Archive for the ‘carburetor’ Tag

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: 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.

 

1 .. damage like this

damage like this

 

2 .. and 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.

 

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

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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