PDA

View Full Version : GP100 engine transplant



richiewendt
12th March 2008, 12:37
I was wondering whether a tf125 engine would work with regards to the electrics in a GP100. And as I would like to keep it road legal for now while also getting it ready to bucket race, would VTNZ cry to much about the 25cc increase? As long as the engine mounts aren't mucked around with too much? Would they even notice?

Slingshot
12th March 2008, 17:48
They probably wouldn't notice. Not to sure about the electrics though.

If you do the transplant, you're going to have a spare GP100 motor...have you got plans for it?

richiewendt
12th March 2008, 18:56
Not really, interested? If the transplant works, i might indeed have an extra engine!

F5 Dave
13th March 2008, 08:47
Read up on the LTSA website carefully. If you want to keep it legal there may be some fish hooks. Certainly my RG50 with TS125 never cut the mustard. You can do engine swaps (they are largely thinking cars) & there are some regulations about power output changes. You may squeak in with this.

Or you may find that no one wants to WOF it as they aren't sure. In which case it's $400 for an LVV inspection/certificate (if it passes) & possibly a re-VIN if they get silly on its current status. Obviously not worth it.

Think about how much to repair the GP engine or find a GP125, now that they would never notice & likely not to care. Further the airbox/exhaust/electrics etc would all line up ok. The details are what makes swaps difficult & I've done a few now.

richiewendt
13th March 2008, 22:23
Just for interest sake... Does anyone race rm125 engines in buckets? And for the moment, would an rm125 engine support the electrics? Anyone have any Suzi 125 engines lying around the place that they don't want, preferably ones that dont need a complete rebuild and that run... new piston I can do... tf motor fell through...

quallman1234
13th March 2008, 22:40
Just for interest sake... Does anyone race rm125 engines in buckets? And for the moment, would an rm125 engine support the electrics? Anyone have any Suzi 125's lying around the place that they don't want, preferably ones that dont need a complete rebuild and that run... new piston I can do... tf motor fell through...

RM engine's not allowed, as they are motocross engines.

Sorry slingshot for asking this but

What would the point be surely the TF engine doesn't put out that much more power than the GP (if less) and the gearbox's wouldn't be suited for on road as well (But may well rock at the bucket track!)

richiewendt
13th March 2008, 22:43
RM engine's not allowed, as they are motocross engines.

SO anything that is competition not allowed? SO what would be a good and common 125 engine be then?

Buddha#81
14th March 2008, 06:21
SO anything that is competition not allowed? SO what would be a good and common 125 engine be then?

Most 125's are bored out 100's, MB,GP,etc. Yamaha do a RH125 and as F5 said Suzuki do a GP125. The old 2 strokers are becomming rear now due to 30 years of bucket racing sucking them up.......Is your 100 dead or are after more power. If your not going to bucket race it and are hell bent on a putting a MX motor in it you will be forever throwing money at it in the way of piston and rings, not to mention rear tyres......be bloody cool though.

F5 Dave
14th March 2008, 08:27
SO anything that is competition not allowed? SO what would be a good and common 125 engine be then?

The general idea was to make the racing competitive but cheap. If they allowed MX engines people would turn up with increasingly new engines (can't buy an MX engine by itself usually, have to buy a whole bike as usually its the engine that gives up first). Then some people would buy a brand new one & no one else would be competitive. MX or other competition engine would roast everything else,but than again would be expensive to run in comparison, especially as they age. On the road they would be very short lived.

100s are allowed any size carb & watercooling mods. 125s must be aircooled & 24mm carb max.

Back to the original engine, 2 strokes are cheap to rebuild, unless it's carved in half.

richiewendt
14th March 2008, 19:03
Most 125's are bored out 100's, MB,GP,etc........Is your 100 dead or are after more power. If your not going to bucket race...

