Building a Superbike out of a Suzuki GP100

[Slingshot]

Hi all, I'm new here and am hoping to tap into some of the collective knowledge of the members...here's the thing:

I've just completed my first season in the F4 class (less than 125 2-stroke/less than 150 4-stroke) on Yamaha RZ50/Suzuki GP100 Hybrid, I went ok and managed to place 6th overall but the big limiting factor at the moment is the RZ50 chassis, it doesn't handle too well. What I lacked in handling and grunt, I made up for with a lack of weight.

So, I'm building a new bike. It'll be a Suzuki FXR150 frame, probably TZR wheels and some old Yokohama slicks and my plan is to use the GP100 motor.

The new FXRGP is going to weigh a bit more so I'm going to lose my weight advantage, I now need to start thinking about tuning the motor. Lots of people have talked about the GP100 Plus that PB did in the '80s and I came across this thread on the old forum but I haven't managed to get my hands on a copy.

What I'm hoping is that someone will be able to give me some specifics around what to do with port timing, enlarging the rotary disk valve, ignition timing...etc. Essentially, anything that will improve the power output without blowing up the motor every race day.

And for those of you wondering about the F4 class (also known as Bucket racing), it's essentially commuter type bikes that are modified to race.

to give you a flavour (I'm the one getting up after a wee slide at 2:49). I've also uploaded a picture of what the RZ/GP hybrid looked like.

[reservoircogs]

Hi,

Heard about this class a few months back, like it. I've got a copy of PB from 1985 with a guide to posrting a GP100 barrell. I'll dig it our. Mark from Raccoon 2 stroke rebuilds is a 2 stroke fiddler...I'm sure he'd like to get involved...eh Mark?

Scottie

[Slingshot]

Hi,

Heard about this class a few months back, like it. I've got a copy of PB from 1985 with a guide to posrting a GP100 barrell. I'll dig it our. Mark from Raccoon 2 stroke rebuilds is a 2 stroke fiddler...I'm sure he'd like to get involved...eh Mark?

Scottie

Thanks, that would be most handy.

I'm guessing that Raccoon 2 Stroke Rebuilds is in the UK? It might be a bit far away....I'm in Wellington, New Zealand.

[raccoon rebuilds]

Thanks, that would be most handy.

I'm guessing that Raccoon 2 Stroke Rebuilds is in the UK? It might be a bit far away....I'm in Wellington, New Zealand.

yes might be a bit of trek for you !

i will give you some porting details and disk valve specs to hot it up a bit !

good luck sounds like fun.... mark...........

[nzknut]

It might be a bit far away....I'm in Wellington, New Zealand.

Hi i race a GP125 up in akl's F4 class, are you at atupo this xmas? i have some tuning advice for you.

[Slingshot]

yes might be a bit of trek for you !

i will give you some porting details and disk valve specs to hot it up a bit !

good luck sounds like fun.... mark...........

That would great, parts for these things are becoming hard to come by so anything I do really needs to be done right the first time, problem is I'm not really sure at the moment on how far to go. Thanks.

Hi i race a GP125 up in akl's F4 class, are you at atupo this xmas? i have some tuning advice for you.

If I manage to get the FXRGP sorted by Xmas then I'm hoping to get to Taupo. There's quite a few Wellington riders heading up and it sounds like a bloody good event.

[intra]

Up the bucket racers!

[Slingshot]

Up the bucket racers!

Thanks for your input

[intra]

Thanks for your input

No worries chief.

[nzknut]

Hi,

machine a 45 deg angle on inlet port side flywheel, machine head flat and use an O ring instead of a head gasket, what size carb are you running? use resin to smooth out the mouth of carb. also, try and rg250 chamber, they work well on the 125

[(F5)]

. . . and we're all here discussing this shit on the other side of the world.

With a air-cooled head as long as it is somewhere near flat Loctite master gasket is all you need to seal it. If you do use an o-ring make sure it is a reasonable (say 4mm) back from the chamber.

Start from basics, I'm guessing you haven't measured the port timings or compression ratio you currently have.

RM ignition or similar would be a good start.

The decisions you should make are how high are you going to rev it safely? (if you want mondo power a new rodkit would be mandatory & if so is there a better road based rodkit you can fit)?

Next if it stays 100 then larger carb & water cooling would be the go, else 125 & go for midrange.

[nzknut]

KX80 ignitions work, have no real advantage except being lighter, my 125 stock revved fine to 9'000, now it's "Race tuned" it should do 9'500 and have around 18horse. are you on KB?? ask tz350, he's very helpful/knowledgeable about the GP100/125

[davej106]

Hi all, I'm new here and am hoping to tap into some of the collective knowledge of the members...here's the thing:

I've just completed my first season in the F4 class (less than 125 2-stroke/less than 150 4-stroke) on Yamaha RZ50/Suzuki GP100 Hybrid, I went ok and managed to place 6th overall but the big limiting factor at the moment is the RZ50 chassis, it doesn't handle too well. What I lacked in handling and grunt, I made up for with a lack of weight.

So, I'm building a new bike. It'll be a Suzuki FXR150 frame, probably TZR wheels and some old Yokohama slicks and my plan is to use the GP100 motor.

The new FXRGP is going to weigh a bit more so I'm going to lose my weight advantage, I now need to start thinking about tuning the motor. Lots of people have talked about the GP100 Plus that PB did in the '80s and I came across this thread on the old forum but I haven't managed to get my hands on a copy.

What I'm hoping is that someone will be able to give me some specifics around what to do with port timing, enlarging the rotary disk valve, ignition timing...etc. Essentially, anything that will improve the power output without blowing up the motor every race day.

And for those of you wondering about the F4 class (also known as Bucket racing), it's essentially commuter type bikes that are modified to race.

to give you a flavour (I'm the one getting up after a wee slide at 2:49). I've also uploaded a picture of what the RZ/GP hybrid looked like.

Hi, I've got the GP100 Plus. I also have all the magazines relating to the specifications. If you send me your email address I'll send the engine mods to you.

[Slingshot]

Thanks everyone for the advice so far.

I pulled the GP100 motor out of the old chassis over the weekend and took the head and barrell off so I could compare it with a stock barrel that I've got.

According to my measurements, the opening/closing timing of the inlet & exhaust ports are the same as stock. The exhaust port has been opened up and it looks like the inlet tracts have been ground a bit.

The barrell has also had about 1mm ground off it, and it had a copper head gasket. Don't know what the compression ratio is, I'll try and measure that tonight.

I'm not sure yet if anything has been done to the rotary valve but the crank looks like it's been knife-edged. The oil-pump has been removed & the ignition isn't stock but I'm not sure what it is.

It's running the stock carb (I think it's a 24mm).

I'll put some photos of the bits up tonight.

Cheers John

[jamietzr250r]

nice hybrid, rz 50 was not available here but the rd80lc was, same chassis. i also did a hybrid and am in the process of rebuilding it:

its an import (to the uk) dt125lc mk3 ypvs in the rd80lc frame.

parts for each are 99% the same, and over here there becoming very rare. what is it like for spares over there?

[Slingshot]

There aren't many RZ50s over here and parts for them are like hens teeth, I'm lucky cause I haven't had to get any RZ specific parts. I just find something that's close and make it fit.

This RZ50 was imported in the early 90s (from Japan I think) and it was raced with some success as an RZ50.

Looks like you've done a nice job of your hybrid.