Archive for the ‘Carb – VM’ Category

XS650: More VM   Leave a comment

Mikuni VM Carbs


Sudco – VM … vm info… parts etc


also see

Carbs … Mikuni

VM34SC Mikuni


Posted May 6, 2012 by xscafe in Carb - VM

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XS650: VM34SC Mikuni   Leave a comment

Yamaha’s Racing Tips Manual, 1974, suggested using VM34SC Mikunis. Round slide. Magnesium bodied. No Power Jet/Accelerator Pump/Beschleunigungspumpe.

Today, TM’s are supposed to provide significant performance improvements. Flat slides and the jet blocks help create a smooth bore effect. Air flows faster and smoother through the TM Series venturi. Higher air flow velocity gives a stronger vacuum at the needle jet needle. Provides more precise metering and better throttle response.

For known geographic and climatic conditions.

However, I do a fair bit of touring. Low to High altitude. For this I actually prefer the original BS series carbs. They automatically adjust themselves for air pressure. Easier to keep tuned.

… VM34SC carb … XS750(650) Racing … adjustments … jet needle 6F9 (#2 clip position)


… carb adjustments


… carb .. VM34SC … adjustment


… VM34SC carb … 3/4-full throttle adj … main jet



… VM34SC carb … 1/4-3/4 throttle adj … needle jet


… VM34SC carb … 1/8 – 1/2 throttle adj … slide adj and cutaway



… VM34SC carb … 0-1/8 throttle adj … air mixture screw – pilot jet



… VM34SC carb … idle speed screw

Posted December 21, 2011 by xscafe in Carb - VM

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XS650: Engine Temp. RHS   Leave a comment

A week or so ago CM wrote asking about operating temp differences left and right. Why does the rhs run hotter?


xsjohns, rip, engine temp recording…with cooler…left and right can run up to 30° or so difference


I’ve been out of town. Away from my computer. Exiled. Disconnected. But not deleted. Basically this is the answer I sent him.

Over the years I’ve heard many postulate. From aerodynamics to exhaust placement.

When you look at the engine design you see all the mechanical parts are focused to the right. As is oil flow.

Hot oil is sucked up through the pump. Slowed at the filter.

Mechanical action from the tacho, oil pump, primary and clutch all add frictional heat.

Aluminium is a good conductor. Heat gravitating upwards meets that from the combustion chamber and exhaust ports.

To the left we have the generator. In an air cell. Dry. Different thermal properties.

Allows the left to act more efficiently and effectively as a heat sink.

When Yamaha built the carbs they did so symmetrically. Jetting specs both the same. Each operating in different thermal environments. Built in imbalance. With appropriate feedback loops.

If you have adjustable needles it is possible to richen the right hand carb by raising the needle.


lowering the clip raises the needle…enrichening the mix


XSJohn, rip, ground needles with different tapers as a set. Left and Right. The canadian needles apparently are similar. Replace needle jets and jet needles together as a pair.


xsjohn, rip … on his needles … no grooves here, his production needles were grooved


John also built wings or foils to deflect airflow over the cylinders.


xsjohn’s wings…notice the deflector under the lower clamp


Others install oil coolers.


remote filter and cooler…so positioned the cooler will reduce air flow over the head


heidens side filter-cooler


Today there are many diagnostic techniques. Years ago I had an interesting conversation over too many beers with an Australian Naval NCO. Have never been fond of military types. Too rigid. Thinking actively suppressed. Later I was to learn the corporate world is, in theory, no different. In practise, a lot worse. Establishment meeting punk – the shock was mutual. He was a vibration analyst. Spent his time listening to machines sing. No different really to listening to your motor through a long solid screwdriver. He had a lot to say about the internal workings of Australia’s navy.

I would like to see a series of  Infra Red pictures of an XS650 motor running. Done in a resolution capable of  differentiating temp gradients within the 80-300° range. You would see some definite nodes on the right and fronts associated with temp assymmetry.

I come from the southern hemisphere. Live in the north. Apart from north and south there are other differences too. Here they drive on the wrong side. When you pull the bath plug, flush the toilet, water runs anticlockwise-the coriolis effect. I wonder what this does to other physical gradients Winking smile.

XS650: Pod Air Filters   3 comments

I lightened down my bike considerably. All the electrics went. Run batteryless. Removed the air-boxes. Left the whole section under the seat empty.

Looks cool. No good for the motor.

Had a pair of el-cheapo pods lying around. More or less the right diameter, according to the label. After fitting I decided I could do better:


now how are you supposed to get a good seal there…also see the cracking along the edges-this was new out of the bag


big inner lip…everything restricted…diaphragm vacuum port at the top: pilot air jet-lower left: main and mid range air jet-lower right: bowl vents in the middle


these filters had never been used


was then given, under protest, a set of these


larger surface area


no restricting step


may not hold up esp. with no support…wonder how long it will last…is the rubber ethanol proof?


  • very thin
  • where-ever you look the filter wants to break out of the rubber
  • see the large hole in the flange lip? WTF?
  • strong enough to hang unsupported?
  • ethanol proof?

Not an optimal solution. But heaps better. Will have a look around for a K n N to compare.

When I think about it I guess I should add here that pods generally increase engine breathing capacity. The immediate effect is to lean the carb mix. More air, less gas. Pistons and valves don’t like this. Get a little intimate with your carbs. Odds are you may have to massage your jetting.

Reading will tell you. Chop those plugs.

XS650: Carbs … Mikuni   Leave a comment

I’m not a carb expert. I can strip, clean and tune my own carbs fine but if I was relying on my powers of fine tuning for a living I’d be in trouble. It’s just as well the original CV carbs are fairly straight forward.

Many of their problems are related to dirty tanks leading to blocked Pilot circuits, float valve, float level setting, airleaks in vacuum barbs, boots, butterfly shaft or air filter.

Good discussions of these have come out of the Garage.

Mikuni_BS-CV_Carburetor_Rebuild_Tutorial … a nice pictorial guide

BS34 and BS38

VM 34-36 Roundslide Mikuni

Here is a Mikuni VM manual

vm carbtune

Interesting read on CV carb setting up

Basic carb theory


jet crossover


Repair hints


VM Jetting chart

Topham resource site

2 types of original carbs, both CV, see the Oz site too

  • BS38 – early types were not linked and bolted perpendicular to the head, later models were linked and the boots were angled to the head
  • BS34 – linked and angled to the head


Basic CV carb ID pics


US XS650 carb guide


pilot jets


float setting


CV cut-away


CV schematic … BS34 pilot jets are fed directly from the main so must be plugged


VM dimensions


Goran Perssons vacuum port mod


This is handy for providing vacuum points to balance your carbs if using boots without vacuum barbs, simply plug when not being used.

Posted January 10, 2011 by xscafe in Carb - Boots, Carb - BS, Carb - VM

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