GN250 Update

[guest1406]

About 6 months a go I bought a GN250 for £325, only frame, forks, swingarm, engine, log book and a handful of parts

Gave it a good grind and weld and paint and fiddle. Not meant to be clean, something that looks like you can throw it about, made by myself, with basic tools, on the cheap.

Left to do is cover the seat, remake the exhaust (third time lucky?), wiring then hoses and cables and what not. Wiring will be sent to Steve Hallam in Croydon, he does a wiked job.

Just a quick one, I'll be running a ram air filter with a small baffle exhaust (so will need to re-jet), any ideas which way i should jet it? Go bigger or smaller jet size? Just wanna have the jets ready

Mackenzie







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[Propellor]

Looking good.

Which carb? What happened to the lectron? Different bike?

I'm no expert with carb tuning, but usually with a more free breathing set up you'd go richer. But it might not just be the main jet. You might need a different needle and a slide if your carb uses cutaways. Then again, it might be ok as is. 

If it's a cv carb...... Oh eck. Anything could happen. 

Should sound the biz. 

[guest1406]

Its the standard BS34SS carb, CV.

The lectron is on another bike. I havent looked at that bike in about 10 month. It stressed me so much i just put a cover over it and sadly ignored it. Lectron carbs are so technical and confusing, and I just dont know who to take it to.

I doubt id need a different slide? Thought just jets? Im no tuner .. .

I just need to get back on the road. Its depressing!

Mackenzie

[Propellor]

Is the underside of the carb diaphragm vented to airbox or to atmosphere?

[timbo]

Ok, so its hardly built to American Chopper standard, but I think the wee bike looks great. fair play to ya 

[Propellor]

Is the underside of the carb diaphragm vented to airbox or to atmosphere?

Before the airbox was taken off I mean. This will be a clue as to whether you may have bother or not.

I have pod filters fitted to my old Beemer and it hasn't affected the running. But when my mate tried fitting them to his er5 it wouldn't run past 4k rpm. We solved the problem in the end but it took some figuring out.

When you say ram intake do you mean velocity stack?

Cheers

[guest1406]

Erm im unsure Propellor. These ram air filters ( www.ebay.co.uk/itm/RAMAIR-MOTORCYCLE-SOCK-FOAM-AIR-FILTER-SINGLE-CARBURETOR-INTAKE-MS-022-/271386728845?pt=LH_DefaultDomain_3&hash=item3f2fe8d98d )

Cheers Timbo

Mackenzie

[Propellor]

Hi.

It's just a sock style filter as far as I can tell. Whether your carb will throw a wobbly remains to be seen. Try it and see. If it causes a problem I can tell you what solution I came up with on the er5. My mate couldn't afford to buy slide carbs so I had to figure out, the best I could, what was happening.

Good luck!

[guest295]

CV carbs need some restriction in order to work. The slide is lifted by vacuum, and if the intake's free-flowing there's no vacuum to lift the slide. An airbox evens out the impulses so the slide doesn't drop between intake cycles.

[Propellor]

CV carbs need some restriction in order to work. The slide is lifted by vacuum, and if the intake's free-flowing there's no vacuum to lift the slide. An airbox evens out the impulses so the slide doesn't drop between intake cycles.

On a mate's kawasaki er5 they seem to have built in a deliberate pressure drop across the intake snout to the airbox. Basically a restriction, as you say. But my own bmw r100 runs well with just a, pod filter on each cv carb.

I did notice a lot of fluttering of the slides on the er5 at low rpm, with the airbox off.

Regarding cv carbs working by vacuum, I'd more say that they work on a pressure difference between the upper side and the lower side of the diaphragm. The upper side is vented to the venturi, around where the needle is. The lower side is usually vented to the outer edge of the carb bellmouth, so airbox effectively.

So whether all cv carbs need a restriction as a principle of operation? I'm not sure I'd go that far.

[guest1406]

Update so far:

- It went off for wiring and its all running electrically well

- Exhaust had to be remade, its all tacked up just needs welding, cleaning then wrap.

- Tank had to be painted a little, the resin from fiberglass seat pan melted through the clingfilm protecting the bottom of the tank. Lesson learned, use tin foil. I scalloped the bottom in black (bare metal/pre paint in pics), came out ok. I mean its slightly rough but i can pass it of with the roughness of the bike. I'll post pics soon.

- The rear wheel had to be changed (there's a cry for help in the problems section) to the original.

- Found some alarming play in the Clutch Release Camshaft too but ill address one thing at a time.

I really need to get a move on, it needs to be finished by 4th of sept for a bike show called "Tripout" wiked show, this is the forth one.

Mackenzie

[guest1406]

Update so far:

- Wheel alignment is sorted and is true as true can be.

- Exhaust is all made, painted, wrapped and mounted

- Clutch is sorted, didn't have the arm on right.

- Did a compression test and got 125 ish i think, all good.

- Next thing after this was to get her running! Then more problems arose, the petcock to carbs nipples are different sizes. Bit of research and it turns out i was given the GN400 carb, 2mm bigger the the 250. Im gonna try and run it and tune it in as i haven't got the time or funds at the moment to get an original. Its also painful and an incentive to use the bigger carb as i had it ultrasonically cleaned and rebuilt which was £80ish. Who knows, maybe a big more power ha!
    Anyway I had a hose that would fit so put it on and used a kitchen funnel taped to the handlebars as a tank! Oil and petrol in and she started! Idle was way high and wanted to cut out a lot, so was tweaking the idle adjustment screw helped a weenie bit. Re jetting is a must (its got a 125 main jet size which is the GN250 factory size), any ideas in which way, high/low? It did backfire through the carb once and cut out and will only run with the choke on.
    Although engine running was cut short as the cdi unit started to get hot and smoke, as well as the coil got hot. When it was wired the guy told me about this and he said its probably the wrong coil or cdi, he said replace one or both. Luckily I have a spare both.

Anyone with a GN400 know if its petcock will fit the GN250 tank? The brass nipples are different diameters, just need to know the bolt hole dimensions.

Mackenzie

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