GN250 Cafe Racer

[underdaseas]

Hello, I have been admiring your GN250. I just purchased a 1991 GN 250 Suzuki and would love to learn how to take it apart and put it back together again. I live near Kaiserslautern Germany and am an American. Would you like to teach an old lady with a fair amount of mechanical ability the ins and outs of the GN 250?

[TheRealWolf]

Hello,
because I live near lake Constance it will be difficult to meet for rebuilding your GN.
Yesterday I tested m moddings at the careburator in Combination with a new intake Air filter and so I got my GN on a test drive up to 135 km/h on a light downhill.



At the end of this video you can see my GN250, it is no fake. The Windshield is not as big as the camera shows, ist because of the wide angle I used. Maybe next year I try the 300ccm kit and an other clutch.
Greetings from Germany, Wolf

[ducatilover]

Go the GN!

With the 300cc kit, be very careful installing the locating dowels under the barrel/cylinder and measure their outer diameter before installing, anything below 12.95mm and the cylinder can twist off center, which will do some damage.

I learned the hard way that not all Chinese made dowels are very good, I ended up using some original Suzuki items.

Or... you could just fit a much larger engine like I have

[JMSR]

Howdy all, I've been lurking around for too long and thought this thread was GN cool enough for jumping in, showing off, and asking questions. Nice bike RealWolf and thanks for adding to the inspiration, the discussion about the spacers under the tank helped heaps. I even went as far as to put sandwiched spacers in the front seat mount to match. I hope you don't mind me hijacking your thread but I thought the title was generic enough for more than 1 bike. My build is more scrambler than cafe racer due to my 6ft frame but the principles are the same, just taller. Here's the build so far, early days yet and plenty of questions to come. Mods, in chronological order, are: 50mm tank and seat spacers (1st photo), straight bars and new tail light, seat cut and shaved (much easier than anticipated), trial tank stripping (looks great most angles but dominates the bike side on, black tank ordered), new rear guard rear mounting bolt holes drilled to lift tail (in line and under original holes, close to the edge gives nice lift), guards and rear shock shrouds cut down, black tank and smaller indicators, new Cycleworks full exhaust system (moved the needle to richen it up to suit) and moved tail light back, removed centre dash between dials and lowered front end 38mm (attacking corners feels much better with this mod). LOTS more mods to come including full engine mods and hot carb. This is easily the most fun I've ever had with 3 weeks and $800!

[JMSR]

Ahem, more than 6 photos.

[ducatilover]

Looks cool dude.
Be warned, get any more power from it and you'll destroy the clutch. don't ask how I know...
For carbs, be careful what you choose, I'd personally try find a Mikuni TM32 for it
You'll want to have the block decked for more compression and you can bore it out to run a DR350 piston, I've got a motor taken to 300cc, but you can go bigger. You can safely run an 84mm bore in it, if you set it up correctly (that leaves 3mm of cylinder liner)
Then if you are to cam it, you'll want to ditch the valve springs, they're not the best and are probably already out of spec...
Then port work, this helps 'em a bit if you keep it simple and smooth the ports out, they're rough as guts and not matched.
You'll need to do the bearings in the motor and make triple sure you clean the oil galleries out (the bearings from factory in Jap ones didn't cope to well with my mods, the Chink ones are worse)

You'll want to upgrade the clutch springs, they're made of cheese.
It's not likely to ever want to rev, the crank and flywheel set up is fucking heavy.

[JMSR]

Hey there, that's all good news! No bad surprises there for sure. I've gone far deeper with mini engines. I'm wasn't going to go any higher than 300cc because I wanted to keep it bullet proof and have yet to decide on re-bore or piston kit but yeah clutch is going to be done with the first engine mods for sure. Thanks for the tip on the carb, I'll start having a look around. You know a good cam guy?

[ducatilover]

There's an excellent guy in Palmy who does great work for a good price, I forget his name though

I'd suggest re-bore and running a DR350 piston, or an early GSX-R 1127 one, but with the Gixxer one you'll need to run teflon caps on the gudgeons, because the 1127 runs a different size pin. You could also look at O/S bandit 1200 slugs, but the early 1127 is flat topped and you don't need to cut in valve reliefs.

I have a spare 300cc kit, but the piston wore out of round when the block locating dowels decided to be crap. so it could be taken out a little and fitted with a DR350 piston.

It will be reliable at 84mm bore if you're got 'er set up correctly. I was contemplating builing one with an alloy piston from a BMW M50B25, but can't be fucked, the DR600 GN I have should work a wee bit better

I've got a brand new cam if you're after a fresh one for a re-grind. I've got GN bits all over my garage...

