Suzuki GN125 | First build

New legislation forces new riders to ride a 125cc for the first two years, this legislation just had its first anniversary so up until recently nobody in their right mind bought a 125cc bike. That is why cheap(-ish) 125cc's are few and far between, 20-year-old Honda CB125's usually go for €1500, so when I found a Suzuki GN125 for half that I bought it. I don't have my license (yet, the 15th of march I should ) but my dad has been using it to scoot around town, and it's running pretty well, but the fun stops at 80 kph.



I didn't want to get too excited too soon, but I'll be able to ride it in a month so I'm starting to prepare for my project of turning it into something a bit more exciting. I won't make any drastic changes to the geometry until I've been able to ride some serious hours and see what works and what doesn't, but I can already start with the rest. First thing will be opening it up and cleaning it. I've also found a stock seat for cheap, which allows me to work on a new seat whilst keeping the bike intact. Last thing is cleaning up the rear triangle and the wiring.


As of now, I have three objectives: (ordered by importance)

• Getting the fucker to do at least 100 kph.
• Emptying the rear triangle.
• Changing the stance to achieve a horizontal line from tank to seat.

I'm not quite sure how hard 1. will be to achieve, but after cleaning everything up, rejetting the carbs and losing all the unnecessary weight I expect to be pretty close.


I made a quick photoshop mockup to see how much the suspension needs to change in order to achieve a horizontal tank, and also did a rough sketch of the seat, and it looks like it will mess with the fork angle pretty bad, so I'll have to see how that turns out.





For cleaning up the rear triangle the two main things I need to get rid of are the battery and airbox. I like the looks of pods, but from what I understood the stock carbs don't like them, so I'll see how they like uni's and consider a carb change if the stock air box really is the only choice. Since the bike doesn't have a kickstarter I'll need a fairly big battery so ,although I'd prefer a skateboard seat, I'll be going for a classic seat pan, probably using the tank as a mold to keep it cohesive, to store the battery in. I really like the look of this seat and will try to recreate it in black with red stitching to match the Uni filter.


I could ramble on for ages, but this is the to-do list for now: (in roughly this order)

• Full tune-up according to Haynes. New oil everywhere, brake pads, the works.
• Working on a seat. I'll do this early because there's not a whole lot that will change about it, it can be done without making significant changes to the bike and will probably take a lot of time because I'll be doing the upholstery from scratch with very little experience.
• Figuring out the suspension, how much fork dropping? New Hagon shocks? Longer / shorter?
• Figuring out the wiring, deciding on a new battery (necessary to decide on the size of the seat cowl) and looking at lights. (I'll probably convert to LED and order from China, want to order everything soon because shipping can take ages.)
• Deciding on a place for the license plate, there are very strict rules about the location and angle, so fabricating a mount a bit like this fender will be the best solution.
• Frame modifications, rear hoop, tabs to get rid off? Tabs needed for rear-set and new seat?
• Rear set? Changing to low bars without a rear-set will kill my back, Viet's clamp-on rearset with custom linkage will probably be the easiest choice.
• Exhaust? Longer / shorter? Different muffler? Taped? Bend it to be horizontal and parallel with the other lines? Will probably mess with the rear-set, so have to consider that too.
• Fenders, the front will stay for sure, but cut down and mostly as a fork brace. Rear will be a the license plate thing and a hugger?
• Bars. Clip-ons or clubmans? Depends on stance and budget, clip-ons will probably be joined by a new speedo and tacho to fill up the triple tree which makes them the more expensive choice.
• Paint. Black with red accents, maybe, but we'll see.
• Try to accomodate for saddle bags, preferrably Ortlieb Speed Bags, which shouldn't be too hard to accomodate for.
• Probably a thousand things more...

So, yeah... There won't be a lot of action going on here for the next month, just me thinking out loud, deciding on the direction this build will go and asking loads of questions. All feedback is appreciated immensely!

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

It should be great fun and very educational to re-do the bike. However, 80 should be adequate for your first couple thousand Ks. As you add power and speed you will subtract tractability and reliability. Removing weight will be useful in getting better acceleration but ultimately it will be easier to reach 100 with a more powerful bike.

Good luck, I can't get over how good your English is, Dutch schools must be better than the ones in the States!

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

rundown said:

It should be great fun and very educational to re-do the bike. However, 80 should be adequate for your first couple thousand Ks. As you add power and speed you will subtract tractability and reliability. Removing weight will be useful in getting better acceleration but ultimately it will be easier to reach 100 with a more powerful bike.

Click to expand...

There might be some truth in that, even on a modern CB125's, which I can get up to 110 without too many trouble, I don't feel really safe on the highway. Might be a good reason to polish up my map reading skills and just stick to B-roads.

rundown said:

Good luck, I can't get over how good your English is, Dutch schools must be better than the ones in the States!

