Mikuni VM34 issues

You don't see these issues with a Mk 1 Amal !  Nuff said.

That's really helpful Neil.  Not!

I respect your knowledge of these things, Peter. But my message was helpful and I offered a solution.

Incidentally, I know they are few and far between but when was the last time a rider on here had issues with the SU conversion?

ATB,

Neil.

Afternoon Rodger

Ignore everything Neil says

I wrote down the jetting in the single VM34 Mikuni after i took it off to change to an SU

Throttle slide 2.5
6DH2 Needle
240 Main Jet
159 P-O Needle jet
Air jet 2.0
Pilot Jet 35

This carb was fitted to my 850Mk3 so jetting might not be correct for a 750 

I think 20 is to small. I have a 30 PJ installed. Works fine on a Mk3, should not be so much difference to 750 in idle or starting.

Roger,

I'm not surprised of your shoddy service from Mikuni Oz.

I put one of their conversions on my Atlas. The manifold was too long so the Aircleaner wouldn't fit, I ended up putting a Unifilter sock on it.

The standard of finish of all the parts including the carburettor was well below what you would get on an OEM fitting. 

I can't remember the exact jet sizes as I gave my records to the bikes new owner. But it was a bit lean on idle and took ages, anout 3 miles before I could close the choke.

As Neil says you don't have these issues with a Mk1 Amal...But I replaced the Amals because one jammed open....you don't have THAT problem with a Mikuni!

Don Anson 

Melbourne  

Peter, I offered a solution, for Roger to decide upon. I don't like being cancelled and I respect everyone's standpoint. But not to be ignored.

Fair point Don, not heard of that before but if it happened to you then it happened.

My point has always been that there appears to be a lot of issues with these particular carbs.

I have a picture of the Late great Kiwi, Mark McLennan fixing the then Chairman's M carb conversion at Oliver's Mount, Scarborough. (Mk3 Commando)  Incidentally, Mark used a Mk 2 Amal on his award winning Commando.

how does one tell a genuine Mikuni from a copy? there seem to be a lot of knock off supposed Jap carbs for sale....

I have just fitted a Mikuni to my 72 750 commando 

could not get it to run right with the jets that were fitted

Peter sent me his settings which work well but had to use the choke For a while on start up

change pilot jet to 32.5 and adjusted pilot screw which now uses less choke and runs really well

A book I found very useful which has these jet settings 

Norton Commando Restoration by Norman White 

Morning Neil

You have not made any helpful suggestions in this thread. I can only describe your first reply as sarcastic and for you to say that it was helpful is complete fantasy.

My reply to your sarcastic comment was sarcastic and meant to be. It could have been worded better, but the intention would have still been the same.

Morning Everybody 

After market suppliers of different carburettors/manifolds i would expect to probably supply the carb/s jetted on the rich side as this would be the safer option. Same as with supplying longer bolts, Imagine if the supplied bolts were too short and the hole stripped when fitting. Hopefully the instructions that came with the carb outlined what needed to be done.      

Commandos are old-fashioned engines with an 89 mm stroke. Amals were developed with this sort of engine in mind, and for the 500cc singles with their 100mm stroke, or the 633ccs at 120mm...

It's a fit athlete rather than the usual type of ageing Norton owner who can spin this sort of engine  when cold fast enough to give sufficient inlet depression to draw sufficient fuel through.

I really don't understand understand why someone would remove proven instruments with ticklers. It's most unlikely that Mikuni did any R&D with this sort of long stroke engine with low-geared kickstarters.

I experimented with a single Mk.2 Amal (very similar to the Mikuni) on my 920 some years ago and although it ran OK, starting was very difficult and I remember thinking at the time that the depression created when (fairly slowly) spinning the engine via the kick-start won't help getting a good slug of fuel mix into the cold cylinders as the 'cold-start' system still relies on said depression. I reverted to a pair of Mk.1 Amals and with a good 'tickle', starting reverted to a one-kick affair. I later fitted an Alton e-starter which probably would have spun the engine fast enough for starting with the Mk.2 Amal (and presumably similar Mikuni).

I know some folk swear by the Mikuni/Mk.2 Amal conversions so it's obviously a somewhat contentious issue ... .

The Mk2 Amal has an enrichening device similar to the Mikuni. These work by the cold start lever getting depressed which lifts the plunger off its seat to allow fuel to bypass the normal metering circuit and create a richer mixture for starting. Both the Mikuni and Amal Mk2 have a starter jet (not usually listed in jetting settings) this is buried in the float bowl on the Amal. I haven't found it on the mikuni yet (its in the loft in a box!). But the drawing in the guide book shows it in the float bowl

Fuel goes through this jet, mixes with air to form an emulsion that then passes though the uncovered port and into the engine. 

The Mikuni carburettor tuning guide i have a copy of states 'it is important that the throttle valve is closed when starting the engine'

From observing the starting techniques of others that have problems on starting. It is the opening of the throttle when kick starting the engine. My usual solution is for the owner to hold the handle bar next to the throttle so avoiding the inadvertant throttle opening.   

I have a single mkII Amal and there is no doubt that if the throttle is opened it will not start, also if it is opened before it has warmed a little, it will die.

Don't touch the throttle when kicking from cold. Operate the cold start device (depress the lever, it does hold itself open?) or open the lever on the handlebar if it has the cable operated cold start.

Then Kick. hold the handle bar next to the throttle to prevent ANY throttle movement.  

Just thought, i wonder if the starter jet is obstructed somewhat. As previously mentioned i think this jet is buried in the float bowl.