

It's difficult to diagnose problems with someone else's bike without being there, any diagnosis would simply be guess work. But I'll offer the following info.
Jetting - read your plugs.
Jetting - read the plugs,
jetting - read the plugs.
I hope that's clear!

The carbs from MikuniOz are pretty well jetted correctly for Australian conditions, and as Oz doesn't differ greatly in altitude that shouldn't present a problem, unless you're at the very top of Kosciuzsko. So, the slide cutaway, needle jet, needle, pilot jet and main jet should all be correct. You do have adjustment available via needle clip position and idle mixture screw and the extra main jets provided by Mikunioz.. Leave the main jet alone for now as that only comes into play from 3/4 to flat out.

I assume you have read Mikunioz's tuning tips.

First things first, clean and gap you plugs, clean and gap the points, gap the valve lash, check your timing, check exhaustmport gaskets, check the float height and bench set the VM settings as per Mikunioz's suggestions.

Always start at the beginning - idle!

Let it idle for a while, without letting it overheat (use a fan positioned in front of the engine), then read the plug. Turning the mixture screw out will make it run leaner, in will make it run richer.

Once you have that sorted, move onto needle position. raising the needle clip up will richen the mixture, lowering the clip will lean the mixture. I would suggest lowering the needle clip to start with. Again, Read the plug once you can get it running on over 1/4 throttle.

Mikuni oz gives basic info on settings: middle clip position on the needle and from memory two turns out on the mixture screw.

Other things you can do. check for leaks, check the condensers, check for a leaking head gasket, check the valve seats.

Do not snap the throttle open until it is running pretty well. Snapping the throttle open on any engine that is not tuned correctly will invariably result in the engine dying.
Click to expand...
Cheers! That’s giving me ideas and things to look at!
 
Oops, I edited my post re raising and lowering the needle clip: Lowering the clip wil raise the needle, thus richen the mixture, raising it will lower the needle thus leaning the mixture.
What's the matter with you people, no one picked up on my 'intentional' mistake. I was just a test - you all failed.
 
Please verify your ignition and charging system are both good. No advance at idle on the timing and a battery that supplies 12.2 -12.5 volts at idle. Cheers
 
ironhawk69 said:
I’ve been thinking more about it and this bike has a 750 kit and a cam in it, how much difference would that make?
Click to expand...
Not much I have VM34's on a 750
Let's wander back to square one for a moment.
First: this has happened to more than a few guys.
It's possible to put the slides in reversed left slide in right carb and vice versa
VERY easy to check, look in the intake, (air filter side) the angled cut away should be plainly visible
Like this;
1712363710086.png

If all you see is slide, virtually no gap, there's your issue. Unscrew the caps, swap slides, done. Go ride.

slides 001.JPG

Yeah don't ask why I KNOW this.............
 
toglhot said:
Oops, I edited my post re raising and lowering the needle clip: Lowering the clip wil raise the needle, thus richen the mixture, raising it will lower the needle thus leaning the mixture.
What's the matter with you people, no one picked up on my 'intentional' mistake. I was just a test - you all failed.
Click to expand...
We come to expect that you do not need proof readers...:)
 
Okay these are what I’m running stock from Mikuni Oz.

MJ: 210
PJ: 42.5
Float - 24mm
Emulsion tube: 159
Slide: 2.5
Jet needle: 6DH4 - second from the top
Air jet - 2.0
 
If it's not running right, change the jetting. You do that by READING THE PLUGS!
 

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