Hey all

my gp3 is fairly smooth compared to my last cag and revs extremely fast as it has quite a thin and lightweight crankshaft. unfortunately it came from the factory with the two lobes offset about 2mm which i have just fixed.

but i thought with everything apart i should have a go at balancing it some so i put the whole assembly together and stuck it all in the lathe between a pair of live centers.



as youd assume the piston always comes to rest at the bottom. but by sticking an M10 nut on the flywheel adjacent the crank pin i can cause the setup to rest at any position! the weight of the nut is somewhere between 10-30g.



my question to the geniuses out there is this. is this setup balanced now it can rest in any position! And if it is done - should i add weights to the opposite end or drill out weight from the pin side? i would prefer to remove the material personally.

im aware also that you can never perfectly balance a single piston setup but never the less im sure it can be better than what it was.

im kicking myself i never tested the setup from the dreambike with the stuffed polini crank thats balanced. bugger.

Glen # 1 is that you will lose primary compression by taking metal off the crank. Balancing the crank is a bit of a misnomer, what you should be looking at is trueing the crank. Run on live centre's measure the run out and align. 0.02 is good but o.o5 is vibrations for sure. I would suggest talking to a kart or bike shop about "crank Balancing"

thats true, you will now loose crank compression so there will be less pressure flowing through the transfer ports, completly changing the airfuel mixture especially when scavaging, the motor will lean off. you will loose a lot of bottom end,

...........

there is heaps of tips on the net, just finding the information is the tricky bit.

I tend to find that more primary suits high revs and low for more torque e.g. motocross. There are some good two stroke software out there MOTA by Ian Williams tuning is one and it is not too expensive.

	Bobalini wrote:
	I tend to find that more primary suits high revs and low for more torque e.g. motocross. There are some good two stroke software out there MOTA by Ian Williams tuning is one and it is not too expensive.

High revs means the charge (fuel air mixture) is delivered in smaller quantities but at a higher rate from the crankcase (primary compression) into the cylinder.
High torque generally means lower revs (relatively), so the crankcase has more time to fill, so can deliver a 'fuller' charge into the cylinder, but at a slower rate.
Hence crankcase compression is an important consideration for your engine. The danger of too much primary compression is that some charge is lost straight out into the exhaust port during the overlap in the port timing. Good cylinder design helps to minimise this.

GRCmoto_Australia wrote:

Bobalini wrote: I tend to find that more primary suits high revs and low for more torque e.g. motocross. There are some good two stroke software out there MOTA by Ian Williams tuning is one and it is not too expensive. High revs means the charge (fuel air mixture) is delivered in smaller quantities but at a higher rate from the crankcase (primary compression) into the cylinder. High torque generally means lower revs (relatively), so the crankcase has more time to fill, so can deliver a 'fuller' charge into the cylinder, but at a slower rate. Hence crankcase compression is an important consideration for your engine. The danger of too much primary compression is that some charge is lost straight out into the exhaust port during the overlap in the port timing. Good cylinder design helps to minimise this.

AND then there`s the tuned expansion chamber.
But none of this answers the balancing question.

ok your saying balancing? it does not look like the tools used above are good enough for precise balancing, it looks more like your using a metal lathe? as i said before removing too much metal from the crank will lower the crank compression and dramatically affect the motors performance. it is like porting it, if too much is removed it will wreck your motor, if the porting is not equal to get a good air fuel mix it will also affect the power. sign of removing too much metal is boggy to start, wont want to idle, will rev ok up high.

yeah its only a metal lathe, i just had it hanging off it to check the balance.

in the end the lobes where out by 1mm or so, so i didnt bother to touch the crank. time better spent on just buying a stuffed one in the first place i think. but if i was going to drill it out id stuff it with cork and epoxy to keep the compression up. which would keep it up and balance it up i would think?

JUAT BUY A POLINI STUFFED CRANK

i would have but have to keep this one for the chinese class, got the polini blue in the dreambike love it!