[JMSR]

Hmm re-bore and DR350 piston sounds like me. We have a couple of really good engine specialists in Dunners that I would trust with that. When you say 'set up correctly' is there a secret knack here or just the usual 'don't be a plonker' stuff? I did find the TM32 Flatsliders on amazon for a starter so I know they're a go. I'll pass on the cam at this stage thanks but will keep it in mind. Oh and a GN600!! That's just asking for a flux capacitor! And I ain't talking washing powder.

[ducatilover]

Hmm re-bore and DR350 piston sounds like me. We have a couple of really good engine specialists in Dunners that I would trust with that. When you say 'set up correctly' is there a secret knack here or just the usual 'don't be a plonker' stuff? I did find the TM32 Flatsliders on amazon for a starter so I know they're a go. I'll pass on the cam at this stage thanks but will keep it in mind. Oh and a GN600!! That's just asking for a flux capacitor! And I ain't talking washing powder.

Not really much secret knack, just not revving too hard and keeping the oil pressure and temp reasonable, plus do not run it too lean, it'll melt a piston. A DR piston is miles away from the limit of how far you can go though. But, from my experience, you won't get a heap of power from it without reshaping the squish area in the head, or having a much, much lighter crank/flywheel. I was building my 300 to try get 30hp, it went pretty well for what it was, but it shat the bottom end and then punched some valves, oops!

I can sell you most parts you'll want, have a new cam chain, new cam, good cam sprocket (these do wear, I have a spec for them written down at home, can get it to you next week) good tensioners. A bored out block with 1mm taken off the base, so with a DR piston you'll be at around 10.5:1 comp. To run a DR350 piston the block I have needs to be taken out another 1mm, it was running a GSXR1127 piston, but the dowels were undersized (12.95mm, should be more, OEM are 12.97) and it twisted and split an oil control ring during run in

But as I've said, standard roller bearings do not like high comp.
here's the state of the GN6 a few months ago when I was last home should be a giggle when I've finished it, less spastic than the other toy.

[ducatilover]

Oh, and with a TM32, you'll have to run a different intake rubber as the standard carb is a BS34SS and has a larger external diameter. If you find an intake runner from another bike that runs a TM32 then make an adapter plate, you'll be sweet

I have some Kawasaki ZZR400K rubbers for the CVKD32, may work, I'll give you one if you need it.

[JMSR]

Ok so not so much secret knack but more like a series of small, secret explosions . I still want it reliable through the rev range so I'll stick with 300cc and a head job. If I have to worry about heat and revs then it wouldn't be in the spirit of the bike I'm going for.

Is lightening and balancing the flywheel and crank shaft an option? Made my last mini sing at high revs.

Was thinking about the carb and it should be tunable from 250cc-350cc to cover every outcome. I'm guessing Mikuni would have set up for the TM32-1 to cover that?

And for sure I'll look your way for parts and knowledge in the future! Great to get to know someone who has been there, done that, got the parts.

[ducatilover]

Setting the carb up is piss easy, I had mine on the BS34SS and only needed to go up 2 jet sizes and raise the fuel level, no needle change or anything

They don't rev well anyway and I wouldn't bother touching the crank, too much money and then you'll have to either remove the balance shaft or have it balanced to the bastard.

There's really only so far you can go with such a shit motor with roller bearings

It's all a laugh though, I had a ball on mine, would hop on it instead of the ZX6 most times. Will be even better now

[JMSR]

The motor's not that bad. I love how the whole GN principle seems to be a fine balance between cheap and fantastic. Know what you mean about having a ball. Spent an hour riding around the Otago peninsula testing if I like the lower forks (Don't know about you but I thought the bike was much keener in attacking corners). That plus the new exhaust made me just not want to go home!

What set up would you use in the standard carb with a pod filter, the baffled free flow exhaust, and standard piston? A nice set up to keep me happy tweaking while I'm sussing the bigger things out in other words.

[ducatilover]

Mine has the front lowered, just for the looks. They scrape the pegs/run out of grip so fast I'm not too worried about trying to make it handle better, a set of sport demons is the best mod you can do on one.

On the stock carb I was up a jet size or two, can't remember what sizes they are, I think standard is a Mikuni 130.
I have a whole bunch of Mikuni jets at home for a standard carb.

The difference with a hacked up standard exhaust is really not noticeable to me. I'd just get it in tune and beat the shit out of it