Frankie's replacement - FXR/GP100 hybrid

[TZ350]

When you reassemble the motor and are putting the hub that drives the rotary valve back, put a small amount of sealant on the end that is against the main bearing. This stops gearbox oil finding its way under the hub and into the engine.

[TZ350]

Here's a photo of the ignition. I think it's probably from the GP but it's been altered. According to the Haynes manual, it should have points, etc.

Interesting.

GP100/125's had all sorts of systems, points, energy transfer, cdi. This is a cdi unit which in my experience was not as good/reliable as the energy transfer system that had points. My personal choice is the KX80 ignition, there is a picture of one fitted to a GP125 near the beginning of this post.

Energy transfer systems work by:- the flywheel as it rotates, drives a current through the primary windings of the coil and slowly (relatively speaking)builds up a magnetic field within the coil (this is the energy transfer bit) just like setting a rat trap. When the points open its like setting of the trap, the magnetic field quickly collapses inducing a high voltage in the secondary windings which is relieved by a spark across the plug gap.

Modern energy transfer systems now use transistors. Motor mowers and small industrial motors both 4 & 2 stroke mostly use energy transfer systems. Magnetos are similar, unlike the energy transfer system which has a separate charging winding (in the motor) and a primary winding in the coil, a magnetos charging winding and primary windings are the same and the secondary winding is wrapped around all of them. Engine heat is the enemy of the insulation of the secondary winding and energy transfer systems proved more reliable.

[Slingshot]

Right, have made some crude measurements of the rotary disk (see the photo). It looks like the port is open for a total duration of about 142`. Not sure if this is standard or not.


And, it looks like the inlet tract in the case has been opened up a bit, but it's almost like it hasn't been done completely, the bit circled in blue is raised up a bit.


Hmmmm, anyone know of a spare KX80 ignition lying around somewhere?

[TZ350]

Yes it looks like the port has been opened up. measure the width (in degrees) of the port (which is 40 degrees on a STD GP motor) and add half of this to the total cutaway in degrees and you will have the total effective inlet duration, ie 20 + 142 = 162 total "effective" duration.

Measured total duration is 40 + 142 = 182 for simplicity this is the figure used in most books when they talk about timing and duration.

There is a practical difference between effective and measured port timing.

The notch is equal to TDC and you can measure forwards plus 40 (port width) and back plus 40 (port width) to get the opening/closing timing of this valve.

"Mean effective port area" is another way of measuring port area and it is where the area of the port is measured after the crank has moved 50% past opening and the area of the port is measured, this usually turns out to be close to 70% of the total port area. Used for the exhaust/transfer ports and piston port inlets.

[TZ350]

Hmmmm, anyone know of a spare KX80 ignition lying around somewhere?[/QUOTE]

You have to get the right type of KX80 ignition, one has a round backing plate like the GP in the picture above. You don't want this one, because it does not fit easily, wrong size. You need the one that has a triangular backing plate like in the picture at the beginning of the post, it fits really easily.

[bucketracer]

Boy, this is realy good stuff, TZ350 you seem realy on to it!!!

[gav]

Have you guys with the GP100/125's done much to the head and the squish band? A mechanic mate built up a fast GP100 a few years ago, and from all the things he did like 28mm flat slide carb, pipe, rotary timing etc he reckoned altering the squish band made the biggest difference of the lot. it involved welding up the head and reprofiling and changing the shape of the combustion chamber and raising the compression ratio.

[bucketracer]

I have heard about doing this and you have encouraged me to make this a must do!

[Slingshot]

Have you guys with the GP100/125's done much to the head and the squish band?

Nope, not yet. The Performance Bike Mag guys used a GP100 head on a GP125 barrel and machined a squish band into it. Would be an easier option for those running the GP125.

[Slingshot]

Yes it looks like the port has been opened up. measure the width (in degrees) of the port (which is 40 degrees on a STD GP motor) and add half of this to the total cutaway in degrees and you will have the total effective inlet duration, ie 20 + 142 = 162 total "effective" duration.

Measured total duration is 40 + 142 = 182 for simplicity this is the figure used in most books when they talk about timing and duration.

There is a practical difference between effective and measured port timing.

The notch is equal to TDC and you can measure forwards plus 40 (port width) and back plus 40 (port width) to get the opening/closing timing of this valve.

"Mean effective port area" is another way of measuring port area and it is where the area of the port is measured after the crank has moved 50% past opening and the area of the port is measured, this usually turns out to be close to 70% of the total port area. Used for the exhaust/transfer ports and piston port inlets.

I need to get myself a protractor



The crankcase has now been split in two, and I'm glad I did...there's a reasonable amount of sludge.

[TZ350]

And, it looks like the inlet tract in the case has been opened up a bit, but it's almost like it hasn't been done completely, the bit circled in blue is raised up a bit.



I have had a closer look at the flywheels and it looks like the inlet side has been cut back at 45 degrees. Graham Bell's book says this is a really good thing to do. Someone has done some useful work to improve this engine.

[Buckets4Me]

here is an interesting pic for you
compairing standard gp120 and gp100 rotory valves

http://www.kiwibiker.co.nz/forums/at...5&d=1223351570

[Slingshot]

here is an interesting pic for you
compairing standard gp120 and gp100 rotory valves

http://www.kiwibiker.co.nz/forums/at...5&d=1223351570

Yeah, I saw that. Nice one!

I have had a closer look at the flywheels and it looks like the inlet side has been cut back at 45 degrees. Graham Bell's book says this is a really good thing to do. Someone has done some useful work to improve this engine.

The crank has been beveled on both sides, I'll take a pic to show you what I mean. I think this is what the old owner described as the knife-edged crank.


Wee update on the GP100, one of the selector forks has a slight bend. I might try and have it straightened.

The old man is going to take the cases away and dunk 'em in his engine bath then bead blast them. So if I manage to get this thing back together it'll be the best looking GP around.

[Slingshot]

Spent some time yesterday at the workshop. Dad made up a thread insert to fix the stripped exhaust thread in the barrel while I bead blasted the other bits. They came up real nice!! Dad's going to source new bearings and seals and I'm gonna paint the cases then re-assemble it.



The forks were too far gone to be straightened, bugger.

So hopefully, the bearings and seals are fairly standard issue and I'll be able to remember how it all goes back together

[Skunk]

Looking good Slingshot. What colours are you thinking of? Grey with fake rust stains? That would be cool.