Na, I do want to race it, there's no doubt about that, but I am poor and so for the mean time I want something that is still relatively road legal... I would chuck some decent tyres on and rip the lights off every few weekends and take her out... Am in the process of selling my RG150 and have dosh to burn (not too much though)

So, could I buy a 125cc piston and bore the barel out, or is that too much of an increase? I want a 125 so I could use it for a bit of highway use, not long distance though, and also I have little experience in tuning (none at all really) so I reckon it would be easier to just get a 125cc and play around within the restrictions, a lot safer than tuning up a 100 to get the same/more power than a 125 and stuffing it up. You're right, the engines are getting rare as I've found out!! Hence I don't want to learn the hard way and destroy an engine!!

Pumba
14th March 2008, 21:59
So, could I buy a 125cc piston and bore the barel out, or is that too much of an increase?

125 air cooled is the largest two stroke you can have in F4

richiewendt
14th March 2008, 22:43
yeah, but I mean can you bore a 100cc barrel out to a 125cc, is it physically possible, or is that asking too much... 100-125cc is quite an increase...

I've been doing my research and the stroke for each bike is the same, but the bore is 50mm in the 100 and 56 in the 125. is a 6mm difference quite big, esp when the barrel is quite small? thats 3mm ring bored and honed out...

diesel pig
14th March 2008, 23:30
yeah, but I mean can you bore a 100cc barrel out to a 125cc, is it physically possible, or is that asking too much... 100-125cc is quite an increase...

I've been doing my research and the stroke for each bike is the same, but the bore is 50mm in the 100 and 56 in the 125. is a 6mm difference quite big, esp when the barrel is quite small? thats 3mm ring bored and honed out...

In short No, There is not nuff meat in the barrel. If you want a 125cc you gotta get a 125 barrel and head.

richiewendt
16th March 2008, 13:44
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?

Buckets4Me
16th March 2008, 19:18
you need a piston and barrel off a gp125 and that is all
but the timing will be different and not quite as fast as a compleat 125 engine
i dout you will notice the difference

richiewendt
16th March 2008, 20:07
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
17th March 2008, 18:19
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/attachment.php?attachmentid=83710&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
19th March 2008, 12:20
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
19th March 2008, 14:00
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
19th March 2008, 20:06
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
19th March 2008, 22:57
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
19th March 2008, 23:01
Been keeping my eye on a pair of rg250 expansion chambers on trademe...

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

TZ350
29th May 2008, 20:18
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
29th May 2008, 20:40
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
29th May 2008, 21:21
Only when mum lets me.

TZ350
29th May 2008, 21:45
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
29th May 2008, 21:49
easier to hot up with the carb hanging out the back too!
have you considered a ts250er engine transplant? hehe

Slingshot
29th May 2008, 22:00
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 :2thumbsup

F5 Dave
30th May 2008, 09:10
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
30th May 2008, 09:34
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?

F5 Dave
30th May 2008, 09:50
no idea. Save your money & get one or two bikes sorted.

Slingshot
30th May 2008, 10:01
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?

I think my ignition has already been worked. It doesn't look anything like the pictures in the Haynes manual.

What type of ignition is it? CDI?

F5 Dave
30th May 2008, 10:05
no I think it's little miniature elves with switches a can of electricity that all live in a small black box. I think your one was RM so should be fine.

I bought the KX one but wasn't as well suited to the 50 as what I had, but you don't know unless you try.

Skunk
30th May 2008, 10:48
no idea. Save your money & get one or two bikes sorted.
Done. Now where was I? Oh, yes...

TZ350
30th May 2008, 19:23
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?

What kind is it? I have seen two basic types, one has a completely round base plate and an external trigger coil on the side and the other has a roughly triangular base plate. The triangular one can be made to fit GP125's easily and I imagine other Suzuki's. The round one's external trigger coil fouls the case making it more difficult but not impossible to use.

What kind do you have?

F5 Dave
3rd June 2008, 09:39
I must dig it out, I fit it to my RG50 fairly easily, just it didn't work as well on the 50 as my other one so I never used it. I'll drag it out when I have a mo.

F5 Dave
5th June 2008, 10:54
Yeah mine is the round one, I'd made up a plate that bolted on the std RG mounts that I would have thought was same as other suzukis & that put the trigger in the space area at 2 O'clock. Then I re-slotted the woodruff key in the flywheel so timing was in right area to adjust.

