GP100 tuning?

[MikeyBrown]

Hi there,
I got shut of the GP125 in the end as there were just too many problems with it (Clutch, brakes, suspension, tyres etc...)

However my quest for a little bike wasn't over and I've ended up with a GP100 instead.

It runs sweet and is currently fitted with a k&n type cone filter (but the jetting seems bob on - plug is nice and brown). This has prompted me to try and find an expansion chamber for it to release some extra power! (I am aware that there is probably not a lot of horses to be gained but a few is good enough for me). Just want a little project and there's nothing to do with this one really so a bit of tuning is in order

I cannot find an expansion chamber that will fit straight onto a GP100 so does anyone know of one readily available that can be modified to fit?

Also, would filing the exhaust port a little do a lot without having to source an expansion chamber and are there any other little mods I could do to make it a little faster (more than 60mph would be nice really but with standard gearing tbh). I'm expecting the 'its a commuting bike, leave it' responses but I just wanna tinker with it a bit and 2-stroke tuning is 'meant' to be easy... Haha

Many thanks,
Mike
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Past Bikes: '97 CG125, '91 GPZ500S, '01 FZS600, '07 S/Triple, '99 ZX9R, '82 GP125, '83 GP100, '03 SV1000S, '98 ZX6R
Current Bike: '07 FZ1N
Trackdays: Darley Moor - 15/6/12, Donington - 28/8/12

[MikeyBrown]

In addition, do you think the standard airbox would make it run better than the cone filter? Even if the jetting is correct at the mo for the cone filter? Thanks, mike
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Past Bikes: '97 CG125, '91 GPZ500S, '01 FZS600, '07 S/Triple, '99 ZX9R, '82 GP125, '83 GP100, '03 SV1000S, '98 ZX6R
Current Bike: '07 FZ1N
Trackdays: Darley Moor - 15/6/12, Donington - 28/8/12

[stevo as b4]

If you want more mph using the same gearing, I'm assuming you want to push peak power higher up the rev range, so that the motor makes power into the red zone, and thus is how you expect to gain your extra speed?

You will obviously need close gearbox ratio's to be able to keep the engine in the power band which will get quite peaky and if tuned as above, is quite possible that when you rev out in a certain gear, that changing up to the next gear will drop the motor below the powerband, and thus you will lose speed and ruin the acceleration and ability to pull up hills etc.

I'd personally give up on trying to get a 100cc cylinder to give you more top end speed by revving harder, and work on improving midrange acceleration.

I've never worked on a GP100 engine, but I bet say another 10degrees of inlet port duration achieved by cutting the disc valve, and squaring off the inlet port opening into the crank case could be worthwhile, as could a slight increase in compression ratio, if you are prepared to run a cooler plug and use super unleaded fuel that is?

On a 125cylinder I would be thinking about trying to adapt an RG125 Gamma expansion chamber to fit the cylinder mounting point and the exhaust mounts on the bike.

Can you fit a GP125 top end to your motor? More capacity should always be the first and best option to think about when tuning a motor, and when you can't get anymore cc safely then I would turn to other tuning work like porting and carb mods etc.

[_Iain_]

I'm sure Nemo on here had a GP100 with a 125 top end on it. Might have been a complete 125 motor though...perhaps ask him.
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Please be aware that the above post may be full of complete nonsense.
Riding: '07 KTM Duke II, Baotian BT49QT-20 Driving: '88 Volvo 340

[Suntan Sid]

[MikeyBrown]

[MikeyBrown]

[Nobby the Bastard]

From personal experience, you also need to change the disc valve to the 125 one(it has a different opening times to the 100 and gives more power.)

Can't remember whether I had to change the carb, but I do remember that the 125 rear wheel hub is bigger and thus is a better brake.
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Triumph Sprint ST 1050

[Suntan Sid]

[stevo as b4]

https://www.kiwibiker.co.nz/forums/showthread.php/94649-Two-stroke-performance-tuning

Look at this link. It give port timings and disc valve timings for the GP100 and 125.

Based on that you need a GP125 disc valve, or cut yours to match.

But the GP125 has longer port timing and the head and barrel fit on your 100cc bottom end. With that in mind and your goal to increase performance, I'd be scouring breaker's etc to source a 125 top end that you can have re-bored and fit a new piston and rings too.

Once you get it to 125cc capacity and power levels, then other tuning like mucking about with different carbs, filter's and exhausts would start to make more sense.

[MikeyBrown]

[cb1rocket]

put the 125 head on and then send it to Mick Abbey for a tuned exhaust
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Honda H100S2 Rebuild - https://www.bikechatforums.com/viewtopic.php?t=253852

[moonzoomer]

GP 100 bore and stroke is 50 x 50 mm, GP125 and TS125 are 56 x 50 so either head and barrel should fit.