Motorcycle Safety & Driveability: knowing how to ride safely a wild horse, vacuum effect, yamaha xs

Question
hello, William,
thanx for the kind presence on the web and the kind and unselfish
help you are supposed to provide for helpless bikers;-)
I own a 26yr old Yamaha xs 380 and she (!) is a nicely humming cat.
But as soon as real power is needed and I turn up the acceleration
there seems to be a hole - the motor seems to cough or breathe deep
through and the acceleration is gone, I fall back in speed. I then
turn back the handle and give it a moment of balance and it runs
nicely again. But top speeds of 68 mph are obviously below her
abilities...she could run faster if that vacuum effect did not take
place. I wonder if I will have to check the carburettor from the
inside...oh...
Then I have the funny "pri" position in this bike plus a fuel on and
a res position. I wonder if I will always have to run the bike on RES
- since the carburettor runs dry after 200 meters in the "ON"
position and I have never tried the "PRI" position for regular
running. My bike is new to me - but may I kindly ask your help ? If
you would please tell me...and if I read your news I will appreciate
very very much
thank you for the time being
Ric


Answer
Ric,

Don't ever run your bike in the PRI position. That stands for "prime", and it's only used when the carburetor is empty after storage. It allows the free-flow of gas into the carb and if left parked in the 'prime' position it will fill your engine with gasoline- not good.

Sounds like you have a bad petcock (the gas valve on the tank) on your bike. You need to replace it. I think once you do that, your bike will run right. It sounds like it is starving fo fuel as the speed (and fuel needs) increase. Empty the tank, remove the petcock, and see if it is clogged or blocked in some way (should be obvious). Check inside the tank for anything other than gas (a rag, whatever).

If you can't see anything straigh away blocking the petcock valves, it probably has internal problems, which is to be expected after 26 years. If you feel capable of taking it apart and cleaning it, you may be able to fix it. If not, you need to get a replacement. The Pingle company makes a wide range of petcocks, contact them for a suitable replacement.

An easy fix, just don't leave it on PRIME!

Bill Roberson

Bill Roberson