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Thanks!

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

Your photoshop picture was wonky of the bike. To get the tank flat you just need to drop the forks by an inch or a little more and keep the stock rear shock?

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

ManxieRacer said:

Your photoshop picture was wonky of the bike. To get the tank flat you just need to drop the forks by an inch or a little more and keep the stock rear shock?

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Agreed. I think just sliding the forks up in the triple trees an inch or so will help a lot. Looks like the line of the tank flows back and down, so lowering the front a little bit should help to get the angle to get a bit closer to flat. Maybe some longer shocks in the rear could help as well?
I also think lifting the back tank mount up a little bit on the frame could help with leveling it all out. If you're willing to modify the frame then that might be a route to go. It could open a bag of worms for mounting the seat and battery, not sure exactly how the frame on that bike is.

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

Nice plans, but kinda strange it only runs 80 km/h Lets krank that little bitch up
cant wait to see more progress, btw where around Rotterdam do you live? i live in the center, neer the Meent.

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

Nice bike, Tijmen. Looks brand new!

You can easily get 100kph on that little bastard, that won't be a problem. Just make sure to clean and tune the carb.

Looking forward for your build!

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

ManxieRacer said:

Your photoshop picture was wonky of the bike. To get the tank flat you just need to drop the forks by an inch or a little more and keep the stock rear shock?

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Yeah, that's what I thought. I'll just try and drop the triple tree before cutting the forks and will see how it works out.

CALfeRacer said:

Agreed. I think just sliding the forks up in the triple trees an inch or so will help a lot. Looks like the line of the tank flows back and down, so lowering the front a little bit should help to get the angle to get a bit closer to flat. Maybe some longer shocks in the rear could help as well?
I also think lifting the back tank mount up a little bit on the frame could help with leveling it all out. If you're willing to modify the frame then that might be a route to go. It could open a bag of worms for mounting the seat and battery, not sure exactly how the frame on that bike is.

Click to expand...

Changing the tank mounts is definitely an option, but only as a last resort. Weary of longer rear shocks because it will change the fork angle, I'll have to read into rake and all that jazz before making decisions.

cb400f caferacer said:

Nice plans, but kinda strange it only runs 80 km/h Lets krank that little bitch up
cant wait to see more progress, btw where around Rotterdam do you live? i live in the center, neer the Meent.

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I just dropped out of mechincal engineering in Delft and left my room in Delft, so currently living with my father in Kralingen again. I'll be gone for a week, but we should meet up after that!

Montag said:

Nice bike, Tijmen. Looks brand new!

You can easily get 100kph on that little bastard, that won't be a problem. Just make sure to clean and tune the carb.

Looking forward for your build!

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Let's hope so!

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

Took the seat off to get a quick peek before going on holiday for a week, here's what I found:


The previous owner told me he rose the seat 2 inches, turns out he did the same thing to the tank, so that should make getting everything level a bit easier.





The levers have everything integrated, so tank fluid and mirrors are all attached to it, which means I'll be getting new levers because I want bar-end mirrors.





It got weird foot-rests and pedals, I've asked Viet if he can make a rear-set for me.





Wiring isn't half bad, although I'll be able to get rid of couple of things (like that side-stand killswitch pictured above) and the gauges can stay too. I'll probably make new backgrounds for the gauges because there's an extremely ugly vomit / seaweed green now.
[/size]



The plan of buying another stock seat to use as a base for the new seat is scrapped, the plastic will be a bitch to work with, the foam is too soft and there's a load of crap bolted to it for rising the seat. I'll probably only use the thing in to front that attaches it to the tank.





So yeah, gained some insights, will do some sketching this week and will really kick off the build next week!

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

So some Luxembourgish asshat decided to switch lanes without checking his mirrors:





Ruining our car and our holiday, but through some divine intervention a concussion and some neck-pain was the worst that happened to us. Back in Rotterdam now, with loads of free time on my hands. So I can kick off the cleaning and will be able to go to a big motor fair and see if I can pick up some parts.

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

Glad to hear you guys are okay! Good luck on the parts search as well.

