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Carburetors for 1970s TZ250/350 Yamahas

By Nick Parkyn

Phil Schilling in his 1979 article “The TZ Papers” stated “The whole issue of carburetion offered some real alternatives The E-type (TZ250E) came equipped with 34mm taper-bore Mikunis. But the hot tip in carburetors for a year or so had been straight bore 36mm Lectrons……Properly setup Lectrons allow TZ250s to pull strongly and crisply out of corners, there’s not hesitation or softness that’s normally associated with 34mm Mikunis which always seem to carburet rich in the upper mid-range. Thirty-six millimeter Mikunis (which have larger carb bodies than the 34mm instruments) were also reputed to work better than the stock (34mm) devices.”

Phil Shilling researched various (Mikuni) alternatives and chose to bore the 34mm Mikunis out to 35mm.

The bore was slightly off-center (slightly above the center of the old bore) to leave a slight groove in the floor of the throat for the slide’s trailing edge so that the slide would close normally. If the slide cannot close normally or carburetor is not accurately bored the engine will probably not idle. The modified straight bore 35mm Mikuni flow more air than a 34mm bore because of the greater area of the modified carburetor. Surprisingly the modified straight bore 35mm Mikuni flows more air for its size than the 36mm Mikuni. A 34mm Mikuni can be bored to a maximum bore size of 35.3mm (see reference 1).

Standard fitment on the TZ250 A through G and TZ350A through C models was the Mikuni VM34SC 34mm carburetor. Some tuners used different carburetors on the TZ250 A through G and TZ350 A through E in an effort to solve the upper mid-range problems.

Types of carburetors included:

-          36mm Amal carbs

-          36mm Dell’Orto carbs

-          36mm Lectrons

-          36mm Mikuni VM36SC

-          34mm and 36mm Mikuni TM Flat valve (later when these became available)

In general Mikuni carburetors are the most popular carburetors for motorcycles followed by Amal carburetors.

As indicated many tuners used larger than standard carburetors on the TZ250s and TZ350s, using 35mm or 36mm carburetors on the TZ250s and 36mm or 38mm on the TZ350s.

Standard fitment on the TZ350F 1979 model was 38mm "Powerjet" Mikuni carburetors. The Powerjet Mikunis allowed fine-tuning of the midrange mixture and increased power output. When Mikuni introduced Powerjets and  Powerjet versions of the Mikuni VM carburetors, mixture problems in the upper mid-range were a thing of the past and the use of carburetors from other manufacturers became unnecessary and  less attractive. Mikuni Powerjet carburetors were originally designed for and used on snowmobile racing engines as early as 1976, but were only used later on racing motorcycle engines.  Powerjet “upgrade” kits (Power Jet Kit) can be purchased from Mikuni and its distributors and can be fitted to Mikuni VM carburetors from 30mm to 44mm. Fitting Powerjets is fairly easy, but requires milling/filing  of the carburetor body and some drilling and tapping. Jetting of Powerjet carburetors is different, with the main jet size being 70 to 100 smaller, depending on the size of the power jet.

For example on the TZ250D/E:

For a standard (not Powerjet) Mikuni VM34 main jet size range is 320 to 360.

For Mikuni VM34 with Powerjet main jet size range 230 to 270 and power jet size range 60 to 80

Mikuni VM carburetors are still readily available from Mikuni and its distributors with VM34 costing less than US dollars $100 from some US distributors at the time of writing. When restoring TZ 250/350 it may be beneficial to purchase some new carburetors. However if you choose to use the original carburetors, you should have them ultra-sonically cleaned to ensure that all internal passages are clean and clear of dirt.

For further reading or reference.

1.Setup of Mikuni Carburetors including Mikuni Powerjet carburetors and carburetor boring is expertly covered in “Two-Stroke Performance Tuning” by A. Graham Bell.

2.Setup of  Amal, Dell’Orto and Lectron carburetors is expertly covered  in” Tuning for SPEED” P.E. Irving

3.Good overall coverage of carburetion can be found “Motorcycle Tuning Two-Stroke” by John Robinson

Setup of TZ250E is expertly covered in the article “The TZ Papers” by Phil Schilling in his article “The TZ Papers” Cycle magazine February1979.

Thanks go out to Nick Parkyn for providing the above article. 


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03/23/08 10:21 AM +1000    

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