GN250 jetting advice

[Emmerson]

I have a 1983 Suzuki GN250 with the Mikuni BS34 carburettor. It don't go very well above 50 (and my licence says I'm allowed to go 70!).

Idle okay, revs out in neutral. Through the gears it's okay up to half throttle, above that it starts to miss and run really rough, losing power.

Above 60km (40 - 50 uphill), it is really rough. When I pull the plug it is really sooty.

Everything I've read tells me that I need to start with a smaller main jet, the current one is a #130, what should I get to try? 127.5 would be the next, or should I go to 125?

I've tried to change the needle - there's only one clip cutout, but I put a washer on top of the holder to push the needle down a fraction more - hopefully leaning out the mid-range, but it didn't really make too much difference.

Perhaps I should just take it to a shop. Who would you recommend in Dunedin?

[hayd3n]

hymm prolly best to go too suzuki for oe parts

[RobRod]

Sounds like it needs a tune alright. Any of the shops should be able to sort it.

[robinm]

Clean or replace the air cleaner, put a NEW spark plug in it. Check the condition of the needle and seat in the carb,replace if there is any wear on the sealing face, if it is the original it WILL be worn out and be a major cause of it running rich. Check the float level. Do all this before you start messing about with changing jets.

[Emmerson]

Everything I've read also says to start with the basics, so thanks for the advice, I'll do what I can myself and if it doesn't work out (and my limited mechanical experience suggests it won't), then Back To The Shop it is.

The air cleaner started to come apart as soon as I touched it, so that won't be helping much either.

[BASS-TREBLE]

Start with what the others have said.
50kmh is nothing for a gn. they are capable of over 100.

Does it start easily and is there smoke from the exhaust?

[Emmerson]

Well, with a new air cleaner, main jet and needle jet it is now running good. The needle jet looked noticibly worn though I have no real way of measuring that.

Before getting the new parts, I did play around with soldering / drilling the main jet, and after taking it down from the standard #130 to what I think is equivalent to #110 (theoretically a 1.1mm aperture), and this improved things markedly. The bike was fully ridable and would go up to 90, but was still very rich.

Now although it does not go any faster, and still hesitates a bit over 80, the plug is finally the chocolately brown colour that it is meant to be. Time then to look at the valave clearances and check the compression.

[FROSTY:Me]

Dude short version--its VERY unlikely that anyone has rejetted a GN250
-I'd talk to the shop about what is the factory jet size.

In other words Im prety sure the tree your barking under is the wrong one.