Archive for the ‘Carbs’ Category

XS650: Breather   2 comments

All internal combustion engines experience positive crankcase pressure. Primarily a function of blow-by gasses. Unburnt fuel/air, water vapour and combustion byproducts leaking past the rings. Into the crankcase space. As the engine wears the problem gets worse.

  • contaminates engine oil
  • contributes to sludge build-up
  • causes corrosion
  • promotes leaky seals
  • introduces windage (drag on the rotating crank caused by an oil cloud in the crankcase)

The parallel twin configuration – both pistons moving in unison – overlays alternate high/low pressure pulses onto this.

Solutions are either make it easier for the pressure to escape or create a vacuum (optimally 14-15 Hg/7 psi crankcase vacuum. More, sucks oil from the valve guides, rings and bearings). Vacuum can be created by pumping, one-way PCV valves, routing through the intake or exhaust and by dry sumps (the option used for Yamaha’s second foray into 4-strokes, the TT/XT/SR500). Systems can be open or closed.

Being 2-stroke people Yamaha knew a lot about crankcase pressure. Their first foray into 4-strokes, the XS650, gave them a few headaches however.

Originally they opted for easing pressure escape. A baffled, single orifice, open system vented to the atmosphere. The amount of oil/air was rather significant. This leaked like a true brit. Later, after experimenting with venting and baffle options they chose to run with a closed system routed through the air intake. Vacuum.


baffled, single orifice breather vented to the atmosphere … produced excessive oil … crankcase oil capacity spec 3.0 ltr



early single outlet breather

the next three generations of breather … twin orifice straight and angled and single orifice reduced box size, restricted outlet … baffle configurations were also changed throughout the series


The first 9685 machines apparently had a single outlet breather with single hose venting to the atmosphere. This was followed by a straight 2 spigot outlet, same baffles, also vented to the atmosphere. Spigots were changed (angled down) and baffles reworked. No effective change to excess oil leaking. They even tried plugging one spigot completely. Over-pressured the crankcase causing further leaks and blown seals.

Eventually vacuum was added by routing the 2 breather hoses directly to each air filter box. Closed system. Restriction plugs were added to each spigot. Again the baffles were reworked. The air intake becomes the vacuum source. Basically the engine is setup to consume it’s own blow-by gasses. Not brilliant. It coats the intake in oil residue, promote carbon build-up on piston crowns, combustion chamber, valves and lowers the effective fuel octane rating. The vapourized oil ignites at lowerenergy levels than 87 octane gas. The more that enters the cylinder, the higher the potential for detonation. A real issue for forced induction systems. It is, however, efficient, practical and good for the environment.

In the end the baffle box was resized. New baffle. Single reduced outlet. Single hose, split to feed each air box. Easier for carb removal.


steel wool baffle in original breather boxes … if this gums up pressure will have a problem escaping


baffles … left hand required 2 gaskets – this was modified to sit inside the box (later this was again modified, shortened, can just see it in the middle box, and the 2 plates shown were added – no steel wool) … the later box had a simpler single plate baffle as shown


baffle plate modified to sit nside the box … this was later shortened (as shown above) and 2 extra plates added


need to take care when using silicone type sealents – too much can block the return hole … oops!


venting to atmosphere, single hose, exiting above the chain


breather box with restricting plugs


Interestingly, early models specified 3000cc oil. Shell Thuet swore this was too much (and was ignored) only putting 2500cc in his motors. Eventually (’74/75) Yamaha’s engineers saw the light. Solved much of the oil leaking problem.


early cases – cast with oil spec, 3000cc – too much


eventually yamaha accepted Shell’s advice – 2500cc


specs modified again to allow for oil filters (when not removed during oil change)


tech bulletin M5-051 – 1975oil specs and dip stick modification


early open system vented to atmosphere


closed system schematic


early closed system airbox … blow-by gasses introduced after the filter, brfore the carbs – both sides


A common mod today is swapping stock air boxes for pods. Loses the vacuum effect (and may require rejetting). A one-way power brake valve can help here. It allows positive crankcase pressure to escape and prevents air returning when the pistons resume their stroke to TDC. A small vacuum is created helping seal rings and prevents oil leaks.


pod filters with breather pipe routed into the end … closed system


another attempt at replicating the original closed system


I have seen people use a short hose positioning the check valve over a pod filter. Any oil drips on the pod recontaminating the intake (and on only 1 side). Others pipe from the check valve back below the engine so any oil drips on the road. Hopefully off to one side and not on the tyre. OK if you enjoy cleaning your bike. If adding a filter ensure the hose ID and filterinlet OD match (12mm).