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

Not nearly enough progress for the amount of free time I had on my hands, but the bike has to stay in one piece until my father has his own mode of transportation again. This entire weekend was spent cursing at my computer because doing sheet metal in Solidworks is nigh impossible, but I managed to come up with a template. I want to recreate the saddle on this XS360:





So I recreated it in Solidworks so I can throw different dimensions at it and do some rapid prototyping. I hope it can be made out of two pieces of bent aluminum, but I'll discuss the feasibility of that with someone who knows more about metal than I do. Here's a couple of images of my mock-ups so far: http://imgur.com/a/WoNCT


Four things I have to decide on:
[list type=decimal]
[*]Length of the flat part, I really need to have the bars and rearset before I can decide on this.
[*]Length of the cowl, I'll decide on this based on the length of the upholstered part and on the size of the battery. Ideally it wouldn't reach past the rear-axle, but I'm not sure whether that's possible.
[*]Angle of the cowl, unlike the Yamaha my Suzuki has a rounder tank, I think I'll aim at getting it co-linear with the cap on the tank.
[*]Height of the skirt, the PO raised the seat and tank by about 5 cm so that's the most logical length for the skirt, but it will depend on the height of the upholstery because it will have to be about the same size, or preferably a bit smaller than that.
[/list]

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

Point of no return, I suppose:





Week of sunny weather, so a week of wrenching. Will strip the thing, decide on suspension and send out the design for my seat Viet, who will make it along with the rearset. Also gonna order the bars, thinking clipons but I can't imagine how they could have made the triple tree uglier, so I might just stick with clubmans so I don't have to worry about that. While waiting for the parts a big clean-up and rewiring is in order. Not sure if I'll keep the old tree or just start from scratch, I think the latter.

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

Forks will be dropped by 3 inches, which will look this:





Decided not to change the rear-shocks because I don't want to buy cheap shocks and don't feel like shelling out for something fancy because it's my first build and it will be sold as soon as I can ride something bigger. Deciding on suspension means I can finalize my design, so big things will happen soon. In the meantime I'll be drawing a new wiring diagram, getting rid of some stuff and moving some other things. Also gonna buy a bench grinder and will start on cleaning up the covers and forks. Cleaning up the engine will happen once I'm sure the wiring is 100%.

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

You'd probably be better off raising the rear than lowering the front.

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

Agreed. 3 inch drop in the front seems like a lot to me. Although, I'm not a suspension guru or anything. Have you looked in to RFY rear shocks? I've heard a lot of good things about them and they aren't too expensive, although I'm not sure how much they would cost where you're at. I know they're around $90 US here.

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

Sonreir said:

You'd probably be better off raising the rear than lowering the front.

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If I want that horizontal line I'll definitely need both and when I have €150 burning a hole in my pocket I'll definitely buy some Hagon shocks. I made a CAD model so I could see how shorter forks or longer shocks affect the angles and getting it level will require either a bigger drop in forks than I'm currently comfortable with or comically long shocks. So I might buy some shocks when I'm 100% sure on what I need, but for now I'll live with the 5 degree angle the tank and seat will have. Some of the possibilities:


http://imgur.com/a/HF6eJ

CALfeRacer said:

Agreed. 3 inch drop in the front seems like a lot to me. Although, I'm not a suspension guru or anything. Have you looked in to RFY rear shocks? I've heard a lot of good things about them and they aren't too expensive, although I'm not sure how much they would cost where you're at. I know they're around $90 US here.

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I considered them, but mostly read negative stuff about their quality and that they at least require a rebuild to be acceptable. There's a GN400 on the forums (http://www.dotheton.com/forum/index.php?topic=43635.0) with nearly 2.5 inch of spacers and the fork another 2 inches past the triple tree, so I don't really worry about that. The difference in fork angle is minimal (114 -> 110) and the overal difference in rake too.

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

Well, I figure if you aren't f***ing up your rake and trail to the point of ruining the handling, and you've done some math to prove it then go for it! Like I said, I'm not a suspension guru

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.

I just can't figure out a way to fabricate a seat around that ridiculous bend the shocks attach too. I could have known, because nearly every GN has a flat saddle but yeah. Over at the Specials forum there's a dude who did a brilliant rebuild of the GN frame completely eliminating that fuck up, but that's way too much down time and I'm not really planning on investing that much effort into this little scooter. So change of plan, I'm going the way of the user cierrecart of this forum with a flat seat with a red diamond stitch, exactly like this: http://silodrome.com/suzuki-gn400/

Also ordered the plugs and wires, tail light and blinkers and bars. Lights and bars will take at least two weeks to arrive, but hopefully I will be making a new loom tomorrow. Also gonna order a set of gasket, so I can do a top end rebuild and clean all the casing and covers in the progress. The carburetor boot and exhaust header both show quite some cracking, so after cleaning and sealing it up I hope it will be a bit more lively.

http://imgur.com/a/48yuf

The gasket sets for the carbs are nearly impossible to come by, would some liquid gaskets do the trick or would I better off buying some gasket paper and cutting new ones by hand?

Re: The Suzuki GN125 is all about speed. Hot, nasty, badass speed.




Phew, first part of the wiring is done, got rid of a good amount of crap and everything should be working. Mailman just delivered the plugs, wires, heat-shrink and all that jazz so I'll start with wiring tomorrow.