Archive for the ‘Carb – Boots’ Category

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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XS650: Carb Boots and Diaphragms   1 comment

Some time ago I had to rebuild my carbs, BS34s from a 3L1 motor, was really no big deal, they had been sitting a while so I cleaned them and tarted em up. Rejetted, new needles. boots, diaphragms etc.

The original Yamaha boots were hard and cracking after just 30 odd years. Anyway I ordered new boots etc and installed the carbs. After 3 months the boots had cracked to shit on the outside and begun to melt on the inside. WTF? And European fuels dont have the same ethanol content as North American-I see the other day that this will be changing in the near future. This was my first experience with that particular supplier I cant even bring myself to name, but we all know who he is with his cheap chinese crap.


damage like this


and this


Anyway, to cut a short story longer, JBM Industries came to the rescue. Theyve been making stuff for the aircraft industry.

These boots dont use the original metal cage and will secure your carbs unsupported. They make BS38 boots both straight and angled flange and BS34 boots. Also the diaphragms.



NOTE. These will have no problems with ethanol containing fuels.

My BS38s are running great. It’s a shame as I prefer the BS34s. I guess it’s good that Paypal is my friend.

JBM’s first carb diaphragms were for our Mikuni CV BS 38mm. Next were those for the BS 34’s. They’ve gone on to design and manufacture parts and compression molding tooling for a variety of vintage motorcycle carburetors. Don James, head of JBM, uses only a specified quality TSR heat cured compression molded rubber compound. (Not that crap recycled, zero quality control crap we are becoming used to from my favourite low service partner). Molds, tooling and production are all done ‘in house’.

TSR … Technically Specified Rubber, a system for describing rubber compound formulations. Graded according to dirt, ash, nitrogen, volatile matter content and the Plasticity Retention Index (PRI).

A first rate design, tooling and molding operation with excellent customer service!

Cost is very reasonable. Reflects a lack of ‘middlemen, mark-up merchants’  (distribution/wholesale/retail links). Products sold directly at a reasonable margin to the customer/user. A sound business model.

JBM Industries – thumbs-up – keep up the good work guys.

Posted January 10, 2011 by xscafe in Carb - Boots

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