GN250 Flat spot

Hi
Have a problem with a flat spot on my GN250 (1990)
Runs fine on tickover - gets to 4000 revs and flat spots, no matter
what gear in.
Changed the plug first and gapping correct, cleaned the jets in the
carb, airfilter is changed, timing ok.

I am suspecting the carb diaphram but the screws are well stuck
although have sprayed them with wd40 didnt want to force my way in.

The bike has only done 19k miles so would expect that the needle in the
carb should be ok, but pulling my hair out.

Anyone had the same problem and fixed it. I have had the bike about 2
months and it has always had this problem.

One thing if I rev past 4000 revs it picks up no problem

 

What sort of exhaust do you have on the bike?

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

Constant vacuum carburetors run on the idle jet and idle circuit until
the slide lifts above the 3/4th's position, and the engine will never
develop enough vacuum to lift the slide if it doesn't get enough fuel
through the idle jet and idle circuits.

I recommend adding about 3 ounces of Berryman's B-12 Chemtool Choke and
Carburetor Cleaner to a full tank of gasoline and going for a slow
ride. Buy the liquis B-12 in the 15 ounce can for adding to the
gasoline.

If you want to try spritzing out the idle circuits under pressure, you
can use B-12 in the aerosol can. There is probably a small hole on the
right hand side of the carburetor inlet bell.

That hole leads to the pilot air jet and the pilot air passage
downstream of that jet connects with the passage leading from the idle
jet. So, squirting B-12 into the pilot
air jet will cause the strong solvent to come out of idle jet into the
float bowl, and out of the four idle ports in the carburetor throat
downstream of the butterfly.

If you want to check your carburetor diamphragm for leaks, take the
Phillips screwdriver that comes in yout tool kit, put it into the X on
top of each street and whack the screwdriver with a hammer a few times
before you try to turn the screw.

If you want to take the carburetor apart for a thorough cleaning, you
will probably find that the needle jet is an air bleed type that has
cross-drilled air emulsion holes. The needle jet is the brass cylinder
that the needle goes up and down in. The needle jet probably can be
pushed up to remove it. There might be a rubber o-ring on the jet.

To spritz B-12 past the idle mixture screws, you'll have to drill out
the EPA anti-tamper plugs.

I have written about how to remove the EPA anti-tamper plugs in this
forum about a bazillion times. If you follow the procedure for removing
and reinstalling the idle mixture screws after cleaning the idle ports
out, you should be back to the factory setting.

 

as far as I know just the standard exhaust

 

Thanks for that, I have looked for B-12 but can't find it in the UK
there are several similar treatments here including redex advanced so
guess one of those will be ok - will give one of those a go before
stripping down the carb.