Using a catch-can resolves this and is relatively simple.


open system with catch-can


simple catch can


Fred Fleury’s soda catch-can


catch-can with filter … chromed


mine, made from an ikea pepper shaker


Comparison of differing methods of dealing with positive crankcase pressure.








De-clutters the engine space






Simplifies crank case ventilation system






Helps remove more oil from the air






Helps evacuate the crank case






Maintains or improves the stock flow capacity






Helps promote ring seal and prevent blow-by






Helps prevent oil smoke in exhaust






Helps prevent dip stick from popping out






Helps prevent oil leaks due to pressure






Helps keep intake track clean including carb body






Helps keep intake manifold/plenum clean






Maintains or reduces amount of pollutants






Maintains prevention of un-metered air from entering the intake







OF: … open flow … single or twin hose

AF: … routed through air filter

CC … routed through PCV valve and catch-can

AP: … air pump routed through catch-can + filter

DS: … dry sump


Posted October 21, 2013 by xscafe in Carbs, Motor - Breathing

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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: Amal Single Carb   Leave a comment

Melv from Greensand Foundry has produced single carb manifolds for an Amal. Nice simple carbs.

Carb exiting to the left …

I made this manifold because I had lots of requests from people asking for a cast 2 into 1 that will fit in a standard frame loop. I made this to fit an Amal MK1 concentric, but will be making these to fit other carbs in the future. The Amal carb has a rubber o ring that seals against the machined face of the manifold!!!!!

Straight mounted carb …

He states: This is the first time I started the bike in 4 months. It usually starts first kick but the battery was flat!!!!! This is my straight mounted 1-2 manifold made to fit a standard XS CV carb. Internal bore diameter is 33.0mm. I made this because a number of people wanted it … personally I don’t like the standard XS carbs so adapted it to fit a trusty old Amal MK1 930 Concentric. My range of XS parts is listed on UK ebay!!!!!.

The jetting/settings on the twin Amal 930’s is   Main jet #140   Needle jet#106   Throttle valve (or slide) #3 1/2   throttle needle is 2 indent (which is the 2 small engraved lines above the clip grooves) and the clip position is second groove down or midway.

For the straight mounted 1-2 everything is the same apart from the main jet which is a #170.

See his other stuff here and here.

Amal site.

Notes On Rebuilding the Amal Mark 1 Concentric Carburetter.

This old tractor. Amal info.

Hitchcocks Amal.

Posted February 1, 2012 by xscafe in Carb - Amal

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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: The Greensand Foundry   Leave a comment

the greensand logo


A while ago I picked some rather tasty finned covers up from The Greensand Foundry. Look good on the XS.

They’ve been busy lately. Alongside single carb manifolds they’ve extended their catalogue.


more on offer


iron cross generator cover


iron cross points … celtic pentagram generator cover


celtic pentagram generator cover


pentagram cover


pentagram generator and finned points


generator cover


13 generator cover


yamaha tuning forks


and for the points cover too


super sport points and finned clutch covers


super sport points covers


XS points and finned clutch/oil


XS points covers


email at:

fleabay at: hardtail28-2008

These are cast. In his small workshop. A one man band.




getting ready








backyard industry … literally … space is tight


single carb manifold


amal mounted


in frame … here with no air filter … am sure I have a pic with filter-cant find it


Contact details:

email at:

fleabay at: hardtail28-2008

  • click on: ‘Angebotene artikel’ or ‘Items for sale’ … found in the profile box where the seller can post his photo if desired.

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.