GP100 engine transplant

[richiewendt]

And what is the liklihood of finding a gp125 engine with my limited number of contacts? 0/nil?? Any ideas, anyone know of a gp125 engine lying around the place?

[Slingshot]

So do GP100 and GP125s have the same bottom end? And is there enough space under the engine side cover where the carb is located to fit say a 28 mm flat slide carb in? And where do you get these odd litle bits and pieces anyway?

You could always cut the carb cover off.

Here's a pic of what I mean:

http://www.kiwibiker.co.nz/forums/at...0&d=1201509070

And what is the liklihood of finding a gp125 engine with my limited number of contacts? 0/nil?? Any ideas, anyone know of a gp125 engine lying around the place?

Trade me is your friend.

[richiewendt]

So would it affect the engine if you lengthened the piping between the carb and the inlet port at the side of the crank case? Would it be possible to put some piping in at a 90deg angle and have the carb located at the slightly more conventional position? Would that affect the performance? Obviously you don't want to have a bend in the system cause it reduces the air flow... but besides that?

[F5 Dave]

Well you could, there was a chap that ran his carb behind the cylinder with a pipe. Don't know if it worked much different, he seemed to think it was ok. Theoretically there should be a number of different lengths you could run for the same intake timing nodes. But you would expect throttle response to suffer.

I'd keep the carb near the case to be sure.

[speedpro]

Been there, done that, on a AC50. It was also done years ago by some proper racing 2-stroke crowd. They played around with lengths and diameters of tubes. Trouble is you gain in one small rev range or two, and lose in others.

Resonance is your enemy.

[richiewendt]

So where the best place to start on the little GP100? Been keeping my eye on a pair of rg250 expansion chambers on trademe... would they do the job?

[richiewendt]

Been keeping my eye on a pair of rg250 expansion chambers on trademe...

Ok, they're sold now! But still...

[TZ350]

RG250 chambers work well on GP125's The differences between the GP100 and GP125 bottom ends are:- GP100 has a 22mm carb GP125 24mm carb, The GP100 rotary valve cover has a smaller inlet throat which can be ported to match the 125. The GP100 has milder rotary valve timing which can be changed to 125 timing:- inlet opening 145 degrees BTDC and closes 55 degrees ATDC. You can fit a GP125 cylinder piston and head straight onto a 100. Now that ignitions are "open" use a Kawasaki KX80 ignition, it fits right in. The ignition timing is the same as the GP100/125's, 20 degrees BTDC. The hot setup is to machine 1.75mm from the top of the GP125 barrel and "o" ring it. Leave the inlet timing as standard for a GP125 and "no" changes (porting) to the exhaust (opens 86 degrees ATDC) or transfers (opening main 116 ATDC, sec 118 ATDC boost 119 ATDC). Leave the head standard, and no head gasket. Use a B8h plug for short circuits a B9h and go up one jet size for longer tracks like Taupo. Use GP125 24mm carb and look in the bell mouth, you will see it is dished instead of convex like other carbs. I think this is to reduce the power of European learner bikes. Anyway Devcon and reshape the entry to the carb into a bell mouth. Use a RG250 chamber, the left hand one it the easiest to fit. If you get a RG250 chamber with the longer parallel midsection then use a 95 main jet, if you have one without the parallel mid section then use a 100 main Jet. To fit the chamber cut the mounting bracket at the rear off and the header pipe off about 30mm back from the exhaust flange. You can then hold the exhaust flange bit in a lathe and turn down and shorten the exhaust spigot until it fits the GP cylinder. Bolt it to the cylinder, hold the pipe in place while someone tack welds it. Tack weld the rear bracket back in its new place weld it all up and presto you have a GP125 just like the one that came 3rd in the last 2hour at Mt Wellington as well as clocking 31's (Keith Biddel) and another GP125 that clocked 30.5's (Joe Mead) against the top riders who range from 29.5's Honda RS125/MB100 through to 30 to 31.5's on their Honda CB125's bored to 150cc. My times are a bit slower 34's. its all in the riding, a GP125 ridden well can be very competitive. Good luck and hope to see you out racing soon.

