Triumphrat

StaggerLee

ON - RES - PRIME

On and RES are self explanatory i guess but what is prime for? And why do modern bikes not have the PRIME option?

Why would a bike run smoother on prime - does it bypass a gauze fuel filter inside the fuel cock or does it bypass something in the carbs?

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Thai me kangaroo down sport!

Woody1911a1

the petcock is vacuum controled , engine not running , petcock shuts off . the prime position is for when you've had the fuel system or carbs apart and there's no fuel in the float bowls .

"Why would a bike run smoother on prime" it shouldn't , the prime setting just bypasses the vacuum shut off function .

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StaggerLee

thx for the reply

How does the mechanics of the vacuum shut off / open work?

is there a diaphragm that the vacuum opens? or some other mechanism? I wonder if a bit of dirt or such is affecting it opening?

The fuel tap is for a yam vx750 ( friends bike)

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Thai me kangaroo down sport!

Woody1911a1

sorry never had one apart , but there'll be a vacuum line from the engine to a diaphram . now what kind of valve it uses i'd assume a plunger of some sorts .

take a look and see if there's screws holding the diaphram cover on . if so id think just take it apart and inspect/clean and make sure it operating .

also check the vacuum line

ps empty the tank first

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If you love it , let it go . If it comes back to you , you've highsided ;-)

midlifecrises

What's this ON RES PRIME. I have an 07 TBA and have ON OFF RES????

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midlifecrises

Age is no barrier.... born again biker!
07 TBA -- tacho, sissybar, footboards, windshield, AI removed, modified pipes.

KitNYC

On/Off/Res is an old-school, non-vacuum petcock. ON flows fuel, off doesn't, & res is reserve. Much simpler than a vacuum unit, but don't forget to turn it off!

Cheers,
-Kit