Building a bike for Taupo

[Slingshot]

The real deal for GP125’s is Skim and “O” ring the barrel, no porting, KX80 ignition, RG250 chamber, 97.5 main jet.

Read Keith Code's books. You will have to experiment with riding style to find what setup works for you.

Some pictures of our GP's at Mt Wellington where we race wet or fine. In the last 2 hour (run anti clockwise) Keith Biddel was consistently clocked at 31's and Joe Mead at 30.5's on these GP’s. The really hot boys were about 30’s so our bikes are right up there. See you at Taupo.

I thought the GP100 & GP125 bottom ends were essentially the same but looking at that photo they're quite different.

[Buckets4Me]

whats so different ???
apart from the kx80 ignition and the oil pump removed and plated over the hole

or am I missing something

[Slingshot]

whats so different ???
apart from the kx80 ignition and the oil pump removed and plated over the hole

or am I missing something

I've just been tinkering and have realised that what I thought was different probably isn't different on a stock version.

On my GP100, the casing around the carb has been cut away, on further inspection tonight, it looks like the casing for the airbox has also been cut away. The bit that looked different was the bit with the sieve attached to it.

[TZ350]

GP100 and 125 bottom ends are much the same just different carbs 22mm for the 100 and 24mm for the 125. Different rotary valve timings which can easily be changed and the rotary valve cover on the 100 has a slightly smaller inlet tract which can be opened up to suit the 125. Everything else on the bottem end is the same. I did try a 125 rotary valve in a 100 once but it made the 100 motor very peaky.

Here are some pictures of one of our team enjoying a closer look at the Taupo track.

[Slingshot]

GP100 and 125 bottom ends are much the same just different carbs 22mm for the 100 and 24mm for the 125. Different rotary valve timings which can easily be changed and the rotary valve cover on the 100 has a slightly smaller inlet tract which can be opened up to suit the 125. Everything else on the bottem end is the same. I did try a 125 rotary valve in a 100 once but it made the 100 motor very peaky.

I've been thinking about putting a 125 rotary valve in, but may just alter the existing one. I've got an spare uncut case which I suspect is off a 125, the inlet tract looks bigger in diameter than the one on the motor at the moment. I'll have to pull it off and compare them.

What are your thoughts on altering the rotary valve timing on the 100?

Here are some pictures of one of our team enjoying a closer look at the Taupo track.

That nylon is doing a nice job in that first pic.

[TZ350]

I will look up all the timing figures for std 100/125's tomorow and get back to you.

Std. Suzuki GP100 inlet opens 135 BTDC closes 45 ATDC, Ex opens 94 ATDC, Trans1 opens 123 ATDC Trans2 125 ATDC Trans3 126 ATDC.

Std Suzuki GP125 inlet opens 145 BTDC closes 55 ATDC, Ex opens 91 ATDC, Trans1 opens 120 ATDC Trans2 121 ATDC Trans3 122 ATDC.

Std 1975 Suzuki RM125 inlet opens 88.5 BBDC, Ex opens 87.5 ATDC, Trans opens 116.5 ATDC. 23hp at 10500 rpm.

Read Graham Alexander Bell's book on engine tuning. You chose the revs you want to make power at and he gives you the timing figures for the ports.

[Slingshot]

I will look up all the timing figures for std 100/125's tomorow and get back to you.

Cool, thanks.

[F5 Dave]

Re Taupo Pic; Yeah but look at those fancy coloured leathers, that was bound to happen.

Keith can peddle fairly well for an old fella. That's a good time, but anticlockwise is faster that clockwise I believe.

[Buckets4Me]

that engine was sold on trade me as an ex drag bike engine for something or rather it had been played with but not sure what they had done not a lot I think cant remember how the add went I'm not surprised it's is different

good luck

[Slingshot]

that engine was sold on trade me as an ex drag bike engine for something or rather it had been played with but not sure what they had done not a lot I think cant remember how the add went I'm not surprised it's is different

good luck

You're spot on, the guy that I brought it off had built it for the Wanganui drags, it was built to look like an old fashioned drag bike so he'd airbrushed rust onto the motor. I think from memory, he did a 12ish second quarter mile on it. Here's the listing.

The transfer and exhaust ports have been cleaned up a bit and it looks like the inlent tract has been smoothed out too but I can't tell yet if the rotary valve has been altered.

[F5 Dave]

yeah that's authentic, 'cause those ally engine cases rust up real good

[Slingshot]

I've managed to get my hands on the article that Performance Bike Magazine did on the GP100 in the mid-eighties. If anyone wants a copy, PM me your email address and I'll send it through. It's 9 jpgs and they're about 2Mb each.

[F5 Dave]

Yeah I probably have a copy of some of it from back then, but not all & I saw that chap had promised a copy. It wasn't exactly revolutionary hi tech tuning, but worth a read I guess.

[TZ350]

Re Taupo Pic; Yeah but look at those fancy coloured leathers, that was bound to happen.

Keith can peddle fairly well for an old fella. That's a good time, but anticlockwise is faster that clockwise I believe.

You are right, its faster but the times for Keith and Joe compaire well with the fast guys going the same way on the day.

[TZ350]

I've been thinking about putting a 125 rotary valve in, but may just alter the existing one. I've got an spare uncut case which I suspect is off a 125, the inlet tract looks bigger in diameter than the one on the motor at the moment. I'll have to pull it off and compare them.

What are your thoughts on altering the rotary valve timing on the 100?


That nylon is doing a nice job in that first pic.

The skate board wheels do a good job of keeping it all of the track.

The first part of the GP's engine number on the crankcases should tell you if they are from a 100 or 125.