[Buddha#81:Me]

RG250 chambers work well on GP125's The differences between the GP100 and GP125 bottom ends are:- GP100 has a 22mm carb GP125 24mm carb, The GP100 rotary valve cover has a smaller inlet throat which can be ported to match the 125. The GP100 has milder rotary valve timing which can be changed to 125 timing:- inlet opening 145 degrees BTDC and closes 55 degrees ATDC. You can fit a GP125 cylinder piston and head straight onto a 100. Now that ignitions are "open" use a Kawasaki KX80 ignition, it fits right in. The ignition timing is the same as the GP100/125's, 20 degrees BTDC. The hot setup is to machine 1.75mm from the top of the GP125 barrel and "o" ring it. Leave the inlet timing as standard for a GP125 and "no" changes (porting) to the exhaust (opens 86 degrees ATDC) or transfers (opening main 116 ATDC, sec 118 ATDC boost 119 ATDC). Leave the head standard, and no head gasket. Use a B8h plug for short circuits a B9h and go up one jet size for longer tracks like Taupo. Use GP125 24mm carb and look in the bell mouth, you will see it is dished instead of convex like other carbs. I think this is to reduce the power of European learner bikes. Anyway Devcon and reshape the entry to the carb into a bell mouth. Use a RG250 chamber, the left hand one it the easiest to fit. If you get a RG250 chamber with the longer parallel midsection then use a 95 main jet, if you have one without the parallel mid section then use a 100 main Jet. To fit the chamber cut the mounting bracket at the rear off and the header pipe off about 30mm back from the exhaust flange. You can then hold the exhaust flange bit in a lathe and turn down and shorten the exhaust spigot until it fits the GP cylinder. Bolt it to the cylinder, hold the pipe in place while someone tack welds it. Tack weld the rear bracket back in its new place weld it all up and presto you have a GP125 just like the one that came 3rd in the last 2hour at Mt Wellington as well as clocking 31's (Keith Biddel) and another GP125 that clocked 30.5's (Joe Mead) against the top riders who range from 29.5's Honda RS125/MB100 through to 30 to 31.5's on their Honda CB125's bored to 150cc. My times are a bit slower 34's. its all in the riding, a GP125 ridden well can be very competitive. Good luck and hope to see you out racing soon.

Shit do you get out much?

[TZ350]

Only when mum lets me.

[TZ350]

Been there, done that, on a AC50. It was also done years ago by some proper racing 2-stroke crowd. They played around with lengths and diameters of tubes. Trouble is you gain in one small rev range or two, and lose in others.

Resonance is your enemy.

The carb/inlet has a resonance peak just like the expansion chamber but smaller. The trick is to fit a very short( 100mm or so) exhaust pipe that exits into a large( car? ) muffler so there is no expansion chamber effect. Then ride the bike and you will notice a small but pronounced power band/peak from the carb/inlet tract. By changing the carb size, inlet tract length and port timing you can dial the carbs resonance into helping at peak torque, peak power, or longer tract length for pulling out of corners. Your choice how you want the resonance to work for you. Without tuning the carb/inlet tract it just falls randomly some where in the rev range.

I have seen studies done that show a very high and flat torque curve for 4-strokes (and this would work on 2-strokes) that have been equipped with a variable inlet tract that keep'd the carb/inlet in resonance all the time. Like traction control variable inlet tracts have been banned in F1 cars because of the advantage it gave to teams that developed the technology to use it.

[fridayflash]

easier to hot up with the carb hanging out the back too!
have you considered a ts250er engine transplant? hehe

[Slingshot]

There's some excellent info in there...thanks.

I've been meaning to get a bigger carb for my GP100 and that porting of the rotary valve sounds like a good idea.

Might have to have a crack at it

[F5 Dave]

hmm, I happen to have a spare complete KX80 ignition I was just thinking as I lay awake I must dig out list on TM. Anyone need one?

[Skunk]

hmm, I happen to have a spare complete KX80 ignition I was just thinking as I lay awake I must dig out list on TM. Anyone need one?

Would it fit a KH100?