1971 Kawasaki G3SS


This bike will be my daughter's first street bike...
Timid little suckers with a stock HP rating of 10.5...
The good part is this little rotary valve 2 stroke can be modified to make double that HP... :D She weighs in at 174lbs and I'd like to get it down to 160...

Basically hide all the electrics and oil tank in the tail, upgrade the front brake to disc., build the motor and pipe.

I got started on the motor over the holiday's by shopping on eBay...
The stock cast iron 3 port 90cc cylinder was upgraded to a G4TR 5 port cylinder. At the same time, I bought some cases from a 1993 KE100, mainly to save some port work time to adapt the different cylinders...

This is the bike that inspired me on this project... Not mine or anyone I know, I just found the image on the web. I photoshopped my first go for the pipe layout.

With any luck, I'll have it ready for the start of riding season in May... Maybe wishful thinking though!!


New Member
Here you go, the guys name was Mitey F, I haven't seen any posts from him in a while but he may just be lurking rather than talking.


show and shine


Build bikes to ride, not to look at!
How are you Walms? Wondering how that bike would look with a mid pipe like a scrambler. It's too bad the air intake is where it is or you could have the pipe come round from one side and go thru between the frame and cylinders to the other then up a bit then straight back just below and parallel to the seat frame rail. Challenge your pipe design skills ;)


Hey Reds, happy new year! I doubt I'll be using the stock filter location. I doubt the carb will fit under the cover. I'll just have a UNI hanging out the side. :)
Being a 1 lunger, I'll have at least 3 pipes for it!
You can bet that I'll take advise on the cross thru. ;)


New Member
Awesome. I have a 73 sitting in my shop that's next on the chopping block as soon as I finish my Sr500. Picked it up for a song and been riding it around the parking lot after reworking the carb. the stock 10hp is modest but still makes it move along pretty nice even with my 200 pound self on it.


Well things are starting to move forward... Slowly :)
I decided to model this bike as I did my last build. I like the idea for performing FEA plus working out the geometry and of course knowing what seat/ tail will look the best.

My current thinking is to go with this airtech tail but nothing set in stone



More playing around... Not happy yet though but going in the right direction, I think... at this point, I'm going to switch gears and design a high pipe for it... :)

I tried an oil tank in the frame but I'm thinking now I might keep that area open.

On the motor side, I'm still sourcing parts... My Frankenstein assembly will be made up from parts from motors ranging from 1971 to 1996... :)
I'm a sucker for punishment, I swear!
In the end though, I'll have a motor that will have readily available parts. I believe they made the same KE100B from 1981 to 1999
The thinking know is to port to G31M specs and open the intake to handle a 26mm carb.
In the end, I'll probably re-contour the head as well.
I like your engine mods. I restored a '71 G3SS and sold it about 4 years ago - still have a bunch of spares including most of another bike though for a future project. Great motor, odd kickstart mechanism, I bought 6 used carbs before I got one good enough for a rebuild. The aluminum top end you're using would have been a great addition to the motor but I was doing a resto, but maybe for the next one...?
My bike went faster than Kawasaki's literature said it would with very little effort. I made sure there were no manufacturing gaffs that needed attention (on my bike, the cylinder sleeve partially blocked the disc valve's intake port and had to be ground back), I trimmed the cylinder base gaskets so they didn't stick into the transfer ports, made certain that the cylinder base transfer ports matched the crankcase (they needed some matching up), did very careful motor assembly, stuff like that. The cylinder port timing was left alone but I did take about 10 degrees off either end of the disc valve - I finally trimmed a NOS phenolic valve and really polished up the modified edges as I couldn't find a steel valve after looking for almost a year. On my bike, the cylinder head volume was way too high - Kawasaki added about .030 extra material to the head gasket area for some reason. I was going to get a machine shop to mill it but ran out of time, but think its something I'll do on the next one.
Regarding seats to fit the narrow G3 frame, on my bike I shortened the rear (almost) vertical frame tube so that it didn't stick up any higher than the horizontal upper rear frame tubes and allowed candidate seat pans to rest on the frame rails. At various times - before we decided to do a full-on resto and use the rebuilt stock seat, I had trial fitted a Bates TT seat, a '71 Ducati 750S solo seat and a Big Bike racing seat to the bike. All 3 are pretty narrow and all 3 looked good with the stock tank, so it would depend on the "look" you are after for your project. The Airtech Knight Seat you showed the photo off is pretty wide at its max - over 9" I think - and the kickup at the back is pretty substantial, so I don't know if it would look that good on the little G3. Airtech sells a Bates TT seat similar to the one we make, so look on their website for photos if you wish.
Hope your daughter enjoys her bike... it sound like she has a swell Dad.
Pat Cowan,
Vintage Motorcycle Fiberglass


Thanks for the reply Pat!
I as well noticed the sleeve! inlet misalignment on the original 3 port cylinder, the 5 port is much better but still needs to be tweaked a bit.
I've sort of changed direction on the seat at the moment... I've decided to attempt to make one out of sheet metal with the same scalloped look as the pipe.
My daughter decided on British racing green for the colour so that is set in stone now... I just need to get the pipe to fit this darn low slung frame!
I tried a high pipe but it looks too much like a dirt bike...

I'll be going this route for sure now...



Pat, a quick question about the front brake... I've been thinking I'd like to swap to a disk but so far no luck finding a good candidate, other than the Honda CG125 04-08 had a disc brake on a 40 hole rim... Everything else has been a small dirt bike rim but they are only 28 spokes.
Are the stock brakes as weak as they look?


New Member
Looking forward to your progress. I've been putting some miles on my '69 G3TR lately, and have really enjoyed it. Surprisingly powerful with a bit of porting to the rotary valve and a hotter pipe.

Ya the brakes are at least as bad as they look LOL - the same brakes as the trail bike, totally inadequate in 1971 for a street bike and damn near killed me a couple of times in the 21st century - your daughter deserves / needs something a lot better.
You can get quite a bit better braking though by making certain your drums are round (my weren't), deglazed and then radius the shoes to the brake. There are brake shops around that still do this or you can do it yourself (laborious, ask me how I know).
The real fix though is to get a larger front brake - drum is the easiest and indeed that's what I'll do on my next bike. The easiest mod is to look for a drum with the same axle diameter as your G3 - maybe one of the larger Kwackers (mixing in the family seems to work best). Shooting from the hip - check on a DLS front brake from an A1, an S2 or go outside the family and see if the RD125, RD200 and CS3 Yamahas have a similar diameter axle. The brake stay fab is relatively easy, same with the cables and I'd keep the stock back brake and do the aforementioned drum checks.
As for a pipe, see if you can compromise with your daughter and use a G3TR high pipe on the left; doesn't look too bad, the factory heat shield keeps you from getting burnt provided you wear appropriate riding gear, it performs fairly well for a 40+ year-old design, bolts right on and the last time I checked, they were still fairly cheap.
Regarding Green Streak porting; back when the earth was (Kawasaki) green, I rode a Centurian - just once - and it had a power band like a toggle switch. Good fun for an expert MX-er but might be a bit much for a tyro rider on the street.
Have fun with your project.
BTW, tell your daughter she has exquisite taste in colours.
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