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Motorcycle Repair/vintage Triumph electrical issue

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Question
I have a 1972 650 Triumph Trophy.  When cruising at medium to high RPM, the exhaust note will suddenly change to "flat", then one cylinder "goes away". Next, quickly, the throttle becomes totally unresponsive.  If the engine is allowed to go to idle (it never dies), it comes back immediately, and is completely strong.  I have new points, condensers, coils, plug wires, and the battery stays up. Single carburator checks out and both pet cocks are open. It never happens under full throttle, but only at light-load cruise.  Doesn't seem to matter if the lights are on or off.  I'm guessing that it HAS to be the Zener diode or the rectifier because there's nothing left, but I don't know which, or why????  HELP!

Answer
Hi Rich,

It sounds to me like your running low on fuel mixture at
mid throttle.  Since it runs at idle and full speed the idle
and main jet are likely okay. I am not sure how
carefully your carburetor was inspected but there
is a jet above the main jet that has air/fuel emulsion holes
in it. These holes tend to get plugged up and reduce
the flow of fuel from one quarter to three quarter throttle
openings. The holes have to be physically cleared out
to be sure they are not blocked.

Another possibility is a blocked fuel tank cap vent which
can slowly create a vacuum in the tank as the fuel goes
down.

The zener diode and rectifier are there only for the charging system
so if it is charging properly they should have little effect on the ignition.

Since the bike runs at full throttle I don't suspect the ignition
as much as the fuel supply. Float level might be an issue
but again if you have full throttle operation then it is more
likely the needle jet emulsions holes are plugged.

WS
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Motorcycle Repair

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Wayne S.

Expertise

Licensed Motorcycle Mechanic, Knowledge of motorcycles from 1960 up, Japanese, British and most other brand motorcycle repairs.

Experience

Worked for Yamaha, Honda, Suzuki, Kawasaki British: Triumph, Norton, BSA Other: Most Scooters and Mo-Peds

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Canadian Motorcycling magazine article

Education/Credentials
Licensed Motorcycle Mechanic

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