TZ350
7th June 2008, 20:13
Hi F5, does the stator have an external trigger coil? Do you have the complete ignition system, stator, flywheel, black box and coil? How many $$$$ ?

richiewendt
15th June 2008, 16:36
I just realised something after reading the past few comments, I know jack shit about tuning 2 strokes! I don't want to push things too far, as my budget is very limited, so Bang is bad... The 100 is staying just that, a 100. I reckon its a pretty decent engine, had heaps more grunt than my old ax100, with a sloppy shit piston too! I reckon Ill just grind a mm of the top of the exhaust port for now and maybe shave a little off the head to increase compression and get a new piston... That should give it a new lease of life, and then from there I can play with the rotary valve and carby. Besides, no use having something fastish without having much racing experience and with a chinese plastic front tyre.

Buckets4Me
15th June 2008, 19:12
i have found a good stock gp125 to be realy good up here at Mt Wellington
nice tight slow track under 100k/h
much better drive out of the corners than some of the tuned bikes
helps me as I'm no valintino
if i could ride better a faster bike may be an advantage but the pull out of corners when i muck up makes up for that

koba
15th June 2008, 22:30
I just realised something after reading the past few comments, I know jack shit about tuning 2 strokes! I don't want to push things too far, as my budget is very limited, so Bang is bad... The 100 is staying just that, a 100. I reckon its a pretty decent engine, had heaps more grunt than my old ax100, with a sloppy shit piston too! I reckon Ill just grind a mm of the top of the exhaust port for now and maybe shave a little off the head to increase compression and get a new piston... That should give it a new lease of life, and then from there I can play with the rotary valve and carby. Besides, no use having something fastish without having much racing experience and with a chinese plastic front tyre.

If you have a shitty old piston in it now I may (just from what I have read - I am no expert) be a good idea to file from the top of the piston rather than the exhaust port, that will give you a reasonable idea of how changin the port timing will affect the characteristics of the motor.
barrels = expesive.
pistons = cheaper.
and you were going to replace it anyway right?!
you can file a bit off the top in line with the exhaust port and do a little at a time and test to see the effect then if it works ok you may even want to just file the new piston to the same point as the old one and leave it at that.

as for the 2 stroke tuning: read good books!
I just read one by A. Graham Bell and it is farken mint, try get ur hands on some good tech books on the subject and you should be able to build a real fire breather:scooter:

richiewendt
15th June 2008, 23:38
ok, this'll all have to wait for a bit, I just crashed the gp100 doing 80ish kph. Back tyre burst/rapidly deflated in the wet!, and hit the deck... I reckon I was pretty lucky though, just scrapes and bruises!

koba
16th June 2008, 10:24
ok, this'll all have to wait for a bit, I just crashed the gp100 doing 80ish kph. Back tyre burst/rapidly deflated in the wet!, and hit the deck... I reckon I was pretty lucky though, just scrapes and bruises!

scrapes on the GP and bruises on you?

richiewendt
16th June 2008, 22:12
I have that book saved on my computer (Graham Bell one) as PDF... If anyone wants a copy, shout! Though some of it is still a bit too advanced for me, even though its pretty straightforward. Also have a computer expansion chamber designer, where you plug in a whole lot of numbers and it decides an optimum chamber for you bike. Generally speaking do they work??

Yeah, I was pretty sore afterwards, thank god for nurofen though! Was lucky as if it had happened a 100 further down the road, I would have been on a steep downhill with barriers on each side towards the bottom. Would have been f***ed up! Scrapes on the GP, and no skin on me where my jacket rode up my arm! Mind you, nothing compared to some of the stories I could read about no doubt!!

Skunk
17th June 2008, 12:27
Also have a computer expansion chamber designer, where you plug in a whole lot of numbers and it decides an optimum chamber for you bike. Generally speaking do they work?
I'll let you know. Mines 80% built to that program. I used the program then cross checked it with Bell's book. Very similar specs (within my building tolerances anyway).