Suzuki GSX 250 1980 stalls and idle speed drops to zero

I am a newbie riding a Suzuki GSX 250 1980 and have been having
problems with it stalling after a few kms of riding. It only seems to
happen when its climbing. Slight backfire then a loss of power. If
it's hot the oil light comes on and it won't start again until you've
waited five minutes or so. If its cold it can be kept running when it
starts stalling by dropping back a gear or stopping and starting
again. Today I spent my hard earned on a professional mechanics
skills, he cleaned the carbs replaced a nut that some amateur had
plonked in there and got them running together, it has a little more
acceleration but now the stalling problem is exactly the same. After
the tune up I went for a short run to see if the problem had
disappeared and now when it's idling it drops to zero and dies. The
mechanic reckons I may have a faulty cdi unit. I might be pretty
ignorant but does anyone here think the tune up wasn't quite right?
Any suggestions would be appreciated.

 

A pound to a penny the valves are burning or burned out, and the valve
clearances way too tight. These bikes were notorious for it.

 

CDI units are mysterious block boxes that usually either work or they
don't work. If they quit working, they rarely start working again.

I have had experience with two CDI's that did act up before they
finally quit working altogether though.

But, you might check all the wiring connectors from the fuse to the
ignition switch, through the kill switch on to the coil and to the CDI
unit. Also, check any connectors associated with the pulser coil on
the end of the crankshaft. It signals the CDI to fire the spark plugs.

Like TOG said, check the valves for the correct clearances. If the
valves don't have any clearance, you'll lose compression when the
engine is hot. Engines won't run once the compression drops below a
certain point.

You might be just running on one cylinder and the engine struggles and
then the other cylinder quits and you have the problem of hard
starting which you mentioned until the engine cools off.

But hard starting when hot is also a symptom of lean idle mixture,
Your idle jets and idle mixture ports might be getting plugged up with
gum and varnish.

I recommend putting 3 or 4 ounces of a clear carburetor cleaner like
Berryman B-12 or GumOut or STP into a full tank of gasoline and riding
to see if that cleans out all the little passageways and ports inside
your carbs.

If you can't find any of the carburetor cleaners I mentioned, try
mineral spirits (paint thinner in the USA) or a mixture of xylene,
methyl alcohol (like methyl ethyl ketone), acetone, toulene, etc.

 

"...a mixture of xylene, methyl alcohol (like methyl ethyl ketone), acetone,
toluene, etc." also known as "lacquer thinner", available in the paint
department instead of the automotive department. It even smells just like
the Berryman's carb cleaner.